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# It's (DOM) Clobbering Time: Attack Techniques, Prevalence, and Defenses
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It’s (DOM) Clobbering Time: Attack Techniques, Prevalence, and Defenses
Soheil Khodayari, Giancarlo Pellegrino
CISPA Helmholtz Center for Information Security
Saarbrücken, Germany
{soheil.khodayari, pellegrino}@cispa.de
Abstract—DOM Clobbering is a type of code-less injection markups. The research community has only recently started
attack where attackers insert a piece of non-script, seemingly studying the security of these interactions, mainly focusing
benign HTML markup into a webpage and transform it on small code fragments called script gadgets [12] that
to executable code by exploiting the unforeseen interactions react to injected HTML markups and transform it into code.
between JavaScript code and the runtime environment. The Unfortunately, script gadgets are only the tip of the iceberg,
attack techniques, browser behaviours, and vulnerable code and other complex interactions exist that attackers can abuse
patterns that enable DOM Clobbering has not been studied to hijack the program execution, which, to date, are largely
yet, and in this paper, we undertake one of the first evaluations unexplored.
of the state of DOM Clobbering on the Web platform. Starting DOM Clobbering is a vulnerability that originates from
with a comprehensive survey of existing literature and dynamic a naming collision between JavaScript variables and named
analysis of 19 different mobile and desktop browsers, we sys- HTML markups, where browsers replace pre-existing content
tematize DOM Clobbering attacks, uncovering 31.4K distinct of an undefined variable with an HTML element when the
markups that use five different techniques to unexpectedly variable name and the element’s name (or id) attribute
overwrite JavaScript variables in at least one browser. Then, we match. Developers unaware of such behavior may use the
use our systematization to identify and characterize program content of undefined variables for sensitive operations, such
instructions that can be overwritten by DOM Clobbering, and as URLs for fetching remote content, and attackers can
use it to present TheThing, an automated system that detects
exploit it by injecting markups with colliding names. DOM
Clobbering vulnerabilities have been known for over a
clobberable data flows to security-sensitive instructions. We
decade, with the first instance identified in 2010 [13] where
instantiate TheThing on the top of the Tranco top 5K sites,
an iframe named self allowed attackers to overwrite
quantifying the prevalence and impact of DOM Clobbering
the top window location of webpages containing frame-
in the wild. Our evaluation uncovers that DOM Clobbering
busting code, i.e., assignments such as top.location
vulnerabilities are ubiquitous, with a total of 9,467 vulnerable
= self.location. Since then, security researchers have
data flows across 491 affected sites, making it possible to mount
identified new, more subtle attack variants, combining pairs of
arbitrary code execution, open redirections, or client-side re- HTML tags (e.g., [14, 15]) or browser-specific markups and
quest forgery attacks also against popular websites such as attributes (e.g., [16–19]), and clobbering not only variables,
Fandom, Trello, Vimeo, TripAdvisor, WikiBooks and GitHub, but also deep object properties (e.g., [20–22]), nested window
that were not exploitable through the traditional attack vectors. proxies (e.g., [21, 23]) and loops (e.g., [21]). When looking
Finally, in this paper, we also evaluate the robustness of at the possible combinations of tags, attributes, code features,
the existing countermeasures, such as HTML sanitizers and and runtime behaviors, prior works have merely scratched
Content Security Policy, against DOM Clobbering. the attack surface, and, to date, we still miss a systematic
and comprehensive exploration of this threat.
Index Terms—DOM Clobbering, Attack Techniques, Preva-
Recently, DOM Clobbering vulnerabilities in Gmail [22]
lence, Defenses
and Google Analytics [24, 25] revamped new discussions
about defenses, such as proposing to switch off named
1. Introduction property accesses for DOM elements at the browser level (see,
Arbitrary client-side JavaScript execution has been one of e.g., [25–27]), which has been dismissed since, according
the major threats against web applications since the early to Google Chrome telemetry data, about 10.5% of the
days, traditionally achieved by injecting JavaScript code into pages in 2021 use named property accesses to implement
vulnerable pages, e.g., Cross-Site Scripting (XSS) attacks [1– functionalities that could otherwise break [28]. To date, the
11]. However, over the past 20 years, the growth of Web burden of protecting from DOM Clobbering attacks is solely
technology has introduced unforeseen interactions between on developers’ hands, who can use existing countermeasures
JavaScript programs and the execution environment that such as HTML sanitizers tailored to protect against DOM
can result in execution of arbitrary code without injecting Clobbering, e.g., DOMPurify [7], or mitigate the risk of
JavaScript but only by injecting seemingly harmless HTML code execution via Content Security Policy (CSP) [29–31].
1
Unfortunately, DOMPurify protects only from specific DOM Listing 1: Example of DOM Clobbering vulnerability where named proper-
ties overshadow JavaScript variables.
Clobbering cases, whereas CSP cannot prevent the execution
1 var s = document.createElement('script');
of already-present code that reacts to markup injections, 2 let config = window.globalConfig || {href: 'script.js'};
suggesting that existing countermeasures may be incomplete 3 s.src = config.href;
or even insufficient. As a last resort, developers can develop 4 document.body.appendChild(s);
their own defenses, requiring a deep understanding of the
main threat and its variants, which, unfortunately, may not
be the case. For example, as witnessed by recent DOM Listing 2: Example of DOM Clobbering vulnerability where named proper-
Clobbering vulnerabilities discovered in HTML sanitizers, ties overshadow native DOM APIs.
e.g., DOMPurify [17] and HTML Janitor [20], developers 1 var s = document.createElement('script');
2 let b = document.documentElement.getAttribute('baseURI');
may still be largely unaware of the risk posed by DOM 3 s.src = b + '/script.js';
Clobbering vulnerabilities. 4 document.body.appendChild(s);
In this paper, we take a step back and take a deep look
at DOM clobbering, with, to the best of our knowledge, the more breakage, i.e., 2,561 websites, than benefits, i.e., 491
first systematic and comprehensive study on this neglected vulnerable websites, with a cost-benefit ratio of 5.2:1 web-
vulnerability class, covering three main aspects: a systematic sites. In the absence of a browser-level fix, developers need to
exploration of the attack surface, a measurement of affected be particularly careful when choosing a countermeasure, as
and vulnerable websites, and a review and evaluation of they balance protection with usability. For example, 55% of
defenses. Starting from a comprehensive survey of prior the most popular HTML sanitizers across the five most used
DOM Clobbering vulnerabilities, we systematically generate web languages are vulnerable to at least one of the 31.4K
candidate DOM Clobbering markups, and automatically test clobbering markups by default. The remaining 45% sanitizers
desktop and mobile browsers against them, covering all remove named properties, i.e., id and name attributes, which
known HTML tags and attributes–including custom ones–and may interfere with the DOM manipulation operations. Also,
markup relationships. Then, we propose TheThing, a DOM our results show that CSP is insufficient because 85% of the
Clobbering detection tool that combines hybrid program discovered vulnerabilities can cause code execution without
analysis, i.e., [32], for the discovery of potentially-vulnerable manipulating the src attribute. Finally, our results show that
data flows, with forced execution, i.e., [33], for the automated developers can fix vulnerabilities at the code level, and we
vulnerability verification, leveraging the generated DOM identify eight distinct vulnerable code patterns to avoid and
Clobbering markups. We instantiate TheThing against the propose four secure patterns to fix them.
Tranco top 5K websites to quantify the prevalence and
impact of DOM Clobbering vulnerabilities, processing, in Contributions. To summarize, this paper makes the follow-
total, over 24.6B lines of JavaScript code across 18.3M ing contributions:
• We conduct the first comprehensive and systematic
scripts and 205.6K webpages. Finally, we identify, review,
and evaluate defenses, covering existing countermeasures and study of DOM Clobbering, covering vulnerability, attack
secure code patterns. In particular, we first precisely measure techniques, detection, prevalence, impact, and defenses.
• We propose a systematic technique to identify DOM
the cost-benefit trade-off of browser-level countermeasures
and thoroughly test HTML sanitizers. Then, we review the Clobbering markups and test browsers automatically,
vulnerable code discovered by TheThing, identify common identifying 148 previously-unknown ones, 30,803 new
developer mistakes, and distill a list of secure coding patterns. variants, and 114 new browser APIs that can be clob-
Our results show that the attack surface of DOM Clob- bered in at least one browser.
• We present TheThing, an automated detection tool for
bering vulnerabilities is large, with only 481 out of 31,432
generated DOM Clobbering markups are currently known, DOM Clobbering that uncovered 9,467 DOM Clob-
and the remainings are either previously-unknown instances bering vulnerabilities, affecting 9.8% of the Tranco
(148) or variants of known cases (30,803). When grouping top 5K sites, of which 44 that we manually confirmed
markups by browser behaviors, we observe ten different to be exploitable, including popular sites like GitHub,
behavioral groups, showing that while most of the attacks Fandom, Vimeo, Trello, TripAdvisor, and AliExpress.
• We evaluate the robustness of 29 client-side and server-
are shared across browsers, many others work with specific
browsers only. In addition, our experiments discovered 114 side HTML sanitizers and CSP, showing that 55%
new native browser APIs that these markups clobber in of sanitizers are vulnerable and 85% of the DOM
at least one browser, including security-sensitive APIs like Clobbering vulnerabilities cannot be mitigated by CSP.
• We review existing countermeasures, analyze common
cache storage [34] and trusted types [35]. Second, DOM
Clobbering vulnerabilities are quite widespread, affecting mistakes of the 491 vulnerable sites, and distill a list
9.8% of the top 5K websites, including popular sites like of recommendations and secure coding patterns.
GitHub, Fandom, Trello, Vimeo, TripAdvisor, WikiBooks
and AliExpress, leading to severe consequences such as
2. Background
arbitrary code execution, client-side CSRF [32], and open Before presenting our study, we first dissect and introduce
redirections [36, 37]. Third, when looking at the browser- the DOM Clobbering vulnerability in §2.1, and then, we
level defenses, disabling named property accesses can cause present the threat model of this work in §2.2.
2
2.1. DOM Clobbering Vulnerability web attacker [46, 47] can achieve that, e.g., adding a preview
of a post to the client-side webpage by leveraging the URL
DOM Clobbering vulnerabilities originate from a naming
parameters. Another example is the case where the attacker
collision between JavaScript variables and named HTML
can implant a persistent DOM Clobbering payload in the
markups, i.e., markups with an id or name attribute [15,
target webpage, which can lie dormant, and exploited later
38, 39]. When an undefined variable [40, 41] and an HTML
on to attack a victim, e.g., adding persistent comments in the
markup have the same name, the browser replaces the pre-
UI through Gmail’s dynamic email feature [48] which allows
existing content of the variable with the DOM object mirror-
including HTML content [22], or user-generated Markdown
ing the markup type. Listing 1 shows a snippet of vulnerable
descriptions in code repositories that are turned into HTML
code, which loads a script whose URL is stored in a global
content [49, 50]. Finally, a more powerful web attacker
configuration object, i.e., window.globalConfig. In
(e.g., [5, 12]) who is aware of a markup injection vulnerability
more details, the code first creates a script tag (line 1), and
can manipulate the DOM tree.
then, it retrieves the global configuration object and stores
it in a local variable config (line 2). If the configuration 3. Problem Statement
object does not exist, it uses a minimal default configuration,
i.e., {href: ‘script.js’} (line 2). Then, the program This paper aims to answer the following questions:
sets the src attribute of the newly created script tag to (RQ1) DOM Clobbering Attack Techniques. When look-
the href property of the configuration object (line 3) and ing at the evolution of DOM Clobbering attack markups, we
appends the new script to the DOM tree (line 4). observe a consistent complexity growth, starting from a single
The vulnerability originates in the assignment in HTML element [13] that can overwrite a variable, evolving
line 2 because attackers can control the value of with pairs of HTML tags [14, 15] that clobber properties of
window.globalConfig, and ultimately, pick the objects (2013-15), and then advancing into a wide variety
script src value of their choosing by injecting an of browser-specific combinations of different HTML tags
HTML tag with id="globalConfig", e.g., . also native DOM objects (2015-18) [16–19], nested object
When parsing such a markup code, the browser maps properties, and loop elements (2018-22) [21–23]. Despite the
the anchor tag element to the window.globalConfig growth of markups’ complexity, the exploration of the attack
property as mandated by the named property access surface has not been conducted systematically, and to date,
rule of the HTML specifications (see [42–44]). The many of the possible combinations of tags, attributes, markup
escalation to arbitrary code execution happens in line 3, relationships and possible JavaScript object manipulations
when the code reads the href property of the object are not considered. As a first research question, we intend to
window.globalConfig, which no longer contains the fill this gap and exhaustively explore such an attack surface
object with the global configuration but it contains the by generating clobbering markups and testing modern mobile
attacker-controlled anchor tag whose href property value and desktop browsers automatically.
is malicious.js. (RQ2) Detection, Prevalence and Impact. While the ex-
Attackers can abuse named property accesses in other istence of DOM Clobbering is known for more than a
ways, where instead of overwriting variables by HTML nodes, decade [13, 14], we still do not have a measurement about
they can overshadow browser APIs. Listing 2 illustrates an ex- the prevalence, impact, and code patterns of this vulnerability.
ample of such an attack. Similarly to Listing 1, this code also In this paper, we intend to quantify the prevalence of DOM
dynamically creates and loads a script. Instead of fetching Clobbering in the wild, identify vulnerable behaviours, and
the URL from a global configuration object, the code intends examine their impact to shed some light on possible causes
to use the baseURI attribute of the main HTML tag via the and factors hampering web applications’ security.
document.documentElement API (line 2). An attacker (RQ3) Defenses and Effectiveness. As a final question, we
can manipulate the content of src in line 3 by shadow- look at the defenses, their effectiveness, and cost-benefit,
ing the native property document.documentElement leveraging the data generated and collected from the previ-
using an attacker-injected node in the DOM tree [45], ous answers, i.e., DOM Clobbering markups, vulnerability
e.g., a form element with name="documentElement" prevalence, and developer mistakes. In particular, we intend
and the custom property baseURI="malicious.js". to re-evaluate the cost-benefit trade-off resulting from dis-
When parsing the form tag, the browser maps the prop- abling named property accesses in browsers and thoroughly
erty document.documentElement to the JavaScript assess existing solutions such as HTML sanitization [7],
object representing the form tag (an instance of the Content-Security Policy (CSP) [22, 29], and freezing object
HTMLFormElement class) which has a function called properties [51] against DOM Clobbering. Finally, we intend
getAttribute which returns the value of the attribute to review developers’ mistakes and identify vulnerable and
baseURI, i.e., the string malicious.js. secure coding patterns that can fix those issues.
2.2. Threat Model 4. Attack Techniques
In a DOM Clobbering attack, the attacker needs to insert an The first part of this paper addresses RQ1, investigating
ad-hoc HTML payload into a target, vulnerable webpage. A the different ways DOM Clobbering markups can manipulate
3
JavaScript variables, object properties, and native APIs. the target is clobbered. Then, we instantiate each browser
Before presenting our findings (§4.2), we describe the and visit the test pages automatically. For web browsers, we
methodology we followed to answer this RQ (§4.1). used BrowserStack [59] to programmatically control browser
versions, names, and their execution life-cycle in a fully
4.1. Methodology automatic fashion. We evaluated (the latest versions of) all
Our methodology comprises two main steps. First, we mobile and desktop browsers available in BrowserStack (i.e.,
review existing works on DOM Clobbering attacks, looking 16 browsers), and additionally tested the Tor Browser for
for the various techniques to generate markups and at the the sake of completeness. Finally, for Safari, we considered
browser specifications causing the overrides. Then, we apply three different versions that correspond to the three recent
the information gathered to generate markups exhaustively macOS operating systems as Safari cannot be upgraded
and thoroughly test browsers. standalone [60]. In total, we evaluated 19 browsers.
Overall, our generation algorithm produced 3,906,136
4.1.1. Systematization of Known Instances candidate test markups, of which 34,648 are for targets
As the first step, we systematically reviewed the existing ‘x’, i.e., variables or native APIs, and the rest are for object
literature on DOM Clobbering attack markups, i.e., the aca- properties ‘x.y ’ and ‘x.x’. When testing variables, we replace
demic literature [7, 12, 13, 26], HackerOne vulnerability re- the target ‘x’ with the variable name generating in total
ports [52], the CVE database [53], Bugzilla bug reports [16], 34,648 test cases for variables. When testing native DOM
and non-academic resources (see, i.e., [14, 15, 17, 21– APIs, we replace the target ‘x’ with the API function or
23, 39, 54, 55]). Then, for each discovered DOM Clobbering property name (e.g., the cookie property of document),
instance, we extracted the HTML tags, attributes, the clob- obtaining 34,648 test cases per API function. As of October
bered target (e.g., variable or window/document property), 2021, the total number of DOM API objects is 581 [61],
the object type of the clobbered target (e.g., HTMLElement of which 347 are window APIs (i.e., 291 properties and
or WindowProxy), and tags relation (i.e., child, srcdoc, 56 methods) [62], and 234 APIs are for the document
or sibling). Then, we looked for the corresponding browser object (i.e., 178 properties and 56 methods) [63]. In total,
specification rules that explain the reason why the clobbering we generated 20,130,488 test cases for native APIs.
instance works. When the rule defines other variants of
the clobbering instance, we add them to the list of the 4.2. Results
instances. Accordingly, we reviewed the HTML and DOM This section presents the results of our literature review
specifications [56, 57], and GitHub issues in the specifica- and browser testing.
tions’ repositories, i.e., W3C permissions policy [27], WICG
document policy [19, 25], and WHATWG HTML and DOM 4.2.1. Systematization of Known Instances
standard repositories [18, 58]. Finally, we group instances
together based on their similarity, i.e., tags, attributes, target, Table 1 summarizes the DOM Clobbering markups. Our
and the type of the value it refers to. Table 1 shows the review identified 481 DOM Clobbering instances that we
result of our systematization. grouped into 13 classes based on their structural similarity.
Each instance shows how a specific HTML markup (e.g.,
4.1.2. Markup Generation and Browser Testing ) can clobber a specific target, i.e., variable
Starting from our systematization, we derived a list of (e.g., x) or object property (e.g., window.x), and replaced
rules for generating DOM Clobbering markups, covering it with a JavaScript object (e.g., x is shadowed by an
all HTML tags, attributes, tags’ relations, and attack targets HTMLAnchorElement). For each class, the table shows
(i.e., a variable, an object property, or a native browser API). the clobbered target, the HTML code that can overwrite it,
First, we generated candidate HTML markups for a target and the object type stored in the target. Also, the review
‘x’ using all the 142 valid HTML tags, including a custom of the HTML and DOM specifications resulted in the
tag (e.g., mytag), and all the 244 valid HTML attributes, identification of five rules that instruct the browser to store
including a custom attribute. For each tag, we set the value the reference type in the target, which is mapped to each
of each attribute to ‘x’ and add the JavaScript code that known DOM Clobbering instance. The rules are Named
checks whether the markup clobbers the target ‘x’. Then, Access on Window ([56] §7.3.3), DOM Tree Accessors
we generated markups for object properties ‘x.y ’ and ‘x.x’ ([56] §3.1.5), Form Element ([56] §4.10.3), Iframe srcdoc
combining all pairs of the 142 HTML tags considering three attribute ([56] §4.8.5), and HTMLCollection ([57] §4.2.10.2),
relations: sibling tags, parent-child tags, and the srcdoc which we labeled as R1 to R5, respectively. The rest of this
attribute value. The experiments with a single tag showed section details each group of clobbering markups and the
that only name and id attributes create named properties. rules abused by them.
Accordingly, to reduce the number of test cases to a testable Named Access Window. These group of markups leverage
size, the generation of markups for object properties did not a single HTML element whose id or name is set to a
consider combinations of all HTML attributes, but only those target variable ‘x’, clobbering window.x due to browsers’
of the name and id, e.g., id=x, or id=x, name=y. compliance with the Named Access on the Window Object
After generating all markups, we put each of them in a rule (R1) [42]. We reviewed this rule in §2.1. Note that we
test webpage, along with a JavaScript code that verifies if use window.x and ‘x’ interchangeably because all global
4
.
Name Rule(s) Target Reference Type Tag 1 Tag 2 Attribute 1 Attribute 2 Relation Total Reference
Named Access Window R1 win.x, x WindowProxy iframe - name=x - - 1 [27, 42, 55]
win.x, x HTMLElement TS1, TS2 - name=x - - 5 [15, 17, 27, 42]
win.x, x HTMLElement any - id=x - - 141 [14, 21, 22, 27, 42]
DOM Tree Accessors R2 doc.x WindowProxy iframe - name=x - - 1 [21, 43, 54]
doc.x HTMLElement TS1, TS2 - name=x - - 5 [15, 43]
doc.x HTMLElement object - id=x - - 1 [43]
doc.x HTMLElement img, image - id=x, name=any - - 2 [15, 17, 43]
Form Parent-Child R3, R1, R2 win.x.y, doc.x.y HTMLElement form TS2, TS3 id=x k name=x id=y k name=y child 36 [15, 17, 18, 20, 21]
Nested Window Proxy R4, R1, R2 win.x.y, doc.x.y WindowProxy iframe iframe name=x name=y srcdoc attr. 1 [21, 23, 54]
HTMLCollection R5, R1, R2 win.x.x HTMLCollection any any id=x id=x child, sibling 141 [14, 22, 57]
doc.x.x HTMLCollection TS2 TS2 id=x id=x child, sibling 3 [14, 43, 57]
win.x.y HTMLCollection any any id=x, name=y id=x child, sibling 141 [14, 21, 22, 39, 54, 57]
doc.x.y HTMLCollection TS2 TS2 id=x, name=y id=x child, sibling 3 [14, 43, 57]
Legend: R1= Named Access on Window Rule ([56] §7.3.3); R2= DOM Tree Accessors Rule ([56] §3.1.5); R3= Form Element Rule ([56] §4.10.3); R4= Iframe srcdoc Rule ([56] §4.8.5);
R5= HTMLCollection Rule ([57] §4.2.10.2); win=window; doc=document; TS1=form, embed; TS2= object, img; image; TS3=button, fieldset, input, output, select, textarea.
TABLE 1: Overview of known DOM Clobbering markups grouped by their corresponding rules in the HTML [56] and DOM [57] specifications.
variables belong to the global window object by default. Post-processing of Results. As the manual review of 31K
DOM Tree Accessors. The markups of this group can individual instances is infeasible, we group instances by
shadow document properties because browsers comply similar features. We start with preliminary groups based
with the DOM Tree Accessors rule (R2) [43], which instructs on the set of browsers they work in and the target they
browsers how to retrieve properties of the document object clobber. Then, we look at the structural features, i.e., tag1,
(e.g., DOM elements). Similarly to the previous group, these tag2, attribute1, attribute2, and relationship, and we merge
markups use a single named HTML element (e.g., object, two groups when all the structural features but one are the
or embed) to clobber a property ‘x’ of the document. same. Accordingly, we reduced the 31K instances to 74
classes, as shown in Table 2, and map each class to our
Form Parent-Child Relationship. These markups clobber
systematization of known instances. In summary, out of the
properties ‘X.y ’ where ‘X ’ can be any of ‘x’, window.x,
74 classes, 10 classes rely on the Window Named Access,
and document.x. First, they exploit either the rules R1 or
four classes on DOM Accessors, 13 classes on the Parent-
R2 to clobber the base object ‘X ’. Then, they use the Form
Child Relationship, four classes on Nested Window Proxies,
Element rule (R3) to clobber property ‘y ’ of object ‘X ’,
and finally 43 classes leverage HTMLCollections.
i.e., the form elements’ parent-child relationships where the
browser creates a property of the second element for the Findings. By comparing the 74 DOM Clobbering classes in
first element’s accessor variable [21]. DOM Clobbering code Table 2 with the 13 previously identified classes in Table 1,
that rely on this technique comprise a form tag and a child we discovered that the 31,432 DOM Clobbering markups
(e.g., an input) named ‘x’ and ‘y ’, respectively. include 148 new instances, 481 previously known ones, and
30,803 variants of the known ones, which rely on one of the
Nested Window Proxies. These markups use the Iframe
five DOM Clobbering techniques of §4.2.1.
srcdoc rule (R4) to create nested window proxies that
The variants derive from markups that are already known
are named with ‘x’ and ‘y ’, respectively. Similarly to the
for DOM Clobbering according to Table 1, but now have
previous group of markups, it uses the rule R1 or R2 to
one or more additional attributes, or are permuted in part
clobber the base object. Then, the stacked iframes enable
with a different HTML tag. For example, HTMLCollections
attackers to exploit frame navigation features to clobber
clobbering window properties may be formed not only for
object properties like ‘x.y ’ [21, 23].
two similar HTML tags as in Table 1 (e.g., two a tags with
HTMLCollection. The last gour groups of markups rely id=x), but also for certain combinations of dissimilar tags
on a different rule known as HTMLCollection (R5). Specif- (e.g., svg and a), which accounts for a large number of
ically, when two or more elements have the same id in the clobbering instances. Other variants are cases where
the DOM tree, browsers create an array-like object called additional id and name attributes are added to the existing
HTMLCollection [14, 64], which contains all elements clobbering markups. For example, when looking at form
with the same id. Elements inside HTMLCollections can be elements and their childern in Table 1, we observe that each
accessed by (i) their index in the collection and (ii) their tag of the markup has only one id or name. However, as
id and name, enabling attackers to abuse R5 to clobber demonstrated by the results in Table 2, these attributes may
arrays [21] and loop elements (e.g., ‘x’ and ‘x[i]’) as well exist simultaneously on HTML tags and with similar or
as object properties like ‘x.x’ and ‘x.y ’ [22]. Similarly to dissimilar values, resulting in additional clobbering variants.
the previous techniques, rules R1-2 can be combined with In comparison, the new clobbering instances rely on new
R5 to clobber nested object properties like window.x.y . (pairs of) HTML tags and attributes that were previously not
4.2.2. Clobbering Variables and Object Properties known to be applicable for DOM Clobbering. We observed
that 28 out of the 74 identified classes contain at least one
Our browser testing experiments uncovered 31,432 distinct new instance, with a total of 148 new instances. From these,
DOM Clobbering markups that work in at least one browser, 22 classes contain only new instances (i.e., 142 instances).
as summarized in Table 2, from which 145 clobber a variable In the remaining of this section, we briefly describe the new
‘x’, and the remaining 31,287 clobber ‘x.y ’ and ‘x.x’.
5
.
Chrome Firefox Opera Edge Safari TB SI UC
95.0.4638
92.0.4515
65.2.3381
82.0.4227
95.0.1020
96.0.1054
95.0.1020
94.1.2
14.7.1
11.0.1
15.0.6
13.3.8
3.2.3
96.0
95.0
39.0
15.1
14.1
13.1
HTML Markup
Clobbered Tag 1 Tag 2 Attribute 1 Attribute 2 Relation Total New ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿
Named Access Window
win.x TS2 - id=x - - 106
E win.x customtag,iframe,TS5 - id=x - - 8 1
win.x TS6,bdi,bdo,big - id=x - - 6
win.x TS4,embed,form - n=x - - 5
win.x video,wbr,xmp - id=x - - 3
win.x aside,audio,b - id=x - - 3
E win.x applet - n=x - - 1 1
win.x iframe - n=x - - 1
win.x base - id=x - - 1
win.x article - id=x - - 1
DOM Tree Accessors
doc.x TS4,embed,form - n=x - - 5
E doc.x applet - id=x k n=x - 2 2
E doc.x iframe - id=x k n=x - - 2 1
doc.x object - id=x - - 1
Form Parent-Child
win.x.y form TS3,TS4 − fieldset id=x k n=x (& id=y) id=y k n=y child 64
doc.x.y form TS3,TS4 n=x (& id=y) id=y (& n=x k n=y) child 36
win.x.y form TS3,TS4 id=x (& n=y) id=x & n=y child 18
win.x.y form TS3,TS4,embed n=x id=y & n=x child 10
doc.x.y form TS3,TS4,embed, form n=x id=y & n=x child 10
win.x.x form TS3,TS4 id=x id=x & n=y child 9
win.x.y form button id=x k n=x (& id=y) id=y k n=y child 8
win.x.x form TS3 n=x id=y & n=x child 6
doc.x.x form TS3,TS4 n=x id=y & n=x child 6
E doc.x.x, win.x.x form TS4,embed n=x id=y & n=x child 4 1
E doc.x.y form iframe n=x id=y & n=x child 1 1
E win.x.y, doc.x.y form TS4,embed id=y & n=x id=y & n=x child 4 1
E win.x.y form applet n=x id=y & n=x child 1 1
Nested Window Proxy
doc.x.x iframe iframe n=x id=y & n=x srcdoc 1
E doc.x.y iframe iframe n=x id=y k n=y srcdoc 2 1
win.x.x iframe iframe n=x id=y & n=x srcdoc 1
E win.x.y iframe iframe n=x id=y k n=y srcdoc 2 1
HTMLCollection
win.x.y TS1,svg,customtag TS1,plaintext id=x id=x & n=y sibling 787
win.x.y TS1,customtag − TS7,iframe TS1,plaintext id=x id=x & n=y child 774
win.x.y abbr,dl,dt TS13 id=x id=x & n=y child, sibling 274
win.x.y abbr,dl,image,img TS8,TS12,TS20 id=x id=x & n=y child, sibling 392
win.x.y TS18 TS13,TS14 id=x id=x & n=y child, sibling 7,480
win.x.y address,dir,dt TS15 id=x id=x & n=y child, sibling 338
doc.x.y applet TS4,applet id=x id=x & n=y child 4
E doc.x.y TS4,applet,embed,form,iframe TS4,applet,embed,form,iframe n=x id=y & n=x sibling 13 13
E doc.x.y applet,embed,form,image,img TS4,applet,embed,form,iframe n=x id=y & n=x child 11 11
doc.x.y applet,object TS4,applet id=x id=x & n=y sibling 5
win.x.y dir,div,dt,element TS16 id=x id=x & n=y child, sibling 252
win.x.y div TS17 id=x id=x & n=y child, sibling 66
win.x.y div,dl TS12 id=x id=x & n=y child, sibling 186
win.x.y element,em,embed,fieldset TS1,plaintext-iframe id=x id=x & n=y child, sibling 876
E win.x.y embed TS4,embed,form n=x id=y & n=x child, sibling 10 10
E doc.x.y TS4,embed,form,iframe TS4,embed,form,iframe n=x id=y & n=x sibling 11 11
E doc.x.y TS4,embed,form TS4,embed,form n=x id=y & n=x sibling 25 25
E doc.x.y embed,image,img iframe n=x id=y & n=x child 3 3
E doc.x.y embed,image,img T3,TS4,embed,form n=x id=y & n=x child 15 15
win.x.y TS9,iframe TS1,plaintext-iframe id=x id=x & n=y sibling 1,436
win.x.y TS9 TS1,plaintext-iframe id=x id=x & n=y child 1,301
E win.x.y form,image TS4,embed,form n=x id=y & n=x sibling 7 7
E win.x.y TS4,form applet n=x id=y & n=x sibling 4 4
E win.x.y image embed,form n=x id=y & n=x child 2 2
E win.x.y image,img TS4,embed,form n=x id=y & n=x child, sibling 16 16
E win.x.y TS4 applet n=x id=y & n=x child 3 3
win.x.y ins content,data id=x id=x & n=y child, sibling 4
win.x.y TS7, TS8 TS1,plaintext − iframe id=x id=x & n=y sibling 8,848
win.x.y TS8 TS1,TS11,plaintext − iframe id=x id=x & n=y child 7,526
E doc.x.x, win.x.x object TS4,embed,form n=x id=y & n=x child 5 5
doc.x.y object TS4 id=x id=x & n=y sibling 3
E doc.x.y object form,image,img n=x id=y & n=x child 3 3
E doc.x.y object iframe n=x id=y & n=x child 1 1
doc.x.y object image,img id=x id=x & n=y child 2
E doc.x.y object embed,object n=x id=y & n=x child 2 2
E win.x.y object TS4,embed,form n=x id=y & n=x child, sibling 1 1
E win.x.y, doc.x.y object TS4,embed,form id=y & n=x id=y & n=x child 5 5
win.x.y svg iframe id=x & n=y id=x & n=y sibling 1
win.x.y svg TS1,plaintext id=x & n=y id=x & n=y sibling 125
win.x.y svg,table TS1,plaintext − TS19 id=x & n=y id=x & n=y child 157
win.x.x table iframe id=x id=x & n=y child 1
win.x.x table TS1,plaintext,svg − TS10 id=x id=x & n=y child 119 0
win.x.y table iframe id=x & n=y id=x & n=y child 1
Total 31,432 148 59 59 46 35 35 46 59 59 44 59 59 43 38 45 52 37 35 59 59
Legend: win=window; doc=document; TS i= Tag Set in Table 7 of §A.2; n= name; (&p)= optional property p; − = minus operator; TB= Tor Browser; SI= Samsung Internet; UC= UC Browser; = clobbered; = clobbering fails;
TABLE 2: Overview of DOM Clobbering markups. Rows marked with E are classes that contain new DOM Clobbering instances. For all rows, clobbering
window.x also implies clobbering the variable x. Browsers with similar behaviours are grouped with the same color. The table highlights a total of 10
distinct groups of browser behaviours with respect to DOM Clobbering.
6
instances within each DOM Clobbering technique. amount of DOM Clobbering risk is associated with using
Named Access Window and DOM Tree Accessors. We browsers like Firefox Desktop/Android and Chromium-based
discovered that any custom HTML tag (e.g., customtag) browsers on Desktop/Android in which 35 and 59 classes
can be used to clobber a target variable x and window.x in all of DOM Clobbering markups work, respectively.
web browsers. Also, iframe tags with id=x can clobber 4.2.3. Clobbering Native APIs
document.x and named applet elements can clobber both
window.x and document.x. In total, we found five new Overall, we identified a total of 347 DOM APIs (Table 12)
instances across four out of the 14 classes that rely on that can be clobbered in at least one browser using one of
the Window Named Access and DOM Accessors techniques. the markups of §4.2.2, including 233 document and 114
window APIs. We observed that all document methods
Form Parent-Child. We discovered that browsers like and properties except the location property (i.e., 233
Firefox and Safari create accessor properties on JavaScript APIs) can be clobbered in all browsers unanimously, as
objects due to element’s ancestral relationship in the DOM expected by the named property visibility algorithm [45] of
tree for previously unknown pairs of tags and attributes, such the specification [19, 56]. However, this experiment resulted
as a parent form tag with a embed, iframe, or applet in a new finding that for a total of 114/347 window APIs
child with both a name and id attribute. Overall, among the (i.e., 91 properties and 23 methods), named properties can
13 classes that rely on elements’ parent-child relationships, shadow native properties that would otherwise appear on the
we found four new markups in four different classes. object in at least one browser, resulting in DOM Clobbering.
Nested Window Proxy. We identified two new clob- This includes security-sensitive APIs such as the cache
bering markups in two out of the four classes which use storage [34], notification API [67], trusted types [35], and
the Nested Window Proxies technique. In particular, we web storage [68]–to name only a few instances. The complete
discovered that using the id attribute in the nested frames list of clobbered window methods and properties is in
creates a named property on the base frame, referring to Table 13 of §A.2. We observed that for 57/114 clobbered
a WindowProxy, whereas id on the base frame does APIs, there is at least one browser that disagrees with others.
not create a WindowProxy accessible through the global
window or document. 5. Detection and Prevalence
HTMLCollection. We found 137 new clobbering in- The second part of this paper intends to evaluate the impact,
stances (across 18 classes) that lead to the construction prevalence and variety of DOM Clobbering vulnerabilities
of HTMLCollections in a different way. Specifically, we in real-world web applications (RQ2 of §3). In §5.1, we first
discovered that some browsers (e.g., Chrome and Firefox) present TheThing, an automated DOM Clobbering detection
create an HTMLCollection not only when two elements tool. Then, in §5.2, we present our experiment results.
share the same id, but also when they have the same name
value. However, we observed that this happens only for
5.1. Detection
certain (combinations of) HTML tags, e.g., two object We formulate the problem of detecting DOM Clobbering
tags and two form tags with the same name can form an vulnerabilities into a series of data flow analysis tasks
HTMLCollection, but not two div tags. where we identify clobberable JavaScript variables, object
Analysis of Browsers’ Behaviours. Our experiments re- properties, and native APIs whose value ultimately reach
vealed that browsers exhibit divergent behaviours when link- security-sensitive instructions, such as script src and eval.
ing named HTML elements to JavaScript variables (Table 2). Identifying such data flows via pure static analysis is
For example, we observed that for a significant fraction of not an easy task given the dynamic nature of client-side
the clobbering markups (i.e., 31,243 out of 31,432), there JavaScript programs [32, 69, 70] and the scale of the analysis
is at least one browser that disagrees with others, rendering as studying DOM Clobbering vulnerabilities requires the
the task of defending against DOM Clobbering increasingly collection and analysis of hundreds of webpages of real web
more challenging. In summary, we identified 10 distinct applications. Accordingly, we use and extend state-of-the-art
groups of browser behaviours with respect to different DOM property graphs for JavaScript and graph traversals [32] to
Clobbering markups, which are highlighted in Table 2 in col- identify potentially-vulnerable data flows and then use forced
ors, showing that while most of the attacks are shared across execution to confirm the presence of the vulnerability.
browsers, many others only work with specific browsers. Figure 1 shows the architecture of TheThing. At a high
The table shows that all Safari and iOS-based browsers level, it has three main components: (i) a web crawler
have their own distinct behaviours, whereas browsers like to collect webpages’ data and the JavaScript code, (ii) a
Chrome, Opera, and Edge on Desktop and Android exhibit vulnerability analysis component that uses property graphs
the same behaviour. Note that, in general, similarities in and traversals for identifying potential DOM Clobbering
behaviours are expected because some browsers rely on sources and capturing data flows to security-sensitive sinks,
the same underlying engine. For example, Chrome, Edge and finally (iii) a vulnerability verification component that
and Opera on Desktop are all Blink-based browsers [65], dynamically confirms the candidate data flows by instru-
whereas iOS browsers are required to use the WebKit engine menting the code and forcefully executing it in a browser
of Apple [66]. Finally, we observed that the least and highest to check if the flow can occur at runtime. The rest of this
section details each component.
7
Figure 1: Architecture of TheThing.
5.1.1. Data Collection hoisting [41] and double-clobbering [17]. For example, a
runtime error in one script causes the browser to stop parsing
To collect the client-side code of web applications, we that script, and continue with parsing of the rest of the
developed a JavaScript-enabled crawler leveraging Pup- scripts. Hence, variables initialized in the first script are
peteer [71] and Chrome DevTools Protocol (CDP) [72]. treated as undefined and can be a candidate DOM Clobbering
Starting from a seed URL of the website under test, it visits source. Such runtime errors can be caused intentionally by
the webpages following a depth-first strategy, and stops when attackers by a preliminary clobbering, e.g., clobbering a
it doesn’t find new URLs, or the maximum of 100 URLs native DOM function that is invoked in a script shadows its
is reached. During the visit, it collects the page resources value to an HTML element, which is not callable, leading
(e.g., scripts) and runtime state values (i.e., fired events and to a runtime error (Cf. Table 3). Accordingly, we changed
DOM objects’ properties) using the CDP and Puppeteer. the normalization procedure to keep track of the script of
5.1.2. Vulnerability Analysis origin for each AST node.
Second, the semantic types of JAW are tailored for client-
Given the webpages’ data collected by the crawler, side CSRF vulnerabilities and are not sufficient to model
TheThing creates a property graph of the client-side DOM clobbering. Accordingly, we added a new set of generic
JavaScript program leveraging a modified engine of JAW [32]. semantic types for DOM Clobbering sources (Cf. Tables 3
Then, we formulate the problem of finding potential DOM and 9) and security-sensitive JavaScript sinks (Cf. Table 8).
Clobbering data flows into a series of graph traversal queries. Third, JAW does not fully support ES6, resulting in impre-
Hybrid Property Graphs. HPGs are graph-based repre- cise control and data flow models. Accordingly, we applied
sentations of client-side JavaScript programs that unify several enhancements. For example, we added support to
multiple static code representations and runtime state values. bind the function call arguments to their definition parameters
State values are event traces and environment properties, when the code uses the ES6 Rest parameters [76] and the
e.g., the values of cookies and web storage. The static Spread operator [77] which improves the precision of the
code representation comprises several graphs, e.g., Abstract call graph and PDG edges. Also, we created bindings for
Syntax Tree (AST), Control Flow Graph (CFG) and Program the this object depending on the calling context [78],
Dependence Graph (PDG) that model the nesting of the and the binding for the arguments object for non-arrow
syntactical constructs of a program, the order and conditions functions [79] to improve pointer analysis tasks.
for the execution of program instructions, and the data flow Analysis Traversals. After construction of an HPG, we
and control dependencies within the statements of a program, traverse it to identify DOM Clobbering source nodes in the
respectively. HPGs also model the event-driven transfer of graph. Table 3 presents the various types of DOM Clobbering
control within JavaScript programs via the Event Registration, sources and their properties. The table shows that clobberable
Dispatch and Dependency Graph [32]. Finally, they include native DOM APIs discovered in §4.2.3 can act as a DOM
Semantic Types, which are labels initially assigned to source Clobbering source. Identifying these objects in the program
and sink nodes to capture the semantic of those instructions is a matter of searching for a pre-defined syntactic structure,
and then propagated through the graph following the program which is similar to other taint-style vulnerabilities like client-
calculation. These representations are encoded in a directed side XSS. However, contrary to the traditional taint analysis,
graph in which nodes and edges can have labels and key- not all DOM Clobbering sources are pre-defined syntactic
value properties, known as a labeled property graph [32, 73]. objects. Instead, they can be a specific property of a program,
Model Construction. After collecting the webpages’ scripts identifying which requires tracking the propagation of data
and state values, TheThing instantiates an HPG, and imports flows within the program itself. This is because any used
it into a Neo4j docker instance [74], allowing the graph to be variable that is undefined within its execution context (i.e.,
traversed declaratively using the Cypher query language [75]. previously not declared and assigned) can act as a DOM
Unfortunately, we could not use JAW as-is and modified it Clobbering source. To identify such sources, we use PDG
to address several of its shortcomings. First, when building a data dependency edges, which specify that a variable defined
graph, JAW normalizes the webpage code by combining code at a source node is subsequently used at the destination
inside script tags into a single script. However, identifying node. Specifically, we query the graph for Identifier nodes
DOM Clobbering sources may require to distinguish the containing a variable v with no incoming PDG edge from
code across two different scripts due to JavaScript variable any AssignmentExpression or VariableDeclaration nodes that
8
.
Object DOM Clobbering Source When? Threat # Sinks # Flows # Conf. # Pages # Sites
v S1: v ∈ NP, CLOB(v ) Client-side XSS 37,941,540 3,688 3,677 1,572 474
S2: v ∈/ NP, v and window.v are not assigned before, v is not Request Forgery 2,555,147 1,406 1,403 541 398
declared with var, let and const before Storage Manipulation 1,047,512 1,369 1,365 418 382
Open Redirect 1,306,603 1,228 1,227 391 385
window.v S3: v ∈ NP, CLOB(v )
JSON Injection 9,610,162 793 793 345 343
S4: v ∈ / NP, v and window.v are not assigned before, v is
Cookie Manipulation 1,702,340 266 266 204 195
not declared with var afterwards within the same script, or
Websocket Hijacking 21,252 367 367 183 147
anywhere before
RegEx Injection 13,325,791 284 284 98 98
S5: v ∈/ NP, v or window.v is assigned or declared with any of
Doc. Domain Manip. 55,266 85 85 69 69
the var, let and const keywords within any previous script
postMessage Manip. 119,971 0 0 0 0
that contains an invocation of function f such that f ∈ NP,
File Read Path Manip. 57,789 0 0 0 0
CLOB(f )
Total 67,743,373 9,486 9,467 3,821 491
document.v S6: v ∈
/ NP
S7: v ∈ NP, CLOB(v ) Legend: Conf.= Dynamically Confirmed
Legend: NP= native property; CLOB(v )= v is a clobberable NP based on §4.2.3. TABLE 4: Prevalence and impact of DOM Clobbering in Tranco top 5K
TABLE 3: Description of properties of DOM Clobbering sources. sites. The table shows the number of clobberable data flows to security
sensitive sinks of Table 8, the number of affected webpages, and websites.
assign to or declare the variable v . If there is such PDG
the source location.
edge, we further check whether the declaration/ assignment
As the next step, we confirm the existence of the data
statement can hinder the clobberability of v based on the
flow to the sink instructions. To do that, we first taint each
criteria in Table 3, which can depend on the declaration
clobberable source, execute the program by loading it via
scope (i.e., same script or not), declaration position (i.e.,
Puppeteer, and check if we can observe the data flow reported
before or after), and the declaration keyword (e.g., var vs
by the static analyzer. If that is not the case, we forcefully
let) of that statement.
execute the path toward sinks to check if there is an execution
After identifying the source nodes, we associate to each
of the program in which the data flow to the target sink occurs.
of them a label that captures the semantic type of the
We use forced execution to find candidate pages among those
source, e.g., a clobberable native property or custom variable
where Puppeteer could not connect sources with sinks, and
(Cf. Table 9). Then, given a list of JavaScript sinks, we
later validate the presence of the vulnerability manually.
identify each of them in the graph and assign each a
Specifically, for each branch in the path control flow, we
relevant semantic type. Semantic types assigned to sink
forcefully execute the program once for the true and once for
instructions are propagated to other functions that encapsulate
the false branch, until we hit a execution path with the target
the same semantic, e.g., the type WIN_LOC_WRITE is set
data flow, or we exhaustively checked possible execution
for instructions that set the value of window.location,
paths. We observed that the number of branches between
such as window.location.replace(), and is then
DOM Clobbering sources and sinks is in practice small (i.e.,
propagated to all other developer-defined functions that can
less than 10), as we will show in §5.2. Finally, as forced
set its value through one of their parameters. TheThing
execution may also lead to spurious execution paths, we
considers different sink types to enable us to capture the
manually validate the decision reported by TheThing and
potential consequences of DOM Clobbering. The complete
examine the exploitability.
list of sinks is in Table 8, which is derived by surveying and
aggregating the JavaScript sinks considered in prior academic 5.2. Prevalence in the Wild
and non-academic resources (see, i.e., [1, 5, 36, 37, 80–
90]). Finally, we conduct forward data flow analysis by We quantified the prevalence and impact of DOM Clob-
propagating semantic types from sources to sinks, and select bering on the top 5K websites using the Tranco list [91]
those flows where a node with a sink semantic type is of Nov 1st, 2021 (ID: Y3JG), where we first selected the
tainted with a source type (i.e., pick up the attacker-controlled top 5K domains by excluding the duplicates like local
values). The concrete queries are presented in Table 11 of versions of websites (e.g., google.com vs google.de), and
§A.2. This component outputs a set of paths with potential then instantiated TheThing for each of the them.
data flows from a DOM Clobbering source to a sink. Data Collection Statistics. Starting from the 5K seed URLs,
TheThing collected 205,696 webpages, ranging between 1 to
5.1.3. Vulnerability Verification 91 pages per site (41 pages on average). Out of the 205,696
Given a set of potential DOM Clobbering data flows, webpages, 187,280 are unique pages based on their set of
the goal of this step is to verify each flow and eliminate scripts. From the 187K pages, TheThing extracted 18,351,815
potential false positives. To accomplish this goal, TheThing scripts with a total of 24,664,686,928 LoC. Accordingly,
features a light-weight, in-browser dynamic taint analysis TheThing generated 187,280 HPGs by processing an average
engine leveraging Iroh.js [33]. After instrumenting the code of 98 scripts and 131,700 LoC per page.
with Iroh for dynamic analysis, we first check whether the Vulnerability Prevalence. The analysis of 187,280 HPGs
source variable of the data flow is clobberable by creating a resulted in the identification of 20,580,350 DOM Clobbering
suitable HTML clobbering payload for that variable using sources and 67,743,373 sinks, which amounts to an average
the DOM Clobbering classes of §4. We inject the payload of 110 sources and 362 sinks per webpage. Out of these,
to the DOM tree and subsequently verify the clobberability static analysis revealed a total of 9,486 potential data flows
of the source variable by dynamically logging its value at from the sources to the sinks, from which the majority (i.e.,
9
9,467) were confirmed dynamically. We observed that these the high number of affected webpages, we randomly se-
vulnerable data flows affect around 2% of the webpages (i.e., lected two vulnerable pages per each of the 491 affected
3,821 out of 187,280) and 9.8% of the tested websites (i.e., sites, and subsequently checked whether we can insert a
491 out of 5K) in total. Table 4 summarizes our findings. DOM Clobbering markup in the page by leveraging the
Vulnerability Impact. We observed that the 9,467 vulner- functionalities offered by the application, or through URL
abilities can have different security implications, as shown parameters, which could allow us to overwrite the clobberable
in Table 4. The most common consequence is XSS that variable identified by TheThing. To be able to use protected
accounts for around 38.8% of the vulnerabilities, whereas functionalities offered by the websites (e.g., creating posts,
the least common consequence is document domain ma- adding comments, etc) and also prevent any side effects for
nipulation [83, 84] that corresponds to less than 1% of other users, we created our own test accounts for 358 sites
the total vulnerabilities. Other common consequences were that supported this feature without monetary costs, and for
client-side state manipulation (17.2%), client-side request the rest, we limited our tests to the public functionalities (e.g.,
forgery (14.8%) and DOM-based open redirection (12.9%). search) without persisting any data. As a result, we created
Finally, the remaining 15.3% of vulnerabilities had other a proof-of-concept exploit for 44 websites in total, affecting
repercussions like JSON injection and Websocket connection popular sites and functionalities like Trello boards, Wiki
hijack. We provide more information on each of these threats pages in WikiBooks and WikiDot, comments in Vimeo and
in Table 8 of §A.2. VK, reviews in TripAdvisor and OpenTable, posts in Fandom
and JustPaste, surveys in SuveryMonkey, poster designs in
Verification and False Positives. Considering the high
PosterMyWall, and finally item searches in GitHub Shop,
number of reported data flows by the static analyzer (Cf.
AliExpress, AliBaba and Telam News–to name only a few
§5.1.2), it was infeasible to verify all of them manually.
examples. The exploits enable an attacker to achieve XSS,
Instead, we followed a semi-automatic approach leveraging
open redirect, and client-side request forgery in 35, five, and
a combination of dynamic analysis, forceful execution and
four sites, respectively. We refer interested readers to §A.1
manual analysis, as detailed in §5.1.3.
for a few case studies of the confirmed attacks.
We observed that in a large number of cases (46.1%,
i.e., 4,373 flows), the dynamic verification component can 6. Defenses
successfully confirm the existence of the vulnerability by
loading the page and executing it via Puppetter, whereas This section addresses RQ3 of §3. First, in §6.1, we have
in the remaining cases (i.e., 5,113 flows), it needs to force a critical look at the existing countermeasures and evaluate
execute between one to ten conditional branches (four on their robustness and cost-benefit tradeoff leveraging what we
average) before it can confirm or reject the data flow and learned from Sections 4 and 5. Then, in §6.2, we analyze the
terminate. As a result of this process, the verifier eliminated common mistakes of the 491 vulnerable sites (see §5), and
a total of 19 FPs across 11 of the 491 vulnerable sites, and distill a list of recommendations and secure coding patterns
confirmed the rest (i.e., 5,094 flows within 2,643 webpages of that can resolve those issues.
491 sites). We manually verified and investigated the reason
for each FP, and discovered that eight FPs occur during the 6.1. Evaluation of Existing Countermeasures
data flow analysis for identification of DOM Clobbering
Disabling DOM Clobbering Features. DOM Clobbering
sources, and 11 during the data flow analysis from sources
can be solved by disabling named properties [19, 25, 27].
to sinks. The former cases happened because a variable
According to Chrome telemetry [28], disabling named prop-
was declared or assigned using a dynamic code generation
erties for clobbered variable accesses could break ~10.5%
construct for which the statement nodes and PDG edges
of the webpages. Our results of §5.2 are in line with these
were missing in the HPG, and the latter cases occurred due
numbers, and we observed that 13.3% of the webpages use
to dynamically fetched code where the value of the tainted
at least an instance of clobbered variable accesses.
variables changed, inaccurate pointer analysis for dynamic
As webpages tend to reuse code via shared scripts, a patch
property lookups, and removal of event handlers that changed
in a script may fix multiple websites. Accordingly, using the
the tainted variables.
number of webpages may not accurately quantify the cost
Finally, we manually validated the feasibility of the
of fixing breakage. As an alternative, we can measure the
forcefully executed data flows by randomly selecting two
number of affected websites, and our results show that the
pages per site, from the 2,643 pages of the 491 websites
affected pages do not concentrate on a small number of sites,
whose data flows were confirmed by forced execution. Our
but they scatter over 51.2% of the top 5K sites.
random sampling included 491 sites, 982 pages and 2076
While breakage adequately measures the cost of this
data flows, out of which we could not determine a realistic
solution, it may not be a good indicator for the actual
execution path for at least 42 data flows in 42 sites, leaving
benefits, i.e., fixed websites. Our results show that 118
us with 2,034 vulnerable data flows of 491 websites.
websites of 2,561 potentially broken sites will be fixed,
which is about 4.61% of the broken websites (and 2.4% of
5.3. Confirming Exploitability of Vulnerabilities
the total). However, our results also show that a large fraction
We manually examined whether the identified vulnera- of vulnerable websites are not considered by breakage. In
bilities can be effectively exploited by an attacker. Given particular, we found 373 websites (76% of the vulnerable
10
Default
DOM Clobbering markups identified in §4 to each of them,
Strict
HTML Sanitizer Ò Bypassed Pct. Ref.
and for each input vetted whether the sanitizer removes
Client-side JS
1. DOMPurify 8.7K 534 49.7K 7.9M 29,995 95.4% [7]
or changes the named properties in the output markup.
2. Google Closure Lib. 4.3K 1K - 117K - - [92] For each sanitizer, we tested both the default and most
3. JS-XSS 4.4K 584 136K 8.7M 25,592 81.4% [93]
4. Sanitize-HTML 2.8K 316 102K 4.7M 79 0.25% [94]
strict configuration that it offers. We marked a sanitizer as
5. Google Caja 1.1K 123 - - 27,951 88.9% [95] vulnerable if there is at least one clobbering markup that
Node.js bypasses the sanitizer without being altered. Finally, we
1. Insane 394 21 - 55.3K 5 0.02% [96] marked sanitizers as partially vulnerable when they encode
2. Bleach 117 19 - 1.6K 2,288 7.2% [97]
3. Angular-sanitize 100 237 49.1K 936K - - [98] the < and > symbols of HTML tags but do not remove
4. Yahoo html-purify 40 6 - 708 28,807 91.6% [99] or change the DOM Clobbering named properties because
5. Arcgis 11 2 - 32.6K - - [100]
encoding these symbols would not help when applications
Python
1. Mozilla Bleach 2.3K 230 155K 17.5M 31,132 99.05% [101]
expect inputs in an HTML format.
2. LXML 2K 481 216K 29.9M 28,211 89.7% [102] Table 5 summarizes our findings. In total, we observed
3. HTML Sanitizer 61 19 - 17.9K 332 1.06% [103] that 16 and 13 out of 29 sanitizers are vulnerable to at
4. Htmllaundry 27 4 - 1.1K 1,460 4.6% [104]
5. Django-html-sanitizer 20 62 - 2.8K - - [105] least one DOM Clobbering markup in their default and most
PHP strict sanitization configuration, respectively. In both of the
1. Htmlpurifier 2.4K 284 82.7K 2.5M - - [106] configurations, four sanitizers are only partially vulnerable, as
2. Html-sanitizer 333 36 - 30.8K - - [107]
3. Symfony Sanitizer 104 1 - 7 - - [108]
they escape the markup rather than cleansing the named prop-
4. HTMLawed 30 14 - 390K 21,211 67.4% [109] erties. Finally, when looking at the remaining 13 sanitizers,
5. Typo3 Sanitizer 13 10 - 88.9K 23,942 76.1% [110]
we observe that they implement a robust, enabled-by-default
C# defense. However, in all cases, they remove named properties
1. AntiXssEncoder 2.6K 1K - 6.4K 31,390 99.8% [111]
2. HtmlSanitizer 1.1K 162 1.8K 108K 654 2.08% [112] unconditionally, i.e., for all input markups including those
3. AJAX Toolkit 275 133 4.2K 264 - - [113] combinations that do not lead to DOM Clobbering, e.g., an
4. NSoup 147 46 - 72 - - [114]
5. HtmlRuleSanitizer 50 16 30 308 - - [115]
anchor tag with name=x does not clobber the variable x.
Java
While such a strict approach is effective, it may hinder the
1. Jsoup 9.2K 2K 98.4K - - - [116] usability of these libraries in cases where developers need
2. OWASP HTML Sanitizer 647 171 - - - - [117] to use id and name attributes for legitimate functionalities.
3. Antisamy 105 72 - - - - [118]
4. HtmlCleaner - - - 824 28,951 92.1% [119] Content-Security Policy (CSP). When attackers can clobber
Total Vuln. ( + ) 16 13 the src attribute of dynamically created scripts, they can
Legend: = GitHub Stars; Ò= GitHub Forks; = GitHub UsedBy; = Monthly Downloads; load and execute arbitrary JavaScript code. In these cases, the
= Vulnerable; = Partially Vulnerable; = Not Vulnerable
CSP script-src directive [29] can be used to constrain
TABLE 5: Robustness of top five HTML sanitizers of web programming the value of script sources to a set of trusted domains,
languages against the 31.4K DOM Clobbering instances of §4.2. The preventing attacker-loaded code to be executed [12, 22, 30].
table shows the results for both the default and the most strict sanitizer
configurations. The tested sanitizer versions are in Table 10. However, unlike malicious JavaScript injected by the attacker,
injected HTML code is not blocked by CSP. Accordingly,
ones and 7.5% of the total) that will benefit from such a CSP does not mitigate other variants of DOM Clobbering that
solution. Overall, when comparing the cost and benefits, the do not require script src manipulation, e.g., clobbering the
ratio of vulnerable over potentially-broken websites is about parameters of dynamic code evaluation constructs like new
1:5.2 (i.e., 491 vulnerable and 2,561 potentially-broken sites). Function() can lead to CSP-bypassable XSS. Our eval-
HTML Sanitization. HTML sanitizers can sanitize the input uation in §5.2 shows that 37.7% of the DOM Clobbering
markups before adding them to the DOM tree, e.g., by vulnerabilities that lead to XSS (i.e., 1,385 out of 3,677),
removing the id and name attributes from certain (combina- which accounts for 14.7% of the total vulnerabilities can be
tions of) HTML tags (Cf. §4). To assess the robustness of mitigated by CSP, whereas the remaining ones cannot.
the popular HTML sanitizers against DOM Clobbering, we Freezing Object Properties. Another way to mitigate
dynamically tested them against all of the DOM Clobbering DOM Clobbering is to freeze DOM objects [51], e.g., via
instances we identified in §4. First, we selected the top Object.freeze() method [121], which prevents the
five web programming languages based on the GitHub 2021 object to be overwritten by named DOM elements. While
Octoverse report [120], i.e., JavaScript, Python, Java, C# effective, determining all objects and object properties that
and PHP. We considered both client-side and server-side need to be frozen is a non-trivial, error-prone task for web
JavaScript (i.e., node.js). Then, we searched for sanitizers of developers. Also, sealed objects cannot be changed anymore,
each language and selected the top five based on their GitHub hindering the dynamic composition of webpages. Finally,
stars, forks and UsedBy, and the number of downloads in native properties cannot be frozen, rendering this approach
their respective package managers (e.g., npm for node.js, ineffective when the DOM Clobbering source is a clobberable
packagist for PHP, etc). This process led to the identification native property, which accounts for ~21.5% of vulnerabilities
of 29 HTML sanitizer libraries, as for Java, we identified (i.e., 2,037 out of 9,467) in §5.2.
only four sanitizers.
After identifying the popular sanitizers, we input the 31.4K
11
.
# Code Pattern Description # Flows # Pages # Sites
A VAR1 is not declared or assigned yet, thus window.VAR1 is clobberable. 3,134 1,214 143
B BA is a clobberable built-in API (§4.2.3), thus BA, window.BA and document.BA are clobberable. 2,037 832 99
C Assignment to document properties is always shadowed by DOM Clobbering (§4.2.3). 1,896 655 81
D VAR1 is declared with let that does not create property on window, thus window.VAR1 is 367 153 18
clobberable.
E VAR1 is initialized without var in the same script and after the sink, but this does not result in hoisting. 1,635 792 116
F VAR1 is initialized with var, but in a different script and after the sink statement. 121 50 12
G VAR1 is initialized in a script where a built-in method can be clobbered and cause an error in parsing 53 36 7
that script, hence window.VAR1 can be clobbered in a subsequent script (double clobbering).
H VAR1 is initialized in a different script as a property of the window or without any modifiers after the 224 89 15
sink statement, thus window.VAR1 is clobberable.
Legend: BA= Built-in API; WinDoc = Window or Document Object; [code]= Alternative code statement; Red= Clobberable; Yellow = script 1; Orange = script 2;
TABLE 6: Overview of DOM Clobbering code patterns in the wild. Different background colors represent code in two different script tags.
6.2. Secure Code Patterns could patch the patterns A, D, E, F, and H. When the value
needs to be used in multiple scripts, as in patterns F and H,
Our evaluation of existing DOM Clobbering counter- the declaration should be in the same (or a previous) script,
measures in §6.1 revealed that they are not sufficient for but not in subsequent ones.
complete protection in a large number of cases. In this section, Strict Type Checking. Another common mistake en-
we have a closer look at the variety of DOM Clobbering abling DOM Clobbering is treating DOM properties, like
vulnerabilities in real web applications (§5.2), identifying document and window properties as safe, trusted values
vulnerable behaviours and the common types of coding (e.g., patterns B, C, and G). Instead, developers should extend
mistakes. Then, we use these vulnerable behaviours to distill the trust boundary to these properties, verifying their type
a list of recommendations and defensive coding patterns before using them in security-sensitive instructions, e.g.,
that developers could apply to prevent DOM Clobbering. using the instanceof [122] and typeof [123] operators.
To achieve this objective, we extracted the vulnerable lines
Do Not Use Document for Global Variables. Properties of
of code and characterized them based on their high-level
document can always be overwritten by DOM Clobbering,
syntax and semantics, identifying eight distinct vulnerable
even immediately after they are assigned a value, as in
code patterns in the wild.
pattern C. Accordingly, developers should refrain from using
Table 6 summarizes our findings. We observe that the most
document as a mean to store and retrieve global values.
common mistakes are patterns A and E, in which the devel-
Instead, they can declare variables with const or var in
oper references an undefined variable through the window
the global context, or use the globalThis object [124].
object, and then use the result in a sensitive instruction,
whereas the least common, but also more complex mistakes Namespace Isolation. While robust sanitizers in §6.1 re-
are patterns F, G and H where the vulnerability originates move named properties, an alternative solution is to separate
due to the position of the instructions that span across two the namespace of variables defined by JavaScript code and
different script tags. Other common mistakes are patterns B named properties in user-generated markups. For example,
and C, where developers treat custom and native document we observed that the markdown to HTML converter of ap-
and window properties as trusted values that can be safely plications like GitHub and BitBucket prefixes id and name
used in sensitive operations. The rest of this section presents attribute values of user-generated markup with a specific
secure coding patterns that can prevent DOM Clobbering. string. Motivated by this solution, one can monitor runtime
changes in the DOM tree via the MutationObserver
Explicit Variable Declarations. As shown in Table 6, a key
API [125], and prefix named properties of all dynamically
element enabling DOM Clobbering is use of the || operator
inserted markups before adding them to the tree, which
to rely on specific defaults when the primary, intended
patches all patterns in Table 6.
variable or property is undefined. As an alternative solution,
developers can initialize those variables with the default 7. Summary and Discussion
value when they are undefined using var declarations, which
prevents named properties to overshadow them according to Clobbering Markups Come In Many Forms. In this
the named property visibility algorithm [45]. This solution paper, we proposed a systematic technique to identify DOM
12
Clobbering markups, and showed that they come in many fragments of JavaScript code, known as script gadgets, are
forms, with a total of 31,432 attack markups that rely on five unexpectedly executed as a result of a non-script markup
different techniques, including 148 new instances and 30,803 injected by attackers. The authors used a modified browser
new variants. We observed that browsers exhibit divergent engine [1] to measure the prevalence of these gadgets,
behaviours when handling named properties. For example, and demonstrated that they are prevalent and can bypass
for a significant fraction of the markups (i.e., 99%), there existing XSS mitigations, such as HTML sanitizers [7] and
is at least one browser that disagrees with others, making it CSP [29, 30]. Later, Roth et. al. [130] quantified the impact
increasingly more challenging to enforce robust defenses. of script gadgets on CSP in the wild. Similarly, Heiderich
DOM Clobbering is Ubiquitous. DOM Clobbering vulner- et. al. [6] discovered mutation-based XSS attacks (mXSS),
abilities are prevalent, affecting over 9.8% of the top 5K showing how specific browser-based mutations of DOM
sites, with the consequences ranging from XSS to user state content and insecure JavaScript that reads and rewrites HTML
manipulation, request forgery and client-side open redirects elements can transform initially secure DOM markup to code.
in the majority of the cases, i.e., 83.7% (see §5.2). While all these three attacks can transform non-script markup
to executable code, the elements enabling DOM Clobbering
Defenses Helpful but May not Completely Cut it. The
is largely different, i.e., script gadgets rely on event handlers
evaluation of existing DOM Clobbering countermeasures (§6)
and mXSS attacks abuse innerHTML mutations, whereas
suggests that each can only mitigate a fraction of the attacks.
DOM Clobbering is the result of a complex interplay of the
For example, 55% of the popular HTML sanitizers are
default browser behaviors and insecure use of named property
vulnerable to at least one of the 31K clobbering markups by
accesses in JavaScript programs. Contrary to these works, our
default, and CSP cannot mitigate over 85% of the identified
study focuses on DOM Clobbering, systematically testing
vulnerabilities. Protecting such a fraction of the attack
mobile and desktop browsers, identifying insecure coding
surface without switching named properties off completely
patterns using both static and dynamic analysis techniques,
is a more costly task, requiring developers to be aware of
and demonstrating their exploitability.
corner case behaviors of browsers and revisit the design and
Multiple instances of DOM Clobbering vulnerabilities
implementation of their systems, e.g., strict type checking,
have been discovered in the last 12 years by both aca-
explicit variable declarations, or namespace isolation.
demics [7, 13, 131] and security analysts [14, 21–23, 54],
Open Science and Website. To support the future research with the first public instance identified in 2010 by Rydstedt
effort, we publicly release TheThing [126], the automated et. al. as a way to circumvent frame busters [13]. The term
browser testing pipeline [127] that identifies clobbering ‘DOM Clobbering’ itself emerged in 2013, when Gareth
markups (see §4), and an interactive version of markups1 . Heyes [14] demonstrated how this class of vulnerabilities can
Ethical Discussion. Our experiments on live sites do not escalate to client-side code execution. Due to such nefarious
target any real user. Tests requiring to persist data, e.g., consequences of DOM Clobbering, prior academic studies
store a markup, is exclusively restricted to user accounts has primarily focused on its defenses (e.g., [7, 26, 51]).
that we created on those sites. Also, we excluded testing Most notably, Heiderich et. al. proposed the JSAgents
functionalities where we could not control the impact and library [51] and later the DOMPurify sanitizer [7] to mitigate
visibility of the injected markup (e.g., publicly accessible the security implications induced by markup injection, such
posts and comments). Tests on public functionalities was as DOM Clobbering and client-side XSS [1, 5]. Our research
performed without persistently injecting any markup. completes the missing pieces of these works by systematically
The vulnerabilities and security risks identified in this studying DOM Clobbering attack techniques, their preva-
paper affects 491 websites and 16 sanitizer libraries. We lence, and effectiveness of the existing countermeasures.
started the process of notifying the affected parties in March
2022 following the best disclosure practices [128, 129], 9. Conclusion
where we prioritized our reports by severity. We sent an
initial notification that includes the vulnerability details, or a In this paper, we performed, to the best of our knowledge,
proof-of-concept exploit, followed by an additional reminder the first comprehensive study of DOM Clobbering, system-
every three weeks to maximize the remediation rate. At the atically covering clobbering techniques, browser behaviours,
time of preparing the camera-ready, we have notified all vulnerability prevalence, and defenses. Starting with a com-
affected parties at least once, out of which 72 sites have prehensive survey of existing literature and dynamic analysis
already confirmed the issues, and 21 sites patched them, such of 19 web browsers, we presented the first taxonomy of DOM
as GitHub, Vimeo, Fandom, TripAdvisor and SuveryMonkey. Clobbering, uncovering 31K distinct markups that use five
different techniques to clobber JavaScript variables. Then,
8. Related Work we presented TheThing, the first DOM Clobbering detection
Reusing the webpages’ legitimate JavaScript code to obtain tool, and instantiated it on the top of the Tranco top 5K sites,
arbitrary client-side code execution have been the subject showing that DOM Clobbering vulnerabilities are prevalent.
of several research endeavors in the past. Most notably, Finally, we demonstrated that existing countermeasures are
Lekies et. al. [12] described a new attack where small not sufficient to mitigate a significant fraction of the vulner-
abilities, and accordingly proposed several recommendations
1. https://soheilkhodayari.github.io/DOMClobbering and secure coding patterns for developers.
13
Acknowledgments permissions-policy/issues/349.
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[60] S. H., “How to Update Safari without upgrading MacOS?” 2021, html.
https://browserhow.com/how-to-update-safari-without-upgrading- [95] Google Caja Sanitizer. https://code.google.com/archive/p/google-
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[61] Web APIs. https://developer.mozilla.org/en-US/docs/Web/API. [96] Insane HTML Sanitizer. https://github.com/bevacqua/insane.
[62] The Window Interface. https://developer.mozilla.org/en-US/docs/We [97] JavaScript Bleach Sanitizer. https://www.npmjs.com/package/bleach.
b/API/Window. [98] Angular-sanitize Library. https://www.npmjs.com/package/bleach.
[63] The Document Interface. https://developer.mozilla.org/en-US/docs/ [99] HTML-Purify Library. https://www.npmjs.com/package/html-purify.
Web/API/Document. [100] Arcgis HTML Sanitizer. https://www.npmjs.com/package/@esri/arcg
[64] (2021) The HTMLCollection Interface. https://developer.mozilla.org/ is-html-sanitizer.
en-US/docs/Web/API/HTMLCollection. [101] Python Mozilla Bleach Sanitizer. https://pypi.org/project/bleach/.
[65] The Blink Rendering Engine. https://www.chromium.org/blink/. [102] LXML Library. https://pypi.org/project/lxml/.
[66] H. Charlton, “Should Apple Continue to Ban Rival Browser Engines [103] Python HTML-sanitizer Library. https://pypi.org/project/html-
on iOS?” 2022, https://www.macrumors.com/2022/02/25/should- sanitizer/.
apple-ban-rival-browser-engines/. [104] HTMLLaundry Library. https://pypi.org/project/htmllaundry/.
[67] The Notification Web API. https://developer.mozilla.org/en-US/docs/ [105] Django HTML Sanitizer. https://pypi.org/project/django-html sanitiz
Web/API/notification. er/.
[68] The WebStorage API. https://developer.mozilla.org/en-US/docs/Web/ [106] PHP HTML Purifier. https://packagist.org/packages/ezyang/htmlpuri
API/Web Storage API. fier.
[69] S. H. Jensen, P. A. Jonsson, and A. Møller, “Remedying the Eval [107] PHP HTML-Sanitizer. https://packagist.org/packages/tgalopin/html-
that Men Do,” in ISSTA, 2012. sanitizer.
[70] S. Guarnieri and B. Livshits, “GULFSTREAM: Staged Static Analy- [108] Symfony HTML Sanitizer. https://packagist.org/packages/symfony/h
sis For Streaming JavaScript Applications,” in WebApps, 2010. tml-sanitizer.
[71] Puppeteer. https://github.com/puppeteer/puppeteer. [109] HTMLawed Library. https://packagist.org/packages/htmlawed/html
[72] Chrome devtools. https://chromedevtools.github.io/devtools-protocol. awed.
[73] F. Yamaguchi, N. Golde, D. Arp, and K. Rieck, “Modeling and [110] Typo3 HTML Sanitizer. https://packagist.org/packages/typo3/html-
Discovering Vulnerabilities with Code Property Graphs,” in IEEE sanitizer.
S&P, 2014. [111] HTML Encoder of AntiXSS Library. https://docs.microsoft.com/en-
[74] Neo4j. https://neo4j.com/. us/dotnet/api/system.web.security.antixss.antixssencoder.htmlencod
[75] Cypher Query Language. https://neo4j.com/developer/cypher/. e?view=netframework-4.8.
[76] Rest Parameters. https://developer.mozilla.org/en-US/docs/Web/Java [112] C# HtmlSanitizer. https://www.nuget.org/packages/HtmlSanitizer.
Script/Reference/Functions/rest parameters. [113] ASP.NET Ajax Control Toolkit. https://www.nuget.org/packages/Aja
[77] Spread Operator Syntax. https://developer.mozilla.org/en-US/docs/ xControlToolkit.HtmlEditor.Sanitizer/.
Web/JavaScript/Reference/Operators/Spread syntax. [114] NSoup HTML Parser and Sanitizer for .NET Framework. https:
[78] S. Guarnieri and V. B. Livshits, “GATEKEEPER: Mostly Static //www.nuget.org/packages/NSoup/.
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in USENIX Security, vol. 10, 2009, pp. 78–85. on.Web.HtmlSanitizer.
[79] The arguments object. https://developer.mozilla.org/en-US/docs/We [116] JSoup: Java HTML Parser. https://github.com/jhy/jsoup.
b/JavaScript/Reference/Functions/arguments. [117] OWASP Java HTML Sanitizer. https://github.com/OWASP/java-
[80] DOM-based WebSocket-URL poisoning. https://portswigger.net/web- html-sanitizer.
security/dom-based/websocket-url-poisoning. [118] Java AntiSamy Library. https://github.com/nahsra/antisamy.
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ktricks.xyz/pentesting-web/cross-site-websocket-hijacking-cswsh. [120] GitHub Octoverse report. https://octoverse.github.com/.
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postMessage Handlers at Scale,” in CCS, 2020, pp. 493–505. vaScript/Reference/Global Objects/Object/freeze.
[83] Dom-based document-domain manipulation. https://portswigger.net/ [122] The instanceof Operator. https://developer.mozilla.org/en-US/docs/
web-security/dom-based/document-domain-manipulation. Web/JavaScript/Reference/Operators/instanceof.
[84] J. Schwenk, M. Niemietz, and C. Mainka, “Same-Origin Policy: [123] The typeof Operator. https://developer.mozilla.org/en-US/docs/Web/
Evaluation in Modern Browsers,” in USENIX Security, 2017. JavaScript/Reference/Operators/typeof.
[85] T. A. Nideck, “What Are JSON Injections?” 2019, https://www.ac [124] The globalThis object. https://developer.mozilla.org/en-US/docs/We
unetix.com/blog/web-security-zone/what-are-json-injections. b/JavaScript/Reference/Global Objects/globalThis.
[86] Client-side json injection. https://portswigger.net/kb/issues/00200370 [125] The MutationObserver API. https://developer.mozilla.org/en-US/doc
client-side-json-injection-dom-based. s/Web/API/MutationObserver.
[87] P. Saxena, D. Akhawe, S. Hanna, F. Mao, S. McCamant, and D. Song, [126] TheThing. https://github.com/SoheilKhodayari/TheThing.
“A symbolic execution framework for JavaScript,” in IEEE S&P, 2010, [127] DOM Clobbering browser testing pipeline. https://github.com/Sohei
pp. 513–528. lKhodayari/DOMClobbering.
[88] C.-A. Staicu and M. Pradel, “Freezing the Web: A Study of ReDoS [128] B. Stock, G. Pellegrino, C. Rossow, M. Johns, and M. Backes, “Hey,
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[89] J. C. Davis, C. A. Coghlan, F. Servant, and D. Lee, “The impact of [129] F. Li, Z. Durumeric, J. Czyz, M. Karami, M. Bailey, D. McCoy,
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study at the ecosystem scale,” in ESEC/FSE, 2018, pp. 246–256. effective vulnerability notifications,” in USENIX Security, 2016.
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web-security/dom-based/local-file-path-manipulation. gadgets on csp at scale,” in ACM Asia CCS, 2020, pp. 420–431.
[91] V. Le Pochat, T. Van Goethem, S. Tajalizadehkhoob, M. Korczyński, [131] M. Heiderich, “ToStaticHTML for Everyone! About DOMPurify,
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[92] Google Closure Library HTML Sanitizer. https://github.com/google/ [133] (2018) Client-side CSRF. https://www.facebook.com/notes/facebook-
closure-library/blob/master/closure/goog/html/sanitizer/htmlsanitize bug-bounty/client-side-csrf/2056804174333798/.
15
Appendix A. Name Î HTML Tags
TS1 a, abbr, acronym, address, applet, area, article, aside, audio, b, base, basefont, bdi,
A.1. Case Studies bdo, bgsound, big, blink, blockquote, br, button, canvas, center, cite, code, com-
mand, content, data, datalist, dd, del, details, dfn, dialog, dir, div, dl, dt, element,
This section reports on a few manually vetted case studies em, embed, fieldset, figcaption, figure, font, footer, form, h1, header, hgroup, hr, i,
iframe, image, img, input, ins, isindex, kbd, keygen, label, legend, li, link, listing,
of the confirmed attacks. We note that the affected parties main, map, mark, marquee, menu, menuitem, meta, meter, multicol, nav, nextid, nobr,
have been promptly informed of the vulnerability, and have noembed, noframes, noscript, object, ol, optgroup, option, output, p, param, picture,
pre, progress, q, rb, rp, rt, rtc, ruby, s, samp, script, section, select, shadow, slot, small,
already patched them (see §7). source, spacer, span, strike, strong, style, sub, summary, sup, table, template, textarea,
time, title, track, tt, u, ul, var, video, wbr, xmp
GitHub. This vulnerability affects the GitHub Shop and orig-
TS2 blockquote, br, button, canvas, center, cite, code, command, content, data, datalist,
inated when loading the Boomerag JavaScript library [132]. dd, del, details, dfn, dialog, dir, div, dl, dt, element, em, embed, fieldset, figcaption,
In more details, the code followed the vulnerable pattern G figure, font, footer, form, h1, header, hgroup, hr, i, image, img, input, ins, isindex, kbd,
keygen, label, legend, li, link, listing, main, map, mark, marquee, menu, menuitem,
of Table 6, where a variable called BOOMR was defined in meta, meter, multicol, nav, nextid, nobr, noembed, noframes, noscript, object, ol,
an inital script that contained a clobberable, invoked native optgroup, option, output, p, param, picture, plaintext, pre, progress, q, rb, rp, rt, rtc,
ruby, s, samp, script, section, select, shadow, slot, small, source, spacer, span, strike,
method, and a second script that used the object property strong, style, sub, summary, sup, svg, table, template, textarea, time, title, track, tt, u,
ul, var
window.BOOMR.url as the src of a dynamically added
TS3 button, fieldset, input, output, select, textarea
script. Attackers can escalate this vulnerable pattern to client-
side XSS via double clobbering. First, they clobber the TS4 image, img, object
invoked native method, causing a runtime error when the TS5 a, abbr, acronym, address, applet, area
browser parses the first script. Therefore, the browser stops TS6 basefont, bgsound, blink
parsing the rest of the script and BOOMR becomes undefined. TS7 noembed, noframes, noscript, script, style, template, textarea, title, xmp
Then, attackers can clobber window.BOOMR.url and TS8 ins, isindex, kbd, keygen, label, legend, li, link, listing, main, map, mark, marquee,
consequently control the script src by injecting a DOM menu, menuitem, meta, meter, multicol, nav, nextid, nobr, object, ol, optgroup, option,
output, p, param, picture, pre, progress, q, rb, rp, rt, rtc, ruby, s, samp, section, select,
Clobbering markup, e.g., . We discovered that
TS9 fieldset, figcaption, figure, font, footer, form, h1, header, hgroup, hr, i, image
it is possible to inject such non-script markup to the client-
side page leveraging the search functionality and the URL TS10 form, iframe, image, img, script, style, table, template
query parameters, which were reflected back to the page. TS11 caption, col, colgroup, tbody, td, tfoot, th, thead, tr
TS12 p, param, picture, plaintext, pre, progress, q, rb, rp, rt, rtc, ruby, s, samp, script,
Trello. We discovered that Trello uses a global object section, select, shadow, slot, small, source, spacer, span, strike, strong, style, sub,
property called window.ClickTaleScriptSource summary, sup, table, template, textarea, time, title, track, tt, u, ul, var, video, wbr, xmp,
a, abbr, acronym, address, applet, area, article, aside, audio, b, base, basefont, bdi,
to programmatically load a script named wrScript. bdo, bgsound, big, blink, blockquote, br, button, canvas, center, cite, code, command,
However, this property was clobberable as content, data, datalist, dd, del, details, dfn, dialog, dir, div, dl, dt, element, em, embed,
fieldset, figcaption, figure, font, footer, form, h1, header, hgroup, hr, i
ClickTaleScriptSource was an undefined variable
TS13 h1, header, hgroup, hr, i, image, img, input, ins, isindex, kbd, keygen, label, legend,
following the vulnerable pattern A of Table 6. Finally, we li, link, listing, main, map, mark, marquee, menu, menuitem, meta, meter, multicol,
found that it is possible to insert a persistent, non-script nav, nextid, nobr, noembed, noframes, noscript, object, ol, optgroup, option, output, p,
param, picture, plaintext, pre, progress, q, rb, rp, rt, rtc, ruby, s, samp, script, section,
markup to overwrite this object property by editing a select, shadow, slot, small, source, spacer, span, strike, strong, style, sub, summary,
sup, table, template, textarea, time, title, track, tt, u, ul, var, video, wbr, xmp, a, abbr,
comment for a card in Trello boards, which resulted in acronym, address, applet, area, article, aside, audio
arbitrary client-side code execution. TS14 form, b, base, basefont, bdi, bdo, bgsound, big, blink, blockquote, br, button, canvas,
center, cite, code, command, content, data, datalist, dd, del, details, dfn, dialog, dir,
Fandom. We discovered a DOM Clobbering vulner- div, dl, dt, element, em, embed, fieldset, figcaption, figure, font, footer
ability in Fandom affecting the users’ message wall TS15 data, datalist, dd, del, details, dfn, dialog, dir, div, dl, dt, element, em, embed, fieldset,
that resulted in open redirection. Specifically, the figcaption, figure, font, footer, form, h1, header, hgroup, hr, i, image, img, input, ins,
isindex, kbd, keygen, label, legend, li, link, listing, main, map, mark, marquee, menu,
JavaScript program contained an assignment to the menuitem, meta, meter, multicol, nav, nextid, nobr, noembed, noframes, noscript,
location.href property of the top-level window, whose object, ol, optgroup, option, output, p, param, picture, plaintext, pre, progress, q, rb,
rp, rt, b, base, basefont, bdi, bdo, bgsound, big, blink, blockquote, br, button, canvas,
value was tainted with a clobberable object property, center, cite, code, command, content, rtc, ruby, s, samp, script, section, select, shadow
i.e., form.elements.targetUsername.value. At- TS16 rtc, ruby, s, samp, script, section, select, shadow, slot, small, source, spacer, span,
tackers can manipulate the value of this property by, e.g., two strike, strong, style, sub, summary, sup, table, template, textarea, time, title, track, tt,
u, ul, var, video, wbr, xmp, a, abbr, acronym, address, applet, area, article, aside, audio,
nested iframe tags that are named form and elements, b, base, basefont, bdi, bdo, bgsound, big, blink, blockquote, br, button, canvas, center,
and an additional input element in the nested frame. The cite, code, command, content, data, datalist, dd, del, details, dfn, dialog, dir, div, dl, dt,
element, em, embed, fieldset, figcaption, figure, font, footer, form, h1, header, hgroup,
input is named targetUsername, and has a value hr, i, image
containing a malicious URL, which will be set as the window TS17 br, button, canvas, center, cite, code, command, content, data, datalist, dd, del, details,
dfn, dialog, dir, div, dl, dt, element, em, embed, fieldset, figcaption, figure, font, footer,
URL. We found that it is possible to inject non-script markup form, h1, header, hgroup, hr, i
in the page in two distinct ways: (i) attackers can insert TS18 address, applet, area, article, aside, audio, b, base, basefont, bdi, bdo, bgsound, big,
persistent payloads using the post functionality in the profile blink, blockquote, br, button, canvas, center, cite, code, command, content, data,
datalist, dd, del, details, dfn, dialog, dir
message wall, and (ii) a URL parameter in the path was
reflected back to the page without extra validation, enabling TS19 form, iframe, script, style, template
transient insertion of clobbering payloads in the page. TS20 image, img, input, noembed, noframes, noscript
A.2. Additional Evaluation Details TABLE 7: List of HTML tags used in Table 2 that share the same DOM
Clobbering behaviour.
16
Ë Security Threat ! Semantic Type Description Reference ý JavaScript Sink
Client-side Open Redirect WIN LOC WRITE Redirecting the Window URL [36, 37] window.location = T
Websocket Hijacking WEBSOCK URL WRITE Hijacking Websocket Connections [80, 81] new WebSocket(T)
Cookie Manipulation DOC COOKIE WRITE Manipulating Cookie State [1, 5, 82] document.cookie = T
Doc. Domain Manipulation DOC DOMAIN WRITE Bypassing SOP [83, 84] document.domain = T
Client-side JSON Injection JSON PARSE Parsing Untrusted JSON [85–87] JSON.parse(T)
RegEx Injection REGEX BUILD Injecting Regex for ReDoS [88, 89] new RegExp(T)
postMessage Manipulation POST MSG WRITE Manipulating postMessages [82] window.postMessage(T)
Local File Path Manipulation FILE PATH WRITE Manipulating Path of Read Files [90] new FileReader().readAsText(T)
Cross-site Scripting (XSS) CODE LOADING Loading New Scripts [1, 22, 39] script.src = T
CODE EXEC Executing Arbitrary JavaScript [1, 7] script.textContent = T
eval(T)
setTimeout(T)
setInterval(T)
new Function(T)
DOM NODE INJECT Injecting DOM Elements [1, 7, 12, 87] document.write(T)
document.writeln(T)
elm.innerHTML = T
elm.outerHTML = T
elm.insertAdjacentHTML(T)
elm.insertAdjacentElement(T)
elm.replaceChild(T)
elm.append(T)
elm.appendChild(T)
Web Storage Manipulation DOC STORAGE WRITE Manipulating Storage State [1, 5, 82] localStorage.setItem()
sessionStorage.setItem()
Client-side Request Forgery REQ Manipulating Asynchronous Reqs. [32, 133] fetch(T)
XMLHttpRequest.open(T)
asyncRequest(T)
$.ajax(T)
Legend: T= Tainted Variable;
TABLE 8: Summary of primitive JavaScript sinks and semantic types supported by TheThing grouped by the security risk of manipulating the sink object.
The list is obtained by aggregating the client-side JavaScript sinks considered in existing literature.
ý Source ! Semantic Type Graph Queries
S1: variable v CLOB CUSTOM VAR Task: Identifying Source Si as in Table 3
S4, S5: window.v CLOB WIN CUSTOM VAR Qs1 = {n: n ∈ N ∧ n.type == ‘Identifier’ ∧ ∃ v ∈ NP ∧ CLOB(v) ∧ n.name == v }
S6: document.v CLOB DOC CUSTOM VAR
Qs2 = {n: n ∈ N ∧ ∀ s ∈ N, @ e ∈ E, e == edge(s, n) ∧ e.type == ‘PDG’ ∧ ∃ v, v ∈
/ NP
S2: property p CLOB NATIVE PROP ∧ e.value == v ∧ (s.type == ‘AssignmentExp’ ∨ s.type == ‘VarDeclaration’) }
S3: window.p CLOB WIN NATIVE PROP
S7: document.p CLOB DOC NATIVE PROP Qs3 = {n: n ∈ N ∧ n.type == ‘MemberExp’ ∧ n.object == window ∧ ∃ v ∈ NP ∧ CLOB(v)
Legend: Si = case Si in Table 3; ∧ n.property == v }
TABLE 9: Summary of DOM Clobbering sources and their semantic types Qs4 = {n: n ∈ N ∧ n.type == ‘MemberExp’ ∧ n.object == window ∧ ∃ v ∈ / NP ∧ n.property
== v ∧ ∀ s ∈ N, @ e ∈ E, e == edge(s, n) ∧ e.type == ‘PDG’ ∧ e.value == v ∧ (s.type ==
based on the seven cases of Table 3. ‘AssignmentExp’ ∨ s.type == ‘VarDeclaration’ ∧ s.kind = ‘var’) }
Qs5 = {n: n ∈ N ∧ n.type == ‘MemberExp’ ∧ n.object == window ∧ ∃ v ∈ / NP ∧ n.property
Sanitizer Version Sanitizer Version == v ∧ ∃ s ∈ N, e ∈ E, e == edge(s, n) ∧ e.type == ‘PDG’ ∧ e.value == v ∧ (s.type ==
‘AssignmentExp’ ∨ s.type == ‘VarDeclaration’ ∧ s.kind ∈ {‘var’, ‘let’, ‘const’})} ∧ ∃ f ∈
Client-side JS Node.js N, f .type == ‘CallExp’ ∧ f .script == s.script ∧ f .name ∈ NP ∧ CLOB(f .name)
1. DOMPurify 2.3.4 1. Insane 2.6.2
2. Google Closure Lib. 20211201.0.0 2. Bleach 0.3.0 Qs6 = {n: n ∈ N ∧ n.type == ‘MemberExp’ ∧ n.object == document ∧ ∃ v ∈
/ NP ∧
3. JS-XSS 1.0.10 Bower-angular-sanitize 1.8.2 n.property == v }
4. Sanitize-HTML 2.6.1 Yahoo html-purify 1.1.0
5. Google Caja 6015 Arcgis 2.9.0 Qs7 = {n: n ∈ N ∧ n.type == ‘MemberExp’ ∧ n.object == document ∧ ∃ v ∈ NP ∧
CLOB(v) ∧n.property == v }
Python PHP
1. Mozilla Bleach 4.1.0 1. Htmlpurifier 4.14.0
2. LXML 4.7.1 2. Html-sanitizer 1.5.0 Task: Identifying Sink Fi as in Table 8
3. HTML Sanitizer 1.9.3 3. Symfony HtmlSanitizer 1.0.0 Qsinks = {n: n ∈ N ∧ ∃ c ∈ N ∧ hasChild(n, c) ∧ c.type == ‘Identifier’ ∧ c ∈ SI }
4. Htmllaundry 2.2 4. HTMLawed 1.2
5. Django-html-sanitizer 0.1.5 5. Typo3 Sanitizer 2.0.13
Task: Identifying Vulnerable Sinks
C# Java
1. AntiXssEncoder 4.3.0 1. Jsoup 1.14.3 Qvuln = {n: n ∈ N ∧ n.type == ‘ExpStatement’ ∧ ∃ c1, c2 ∈ N ∧ hasChild(n, c1) ∧
2. HtmlSanitizer 7.0.473 2. Java-html-sanitizer 20211018.2 hasChild(n, c2) ∧ c1.semType == ‘SOURCE’ ∧ c2.semType == ‘SINK’ }
3. AJAX Toolkit 20.1.0 3. Antisamy 1.6.4
4. NSoup 0.8.0 4. HtmlCleaner 2.25 Legend: N, E= HPG nodes, edges; SI= sinks in Table 8; NP= native property;
5. HtmlRuleSanitizer 1.6.0.1 CLOB(v)= v is a clobberable NP according to §4.2.3.
TABLE 10: The specific versions of HTML sanitizers tested in §6 TABLE 11: Excerpt of DOM Clobbering detection queries.
17
.
Chrome Firefox Opera Edge Safari TB SI UC
95.0.4638
92.0.4515
65.2.3381
82.0.4227
95.0.1020
96.0.1054
95.0.1020
94.1.2
14.7.1
11.0.1
15.0.6
13.3.8
3.2.3
96.0
95.0
39.0
15.1
14.1
13.1
API ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿ Total
Method 75 72 77 70 69 77 75 72 77 75 72 77 76 77 77 77 69 75 75 79
Property 246 244 256 240 238 255 246 244 254 246 244 255 255 258 260 255 244 246 251 268
Total 321 316 333 310 307 332 321 316 331 321 316 322 331 335 337 332 313 321 326 347
Legend: TB= Tor Browser; SI= Samsung Internet; UC= UC Browser;
TABLE 12: Count of clobbered native DOM APIs in mobile and desktop browsers. Browsers with similar behaviours are grouped with the same color.
.
Chrome Firefox Opera Edge Safari TB SI UC Chrome Firefox Opera Edge Safari TB SI UC
95.0.4638
92.0.4515
65.2.3381
82.0.4227
95.0.1020
96.0.1054
95.0.1020
95.0.4638
92.0.4515
65.2.3381
82.0.4227
95.0.1020
96.0.1054
95.0.1020
94.1.2
14.7.1
11.0.1
15.0.6
13.3.8
94.1.2
14.7.1
11.0.1
15.0.6
13.3.8
3.2.3
96.0
95.0
39.0
15.1
14.1
13.1
3.2.3
96.0
95.0
39.0
15.1
14.1
13.1
API ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿ API ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿
cancelIdleCallback() sidebar
clearImmediate()
Notification
convertPointFromNodeToPage()
dataLayer
convertPointFromPageToNode()
XDomainRequest
createImageBitmap() ActiveXObject
dump() attachEvent
getDefaultComputedStyle() EventEmitter
home() ReportingObserver
minimize() chrome
openDialog()
safari
print() firefoxAccessException
requestIdleCallback() trustedTypes
routeEvent() NaN
scrollByLines() SharedArrayBuffer
scrollByPages() afterprint
setCursor() animationcancel
setImmediate() animationend
showDirectoryPicker() animationiteration
showModalDialog() beforeprint
showOpenFilePicker() beforeunload
showSaveFilePicker() copy
sizeToContent() cut
updateCommands() DOMContentLoaded
error
caches hashchange
controllers languagechange
crossOriginIsolated load
dialogArguments message
directories messageerror
fullScreen offline
mozAnimationStartTime online
mozInnerScreenX orientationchange
mozInnerScreenY pagehide
pageshow
onappinstalled
paste
onauxclick
popstate
onbeforeinstallprompt
rejectionhandled
oncancel Storage
onclose transitioncancel
ondeviceorientationabsolute unhandledrejection
ondragdrop unload
onformdata vrdisplayconnect
ongamepadconnected vrdisplaydisconnect
ongamepaddisconnected vrdisplaypresentchange
onloadend onanimationcancel
onmessageerror ontouchcancel
onpaint ontouchstart
onvrdisplayactivate defaultStatus
onvrdisplayblur Touch
onvrdisplayconnect TouchEvent
onvrdisplaydeactivate ondevicemotion
onvrdisplaydisconnect ondeviceorientation
onvrdisplayfocus ontransitioncancel
onvrdisplaypointerrestricted speechSynthesis
onvrdisplaypointerunrestricted onselectionchange
onvrdisplaypresentchange MediaSource
pkcs11
onselectstart
scrollMaxX
scrollMaxY Legend: TB= Tor Browser; SI= Samsung Int.; UC= UC Browser; = successfully clobbered; = clobbering fails;
TABLE 13: List of clobbered Window methods and properties in web browsers.
18