---
type: Article
title: On the Incoherencies in Web Browser Access Control Policies
description: Browsers label principals differently for DOM, cookies, XHR and display, so their policies conflict wherever those resources interplay, and runtime document.domain changes are ignored by most of them. User-owned resources such as clipboard and geolocation are reachable from scripts. WebAnalyzer, an instrumented IE crawler, measured each unsafe feature across the Alexa top 100,000.
resource: "https://www.microsoft.com/en-us/research/publication/incoherencies-web-browser-access-control-policies/"
tags: [article, webseclist-reference, same-origin-policy, sop-bypass, cookie, dom, measurement-study, large-scale-scan, info-leak, clickjacking, owasp-a01-2021, owasp-a04-2021, owasp-a07-2021]
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resource: "https://www.microsoft.com/en-us/research/publication/incoherencies-web-browser-access-control-policies/"
title: On the Incoherencies in Web Browser Access Control Policies
author: Kapil Singh, Alexander Moshchuk, Helen J. Wang, Wenke Lee
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authors:
- Kapil Singh
- Alexander Moshchuk
- Helen J. Wang
- Wenke Lee
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# On the Incoherencies in Web Browser Access Control Policies
**On the Incoherencies in Web Browser Access Control Policies** - Kapil Singh, Alexander Moshchuk, Helen J. Wang, Wenke Lee, Publisher not stated.
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On the Incoherencies in Web Browser Access Control Policies
Kapil Singh∗ , Alexander Moshchuk†, Helen J. Wang† and Wenke Lee∗
∗ Georgia Institute of Technology, Atlanta, GA
Email: {ksingh, wenke}@cc.gatech.edu
† Microsoft Research, Redmond, WA
Email: {alexmos, helenw}@microsoft.com
Abstract—Web browsers’ access control policies have evolved Inconsistent principal labeling. Today’s browsers do
piecemeal in an ad-hoc fashion with the introduction of new not have the same principal definition for all browser re-
browser features. This has resulted in numerous incoherencies. sources (which include the Document Object Model (DOM),
In this paper, we analyze three major access control flaws in
today’s browsers: (1) principal labeling is different for different network, cookies, other persistent state, and display). For
resources, raising problems when resources interplay, (2) run- example, for the DOM (memory) resource, a principal is
time changes to principal identities are handled inconsistently, labeled by the origin defined in the same origin policy
and (3) browsers mismanage resources belonging to the user (SOP) in the form of [4]; but
principal. We show that such mishandling of principals leads for the cookie resource, a principal is labeled by . Different principal definitions for two resources are
A unique contribution of this paper is to identify the com- benign as long as the two resources do not interplay with
patibility cost of removing these unsafe browser features. To do each other. However, when they do, incoherencies arise. For
this, we have built WebAnalyzer, a crawler-based framework example, when cookies became accessible through DOM’s
for measuring real-world usage of browser features, and used “document” object, DOM’s access control policy, namely the
it to study the top 100,000 popular web sites ranked by Alexa.
Our methodology and results serve as a guideline for browser SOP, undermines some of cookie’s access control policies
designers to balance security and backward compatibility. (Section II-C1 gives a more detailed analysis).
Inappropriate handling of principal label changes. A
web application is allowed to change its principal’s label
I. I NTRODUCTION
at runtime through the use of the document.domain
Web browsers have gradually evolved from an application DOM property. Nevertheless, the access control state is
that views static web pages to a rich application platform often kept static and such “effective” principal IDs set by
on which mutually distrusting web site principals co-exist document.domain are disregarded. This leads to access
and interact [1]–[3]. Along the way, the browsers’ access control incoherencies.
control policies have also been evolving, but unfortunately Disregard of the user principal. In this paper, we
this happened in a piecemeal and ad-hoc fashion alongside introduce the concept of a user principal in the browser
the introduction of new browser features (such as AJAX) setting. The user principal represents the user of a browser.
or resources (such as local storage). There have been no Sometimes, the user principal is disregarded in existing
principles or invariants that a new access control policy must browsers’ access control policies. Certain resources should
follow or maintain. Consequently, numerous incoherencies belong to the user principal exclusively. They include the
in browsers’ access control policies exist, presenting hurdles user-private state such as clipboard and geolocation, user
for web programmers to build robust web applications. actions like navigating back and forward in browsing history,
In this paper, we examine the current state of browser ac- and a browser’s UI including the current tab. These resources
cess control policies, uncover and analyze the incoherencies should not be accessible by web applications without user
in these policies, and measure the cost of eliminating them permission; otherwise, a web site could impersonate the user
in today’s web. and violate user privacy. Unfortunately, today’s DOM APIs
An access control policy configures how a principal expose some of these resources to web applications.
accesses certain resources. This involves defining how prin- To systematically analyze and uncover the incoherencies
cipals are identified, how resources are labeled with principal created by these three problem areas, we have devised a set
IDs, and how these labels may be changed and handled at of coherency principles and constructed tests to check major
runtime. Unfortunately, browsers often mismanage princi- browsers (including Internet Explorer, Firefox, and Google
pals, resulting in access control inconsistencies. We focus on Chrome) for violations of these principles and to uncover
three major sources of these problems: inconsistent principal the incoherencies that ensue.
labeling, inappropriate handling of principal label changes, A major goal of our work is to evaluate the compatibility
and disregard of the user principal. cost of removing unsafe browser features that contribute to
the incoherencies. To this end, we have built WebAnalyzer, a 1) Each shared browser resource, i.e. a resource shared
scalable, crawler-based browser-feature measurement frame- among multiple principals, should have a principal
work that can inspect a large number of web pages by definition (labeling of principals that share the re-
rendering them in instrumented browsers. WebAnalyzer cap- source) and have an access control policy.
tures the DOM interactions of a page by interposing between 2) For each non-shared browser resource that is explicitly
the JavaScript engine and the DOM renderer, captures the owned by a single principal, the resource should have
protocol-level behavior through an HTTP proxy, and ana- an owner principal with a specific label or be globally
lyzes the visual appearance of a page by extracting its page accessible.
layout. 3) When two resources interplay, both resources should
Armed with WebAnalyzer, we have conducted measure- have the same principal definition.
ments on the prevalence of unsafe browser features over the This is because when two resources have different
most popular 100,000 web sites as ranked by Alexa [5]. Our ways of labeling principals and when they interplay,
results pinpoint some unsafe features that have little back- their respective access control policies can be in con-
ward compatibility cost and are thus possible to remove from flict.
current browsers without breaking many sites. For example, 4) All access control policies must consider the runtime
we find that most APIs controlling user-owned resources, label of the principals, namely, the “effective” princi-
descendant navigation, and incoherencies in XMLHttpRe- pal ID.
quest’s principal labeling have low compatibility costs, 5) The user principal’s resources should not be accessible
whereas a substantial number of sites depend on “dangerous” by web applications.
functionality provided by document.domain or transpar- This is because when the user principal’s resources
ent cross-origin overlapping frames. Overall, we believe that are accessible by web applications, the user’s privacy
by estimating the prevalence of unsafe features on the web, may be compromised or a web application could act
our measurements can guide future browsers to make better on the user’s behalf without the user’s knowledge.
security vs. functionality trade-offs. We look for violations of these principles and check for
In summary, this work makes the following contributions: incoherencies when violations take place. The pseudocode
• A systematic, principal-driven analysis of access con- below illustrates our manual analysis process.
trol incoherencies in today’s browsers.
• Introduction of the user principal concept for the 0 foreach (browser resources) {
browser setting. 1 if exists (access control) {
• A comprehensive, extensible compatibility measure- 2 if !considers (effective principal ID)
ment framework. 3 check improper principal ID changes
• The first large-scale measurements on the compatibility 4 } else
cost of coherent access control policies. 5 check if lack of policy is appropriate
The rest of the paper is organized as follows. Section II 6 }
presents our systematic analysis of today’s browser access 7
control policies and enumerates access control incoheren- 8 foreach (pairs of resources) {
cies. Section III discusses our measurement motivation, 9 if (they interplay &&
tools, and infrastructure. Section IV presents our measure- 10 the principal/owner labeling differs)
ment results and gives recommendations on which unsafe 11 check resource interplay incoherencies
policies can be eliminated with acceptable compatibility 12 }
cost. Section V discusses limitations of our approach, Sec-
tion VI presents related work, and Section VII concludes.
For each resource, we check whether it has an access
II. A N ANALYSIS OF BROWSER ACCESS CONTROL control policy. If not, we check whether the lack of policy
INCOHERENCIES
is appropriate (line 5, for example, Section II-E illustrates
on how some resources that belong to the user principal
In this section, we present our systematic analysis of lack access control considerations). If yes, we further check
today’s browser access control policies and enumerate their whether the access control policy considers the effective
incoherencies. principal ID that sites can change dynamically at render-
time. If it does not, then we check for incoherencies there
A. Methodology (line 3, Section II-D).
For a systematic analysis, we establish the following In addition, we go through all pairs of resources; if they
access control coherency principles to guide our search for interplay and if they have the different principal definitions,
incoherencies: we check for incoherencies (line 11, Section II-C). Careful
Shared resources Principal definition
documents using the Document Object Model (DOM),
DOM objects SOP origin which is the platform- and language-neutral interface that
cookie domain/path allows scripts to dynamically access and update the content,
localStorage SOP origin
structure and style of a document [6].
sessionStorage SOP origin
A cookie is a persistent state resource. The browser
display SOP origin and dual ownership *
ensures that a site can only set its own cookie and that
Table I a cookie is attached only to HTTP requests to that site.
S HARED BROWSER RESOURCES AND THEIR RESPECTIVE PRINCIPAL
DEFINITIONS . *D ISPLAY ACCESS CONTROL IS NOT WELL- DEFINED IN TODAY ’ S By default, the principal is labeled with the host name
BROWSERS . and path, but without the protocol and the port num-
ber [7], [8], unlike SOP origins. For example, if the page
a.com/dir/1.html creates a cookie, then that cookie
Non-shared resources Owner is accessible to a.com/dir/2.html and other pages
XMLHttpRequest SOP origin from that dir/ directory and its subdirectories, but is not
postMessage SOP origin accessible to a.com/. Furthermore, https://a.com/
clipboard user* and http://a.com/ share the cookie store unless a
browser history user*
cookie is marked with a “secure” flag. Non-HTTPS sites
geolocation user
can still set “secure” cookies in some implementations, but
Table II cannot read them back [9]–[11]. A web programmer can
N ON - SHARED BROWSER RESOURCES AND THEIR RESPECTIVE OWNER
PRINCIPAL . *ACCESS CONTROL IS NOT WELL- DEFINED IN TODAY ’ S BROWSERS .
make cookie access less restrictive by setting a cookie’s
domain attribute to a postfix domain or the path name to
be a prefix path.
Local storage is the persistent client-side storage shared
readers may wonder what happens to the interplay of more among principals defined by SOP origins [12].
than two resources. Coherency in this context is a transitive Session storage is storage for a tab [12]. Each tab has a
property. That is, if a Resource 1 and Resource 2’s access unique set of session storage areas, one for each SOP origin.
control policies are coherent (namely have the same princi- The sessionStorage values are not shared between tabs. The
pal definitions) and that of Resource 2 and Resource 3 are lifetime of this storage is the same as that of the tab.
coherent, then the access control policies of Resource 1 and Display does not have a well-specified access control
Resource 3 are also coherent since their principal definitions policy in today’s browsers and standards (corresponding to
should also be the same. line 5 in our pseudocode). Our earlier work Gazelle [3]
The enumeration of resources is done by manually brows- specified an access control policy for display (and Gazelle
ing through IE’s source code (more in Section II-B). Our further advocated that this policy be enforced by the browser
incoherency checks are done through test programs on major kernel, unlike existing browsers). In Gazelle’s model, a web
browser versions. site principal delegates its display area to another principal in
Despite our effort to be comprehensive, it is possible that the form of cross-domain iframes (or objects, images). Such
we miss some browser resources or miss some interplays an iframe (window) is co-owned by both the host page’s
among the resources. We hope our work to be a start for principal, called landlord, and the nested page’s principal,
a community effort on mapping out the full set of browser called tenant (both labeled with SOP origins). Principals
access control policies. other than the landlord and the tenant have no access per-
missions for the window. For the top-level window, the user
B. Browser resources principal owns it and plays the role of its landlord. Gazelle’s
In this section, we enumerate all types of browser re- policy further specifies how landlord and tenant should
sources. A browser resource may be shared among (some access the four attributes of a window, namely the position,
definition of) principals or may not be shared and is ex- dimensions, pixels, and URL location. This specification
plicitly owned by some principal. Table I shows the shared guarantees that the tenant cannot interfere with the landlord’s
resources and their respective principal definitions. Table II display, and that the tenant’s pixels, DOM objects, and
shows non-shared resources and their respective owners. navigation history are private to the tenant. Gazelle’s policy
We now describe each resource, their principal or owner is coherent with SOP. In Table III, we summarized the access
definition, and its access control policy in turn. control matrix for Gazelle, IE 8, Firefox 3.5, and Chrome 2.
A DOM object is a memory resource shared among The access control of the URL location attribute corresponds
principals labeled with SOP origins, namely, . The access control policy of DOM objects is policy allows navigating a descendant window regardless
governed by SOP [4], which mandates that two documents of its origin; this was advocated and implemented over
from different origins cannot access each other’s HTML several browsers [13]. Gazelle’s policy is child navigation
Landlord Tenant
Gazelle IE FF/Chrome Gazelle IE FF/Chrome
position (x,y,z) RW RW RW RW
dimensions (height, width) RW RW RW R RW R
pixels W* W* RW RW RW
URL location W W RW* RW RW RW
Table III
A CCESS CONTROL POLICY FOR A WINDOW ’ S LANDLORD AND TENANT ( BEING A DIFFERENT PRINCIPAL FROM THE LANDLORD ) ON G AZELLE , IE 8,
F IREFOX 3.5, AND C HROME . RW*: T HE URL IS READABLE ONLY IF THE LANDLORD SETS IT. IF THE TENANT NAVIGATES TO ANOTHER PAGE, LANDLORD WILL NOT
SEE THE NEW URL. W*: THE LANDLORD CAN WRITE PIXELS WHEN THE TENANT IS TRANSPARENTLY OVERLAID ON THE LANDLORD .
policy. (We elaborate in Section II-C3 that the descendant
navigation policy is at conflict with DOM’s SOP.) Our tests
indicate that Firefox 3.5 and Chrome 2 currently support
the child policy, while IE 8 supports the descendant policy.
All major browsers allow any window to navigate the top-
level window, while Gazelle only allows top-level window
navigation from the top-level window’s tenant and the user.
XMLHttpRequest allows a web site principal to use scripts
to access its document origin’s remote data store by issuing
an asynchronous or synchronous HTTP request to the remote
server [14]. XMLHttpRequest2 [15] and XDomainRequest
have been recently proposed and implemented in major
browsers to allow cross-origin communications with remote
servers, where HTTP authentication data and cookies are not
Figure 1. Incoherency arises from the interplay between the access control
sent by default. These networking capabilities are not shared policies of DOM and cookies
and strictly belongs to a web site principal labeled with a
SOP origin.
PostMessage is a recently proposed client-side cross-
origin communication mechanism that is now implemented by setting the cookie with the “secure” flag. However, a
in all major browsers. This is also a web site principal’s ”secure” cookie can still be set by an HTTP response
capability which is not shared with any other principals. and be accessed by scripts belonging to an HTTP page as
The last three resources in the non-shared resource table, long as their domains are the same. Additionally, different
namely clipboard, browser history, and geolocation, all be- services running on different ports of the same domain can
long to the user principal, and web applications should not be access each other’s cookies. Moreover, the path protection
able to access them directly. However, they are all accessible of cookies becomes ineffective as a script from a different
by scripts through the DOM API, causing problems that we path can access the cookie based on SOP.
describe in Section II-E. The interplay between DOM and cookies also allows the
scripts to set the effective domain of a cookie to any suffix
C. The interplay of the resources of the original domain by setting the domain attribute of
From the enumeration of the resources and their respective the cookie. This can lead to inconsistencies in the current
principal or owner definition in the above section, we derived browsers. Figure 1 shows a scenario in which such inconsis-
the following problematic pairs of resources, where the two tencies lead to an undefined behavior in the browsers. In this
resources interplay and their principal or owner definitions example, a cookie named “stockCookie” with value “buy”
differ: DOM-cookie, cookie-XMLHttpRequest, and DOM- is stored in the cookie store for the domain a.com. A script
display. We elaborate on these interplays below. injected into a compromised page belonging to x.a.com
1) DOM and Cookies: DOM and cookies interplay be- can create another cookie with the same name but with a
cause scripts are able to create or modify cookies by using different value “sell” while setting its domain attribute to
the document.cookie property in the DOM API. a.com.
With no protocol in cookie’s principal definition, cookies While this leads to a compromised state in the current
are vulnerable to information leaks. A cookie intended for browsers, different browsers deviate in their behavior cre-
a secure HTTPS principal can be passed over HTTP and ating further inconsistencies in the web applications sup-
be exposed to network attackers. This can be prevented porting multiple browsers. Firefox 3 sets this cookie with
a domain value of .a.com resulting in multiple cookies a window, a resource created by its descendant through a
with the same name in browser’s cookie store. The browser DOM API, even if the landlord and the descendant are
attaches both cookies (genuine cookie with domain a.com different principals. This gives a malicious landlord more
and evil cookie with domain .a.com) to any server requests powerful ways to manipulate a nested, legitimate sites than
to a.com. The server only receives the cookie’s name- just overdrawing: with overdrawing, a malicious landlord
value pair without any information about its corresponding can imitate a tenant’s content, but the landlord cannot send
domain. This results in the server receiving two cookies with messages to the tenant’s backend in the name of the tenant.
the same name. Since server-side behavior is not defined in As an example attack, imagine that an attacker site nests
case of duplicate cookies [9], it leads to inconsistent state at a legitimate trading site as its tenant. The trading site
a.com’s server. In case of IE 8, the original cookie value further nests an advisory site and uses a script to interact
is overwritten and only the wrong cookie value is received with the advisory window to issue trades to the trading
by the server. site backend (e.g., making a particular trade based on the
2) Cookies and XMLHttpRequest: Cookies and XML- advisory’s recommendation shown in the URL fragment).
HttpRequest interplay because XMLHttpRequest can set With just one line of JavaScript, the attacker could navigate
cookie values by manipulating HTTP headers through the advisory window (which is a descendant) and create
scripts. XMLHttpRequest’s owner principal is labeled by the unintended trades.
SOP origin, while cookie has a different principal definition Another conflict lies in the access control on the pixels
(Section II-B). of a window. DOM objects are ultimately rendered into
If a server flags a cookie as “HttpOnly”, the browser the pixels on the screen. SOP demands non-interference
prevents any script from accessing (both reading and writing) between the DOM objects of different origins. However,
the cookie using the document.cookie property. This existing browsers allow intermingling the landlord’s and
effectively prevents cookies being leaked to unintended tenant’s pixels by overlaying transparent tenant iframes on
parties via cross-site scripting attacks [16]. the landlord, deviating from the non-interference goal of
The purpose of HttpOnly cookies is that such SOP. This enables an easy form of clickjacking attacks [19].
cookies should not be touched by client-side In contrast, Gazelle advocates cross-principal pixel isolation
scripts. However, XMLHttpRequests are created and in accordance with SOP (Table III, row “pixels”).
invoked by client-side JavaScript code, and certain
methods of the XMLHttpRequest object facilitate D. Effective Principal ID
access to cookies: getResponseHeader and Browsers allow cross-principal sharing for “related” sites
getAllResponseHeaders allow reading of the by allowing sites to change their principal ID via the
“Set-cookie” header, and this header includes the value of document.domain property [4]. This property can be
HttpOnly cookies. Another method, setRequestHeader, set to suffixes of a page’s domain to allow sharing of
enables modification of this header to allow writing to pages across frames. For example, a page in one frame
HttpOnly cookies. from x.a.com and a page from www.a.com initially
Some of the latest browsers have tried to resolve this issue cannot communicate with each other due to SOP restrictions.
with varied success. IE 8 currently prevents both read and This is one of the few methods for cross-origin frames to
write to cookies via “Set-cookie” header, but still allows communicate before the advent of postMessage [20]. How-
access via “Set-cookie2” header [17]. Firefox has also recog- ever, changing document.domain violates the principle
nized and fixed the issue for cookie reads: their fix prevents of least privilege: once a subdomain sets its domain to its
XMLHttpRequest from accessing cookie headers of any suffix, there is no control over which other subdomains can
response, whether or not the HttpOnly flag was set for those access it.
cookies [18]. This is a bold step taken by Firefox, as our Furthermore, almost no existing access control policies
results show that a considerable number of web pages still of today’s browsers take such “effective” principal IDs into
read cookie headers from XMLHttpRequest (Section IV). consideration. In the following subsections, we examine
However, we have still observed the writing issue with how the disregard of effective principal IDs leads to dual
HttpOnly cookies using Firefox 3.5. A script can set a cookie identities and incoherencies exploitable by attackers. In our
with the same name as the HttpOnly cookie and can have a attack model, an attacker owns a subdomain (through third-
different value set using the setRequestHeader method. party content hosting as in iGoogle or by exploiting a site
This results in a duplicate cookie being sent to the server, vulnerability). As we will show in the following sections,
thus creating an inconsistent state on the server side. the attacker can leverage document.domain to penetrate
3) DOM and Display: One incoherence takes place on the base domain and its other subdomains.
URL location of a window. The descendant navigation 1) Cookie: Any change of origin using
policy (Section II-B) is at conflict with DOM’s SOP. De- document.domain only modifies the effective principal
scendant navigation policy allows a landlord to navigate ID for DOM access and does not impact the domain for
Figure 2. Lack of effective principal ID consideration in cookie’s access Figure 3. Lack of effective principal ID consideration in XMLHttpRe-
control policy quest’s access control policy
cookie access. Figure 2 shows an attack to exploit this into 1.html via DOM access, and this script can then
inconsistent behavior of browser policy design. In this make XMLHttpRequest calls to the original domain of the
scenario, a page 1.html in domain x.a.com changes it page. Since a well-crafted XMLHttpRequest can change the
effective domain to a.com. As a result, it can access the server-side state for the web application, and this state might
DOM properties of other pages belonging to a.com, but be shared between other pages within the domain x.a.com,
it can no longer access the pages of its original domain such attack can possibly impact all pages belonging to
x.a.com. However, since the effective domain does not x.a.com.
change for cookie access, the page still maintains access 3) postMessage: postMessage also ignores any
to the cookies belonging to its original domain. This document.domain changes: if x.a.com changes do-
inconsistent dual identity possessed by the page acts as a main to a.com and sends a message to y.b.com,
bridge to access cookies from both the original domain and y.b.com still sees the message’s origin as x.a.com. Also,
the effective domain. if y.b.com changes its domain to b.com, x.a.com still
In order to launch the attack, an attacker (after owning has to address messages to y.b.com for them to be deliv-
a subdomain page) first assumes the identity of a.com ered. This gives the attacker (with a compromised subdo-
and subsequently injects a script into the page 1.html. main) an opportunity to send messages while masquerading
This injected script can now read and write the cookies under the identity of another subdomain (Figure 4).
belonging to x.a.com including any cookies created later. 4) Storage: Based on our tests, IE 8 does not take any
Effectively, if the attacker can compromise a page in one document.domain changes into consideration for both
of the subdomains, he can access the cookies of any other local storage and session storage. Firefox 3.5 also ignores
subdomains that change their effective origin to the base effective principal ID for local storage. However, for session
domain. storage, any domain changes via document.domain are
2) XMLHttpRequest: Change of origin for scripts does considered: the old session storage is lost for the original
not change the effective principal ID for XMLHttpRequest domain and a new session storage is created for the effective
usage. This enables a (malicious) script in a (compromised) principal.
subdomain to issue XMLHttpRequest to the servers be- Inconsistency arises when document.domain changes
longing to the base domain and its other subdomains. The are ignored (for both session storage and local storage in IE;
attack scenario is illustrated in Figure 3. Page 1.html for only local storage in Firefox). An attacker (being able to
has changed its effective domain value to a.com from the inject a script into one of the pages of any subdomain, say
original value of x.a.com. With no effect on XMLHttpRe- x.a.com) can change its origin to the base domain a.com
quest usage, scripts in 1.html can still make requests and can successfully inject a script into the DOM of the base
to the server belonging to x.a.com. This again gives a domain or any other origins (e.g., y.a.com) that change
script a dual identity – one for DOM access (a.com) and identity to the base domain. Since access control checks
another for XMLHttpRequest (x.a.com). Therefore, an on storage rely on original domain (i.e., y.a.com), the
attacker compromising any subdomain can inject a script malicious script can now freely access the storage belonging
The window object has a history property with an
array of user-visited URLs. Browsers have been denying any
site’s access to this array to protect user privacy, but they
do allow a site to navigate the browser back and forward in
history through the back() and forward() methods [8].
Worse, our tests indicate that Firefox 3 and Google Chrome
2 allow any child window to navigate the top-level window
back or forward in history irrespective of the origin. In
many cases this is just a nuisance, but some properly-crafted
history navigation by a malicious application can lead to
more severe damage. For example, the user might be tricked
to make multiple purchases of the same product.
We have also investigated synthetic event creation. The
DOM API allows a site to generate synthetic mouse or key-
board events through the document.createEvent()
method (or document.createEventObject() in IE).
Figure 4. Lack of effective principal ID consideration in postMessage In IE, a programmer could directly invoke a click()
method on any HTML element to simulate user clicks. These
techniques are useful for debugging purposes. To our delight,
all major browsers are careful not to let a web site to manip-
to y.a.com. ulate another site’s user experience with these synthetic user
events. Note that it is benign for a site to simulate the user’s
E. The User Principal actions for itself, since loading and rendering site content
can by itself achieve any effects of simulating user actions
In this paper, we introduce the concept of the user prin- (e.g., simulating a mouse click is equivalent of calling the
cipal in the browser setting. The user principal represents onclick function on the corresponding element).
the user of the browser. Unfortunately, it has often been 2) Browser UI: An important part of the browser UI
neglected in browser access control policies. is the current tab window, or top-level window. In today’s
While a web application does manage the user’s data browsers, any web site loaded in any window is able to repo-
and experience for that particular application (e.g., a user’s sition and resize a top-level window through the moveTo,
banking data at a banking site), certain browser resources moveBy, resizeTo, and resizeBy properties of the
or data belong to the user exclusively and should not be top-level window. Resizing the currently active top-level
accessible by any web site without user permissions. Such window effectively resizes the browser window. Firefox 3
resources include: user’s private data, such as clipboard allows an application to resize a browser window even in the
data and geolocation; user actions, such as clicking on presence of multiple tabs, while IE 8 and Chrome 2 do not
the forward and back button; devices, such as camera and allow this. A site can also open and close a top-level window
microphone; and browser UI, including the current tab using open and close methods. The use of open method
window (top-level window). has been mitigated through built-in popup blockers. IE 8
Unfortunately, in today’s browsers, some of these re- allows any frame to close a top-level window irrespective of
sources are directly exposed to web applications through the origin, while Firefox 3 and Chrome 2 prevent this from
the DOM API. This breaks the fundamental rule of pro- happening. These capabilities allow an attacker site (even
tecting resources belonging to different principals from one when deeply nested in the DOM hierarchy, say a malicious
another, as the user principal’s resources can be accessed ad) to directly interfere with the user’s experience with the
and manipulated by site principals. This can result in pri- browser UI.
vacy compromises, information leaks, and attacks that trick Some of the other loopholes in browser UI have already
users into performing unintended actions. In this section, been fixed. For example, the status bar can no longer be set
we examine the user principal resources and describe our by a web site.
findings on how they may be accessed improperly by web 3) User-private state: Jackson et al. have shown that a
applications. user’s browsing history can be exposed by inspecting the
1) User actions: The focus and blur properties of the color of a visited hyperlink [21], raising privacy concerns.
window object allow web sites to change focus between the The hyperlink’s color is intended for the user, and it is not
windows that they opened irrespective of the origins. This necessary for web sites to be able to read it.
enables an attacker site to steal focus or cause the user to The clipboard data also belongs exclusively to the
act on a window unintentionally. user principal. All versions of IE since 5.0 support
APIs to access clipboard data. A web site can get III. T HE W EBA NALYZER M EASUREMENT F RAMEWORK
contents of a user’s clipboard by successfully calling
window.clipboardData.getData("Text"). To achieve consistent browser access control poli-
Depending on the default Internet security settings, the cies, browser vendors need to remove or modify the
browser may prompt user before getting the data. However, features that contribute to incoherencies. For exam-
the prompt does not identify the principal making the ple, disallowing domain-setting for cookies, eliminating
request (simply using the term “this site”). As a result, a document.domain, and removing support for access-
malicious script embedded on a third-party frame may trick ing user principal resources are steps towards secure new
the user into giving away his clipboard because he thinks browsers. However, this begs the question of what the cost of
that such access is being requested by the trusted top-level these feature removals is and how many web sites will break
site. as a result. In today’s highly competitive browser market,
backward compatibility with the existing web is paramount.
Geolocation is one of the latest browser features that
To help browser vendors balance security and compati-
allows a site to determine the client’s location by using the
bility, we set off to build a measurement system to measure
navigator.geolocation [12] interface. At the time
the cost of security. Many previous web compatibility studies
of writing, Firefox 3.5 is the only stable production browser
have been browser-centric: they have evaluated the degree to
supporting this HTML5 feature. Geolocation is user-private
which a given browser supports various web standards or is
data. Today’s browsers do ask user permission before access-
vulnerable to attacks [22], [23]. In contrast, we take a web-
ing it. However, issues arise when a site embeds content
centric perspective and actively crawl the web to look for
from multiple principals (i.e., in frames), and more than
prevalence of unsafe browser features on existing web pages.
one origin needs access to geolocation information. The
Compared to existing crawlers, however, static web page
geolocation dialog is active for only one origin at a time;
inspection is insufficient. Dynamic features such as AJAX
if there is a request to access geolocation from b.com
or post-render script events require us to actively render a
while the dialog for a.com is still active, it is ignored
web page to analyze its behavior at run time. Moreover, the
— the principal that succeeds in invoking the geolocation
incoherencies we identified in Section II require analysis of
dialog first wins. Therefore, if a malicious party manages to
not just a page’s JavaScript execution [24], but also DOM
embed a script (or a frame) on the page, it can prevent the
interactions, display layout, and protocol-layer data.
main site from triggering the geolocation dialog by invoking
the dialog first. As a result, the malicious party can create To address these challenges, we have constructed a scal-
denial-of-service against the main site, preventing it from able, execution-based crawling platform, called WebAna-
retrieving a user’s geolocation information. Additionally, it lyzer, that can inspect a large number of web pages by
could trick the user into giving away location to itself rather rendering them in an instrumented browser. The platform
than the main site (e.g., using phishing domain names like consumes a list of URLs (defined by a human operator or
www.gooogle.com). generated by a traditional web crawler), and distributes them
among virtual machine workers, which renders them using
Changing document.domain also generates inconsis-
IEWA , a specially instrumented version of Internet Explorer.
tencies. The geolocation prompt is designed to work only
IEWA provides dynamic mediation for all browser resources,
with the original principals, and even if a site changes
and detects when a resource invocation matches one of
identity, the prompt still displays the original domain as
preset policy rules. Even though our framework is extensible
the requesting domain. For an example site good.a.com
to a large variety of browser policies, we concentrate on “un-
that changes its document.domain to a.com, this causes the
safe feature” rules derived from our analysis in Section II.
following problems:
To build IEWA , the central piece of our measurement
• If an attacker site evil.a.com changes its platform, we leverage public COM interfaces and exten-
document.domain to a.com, it can steal position sibility APIs exported by Internet Explorer 8. Figure 5
information from good.a.com, if good.a.com shows the architecture of IEWA , which centers around three
has stored or displayed this information in major interposition modules: (1) a script engine proxy, which
a place that is accessible via the DOM (e.g., provides JavaScript and DOM interposition, (2) a network
using parent.document.getElementById( proxy based on Fiddler [25], and (3) display dumper, which
"coords").innerHTML). enables custom analysis of a page’s layout as it is visible to
• If another site evil.a.com also changes its domain the user. Next, we discuss each module in turn.
to a.com, it could impersonate good.a.com, Script engine proxy. We build on our earlier system
by using parent.navigator.geolocation in MashupOS [1] to implement a JavaScript engine proxy
.getCurrentPosition, which would trigger (called script engine proxy (SEP)): SEP is installed be-
the access prompt using good.a.com, instead of tween IE’s rendering and script engines, and it mediates
evil.a.com. and customizes DOM object interactions. SEP exports the
Figure 5. High-Level Architecture of IEWA .
script engine API to IE’s renderer, and it exports the DOM consecutive URLs, detects when sites fail to render (e.g.,
and rendering interfaces to IE’s script engine. Each DOM 404 errors), and recovers from any browser crashes.
object is interposed by a corresponding object wrapper. Visiting a site’s home page is sometimes insufficient to
When IE’s script engine asks for a DOM object from the invoke the site’s core functionality. For example, a feature
rendering engine, SEP intercepts the request, retrieves the may be accessed only when the user clicks on a link, types
corresponding DOM object, associates the DOM object with search queries, or causes mouse event handlers to run.
its wrapper object inside SEP, and then passes the wrapper It is difficult and time-consuming to fully automate a site’s
object back to the original script engine. Any subsequent analysis to study all possible features and pages that could
invocation of wrapper object methods from the original be invoked using all combinations of user input. Instead of
script engine passes through SEP. SEP is implemented as aiming for complete coverage within a particular site, we
a COM object and is installed into IE by modifying IE’s enhanced our navigation engine with simple heuristics that
JavaScript engine ID in the Windows registry. simulate some user interaction. After rendering a site’s home
Network interposition. In addition to SEP, we route the page, IEWA will find and simulate a click on at most five
browser’s network traffic through a proxy to monitor all random links, producing five random navigation events. In
HTTP/HTTPS requests and analyze cookie transfers as well addition, IEWA will check for presence of a search form,
as network APIs like XMLHttpRequest. Our network proxy fill it with random keywords, and submit it. We restrict all
is implemented using the FiddlerCore interfaces provided by simulated navigations to stay within the same origin as a
the public-domain Fiddler web debugging proxy [25], [26]. site’s home page.
Display analysis. In order to evaluate display policies, it These simple enhancements maintain our ability to ex-
is necessary to analyze a browser’s visual output as seen by amine a large number of sites while adding the ability to
the user. For this purpose, we use a customized version of properly handle many (but not all) sites with home pages
IE’s rendering engine that exposes COM interfaces to extract that do not invoke the site’s main functionality. For example,
a textual representation of a particular page’s visual layout we can navigate to a random article on Wikipedia, a random
at any stage of rendering. In our current evaluation, we use video on YouTube, a random profile on MySpace, a random
these COM interfaces to save a snapshot log of IE’s display Twitter feed, and a random search query on Google. We
after a page has fully loaded. Because some pages have evaluate the success of this methodology against a user-
post-render events that alter layout, we wait an additional driven browsing study in Section IV-G and discuss its
5 seconds before taking a display snapshot. Snapshot logs limitations in Section V.
provide a mapping between a page’s objects and their layout Performance. We deployed our system on several desktop
properties, such as position, dimensions, or transparency. machines, each with an Intel 2.4 GHz quad-core CPU and
They can be analyzed offline for the presence of unsafe 4 GB of RAM. Our IEWA workers run inside a Windows
frame overlapping behavior or other dangerous page layouts. Vista VMware virtual machine to prevent malware infection.
Navigation. To facilitate automatic analysis for a large We executed multiple workers in each VM, isolating them
number of URLs, IEWA includes a URL navigation en- from one another using different UIDs and different remote
gine, which utilizes IE’s extensibility interfaces, such as desktop sessions.
IWebBrowser2, to completely automate the browser’s nav- On such a setup, one IEWA worker is able to analyze about
igation. In addition to pointing the browser to new URLs, 115 typical web sites per hour. Each site’s processing time
this module also cleans up state such as pop-ups between includes the home page, five random link clicks, and one
form submission, as well as overheads introduced by IEWA ’s Tables IV, V, and VI present the results of our analysis,
three interposition modules. We found that we could execute showing how frequently each feature we analyzed earlier is
up to eight parallel workers in one VM, for a throughput of encountered. Next, we organize our findings according to
900 sites per VM, before saturating the CPU. Optimizing our discussion in Section II and discuss their implications
this infrastructure for performance was not a goal of this on future browser security policies.
paper and is left as future work.
B. The interplay of browser resources
IV. E XPERIMENTAL R ESULTS
1) DOM and Cookies: Cookie usage is extremely pop-
Our analysis in Section II provides an understanding of the ular, and so is their programmatic DOM access via
security characteristics of the current access control policies document.cookie, which we found on 81% web sites
in browsers. In this section, we complete the other half for reading and 76% of web sites for writing cookie values,
of the equilibrium by using the measurement infrastructure respectively. The use of the cookie’s domain attribute is
presented in Section III to study the prevalence of unsafe also widespread (67% of sites), with about 46% of sites
browser features (analyzed in Section II) on a large set of using it to actually change the domain value of the cookie.
popular web sites. By presenting both sides, we enable the As a result, the issues described in Section II-C1 cannot
browser vendors to make more informed decisions about be solved by simply deprecating the usage of this attribute
whether or not to continue supporting a particular unsafe and changing the principal definition of cookies. One pos-
feature based on its real-world usage. sible approach to solve the inconsistency issue with cookie
handling is to tag the cookie with the origin of the page
A. Experimental overview setting the cookie. This information should be passed to the
1) Choosing the sites for analysis: Instead of randomly server to allow the server to differentiate between duplicate
crawling the web and looking for unsafe features, we de- cookies.
cided to focus our attention on the “interesting” parts of the Section II-C1 also identified inconsistencies pertaining
web that people tend to visit often. Accordingly, to seed our to cookies and HTTP/HTTPS, which we now support
analysis, we take the set of 100,000 most popular web sites with measurements. First, 0.07% of sites alarmingly send
ranked by Alexa [5], as seen on November 9, 2009, as our secure cookies over HTTP. This effectively tampers with
representative data set. The data collection and analysis were the integrity of cookies that may have been intended for
completed in the last week of February 2010. HTTPS sessions [10]. Fortunately, it appears that this func-
2) Defining the compatibility cost: We define the cost of tionality can be disallowed with little cost. Surprisingly, a
removing a feature to be the number of Alexa-ranked, top much larger number of sites (5.48%) sent HTTP cookies
100,000 sites that use the feature. over HTTPS. The HTTP cookies cannot be kept confidential
We conservatively assume that disallowing a feature will and are accessible to HTTP sessions. Our recommended
significantly hinder a site’s functionality, whereas it could solution to this problem is that the “secure” flag should be
simply cause a visual nuisance. A more detailed analysis on enforced for any cookies passed over an HTTPS connection
the effect of policy changes on page behavior is promising even if the web developer fails to set the flag. This would
but is left as future work. still enable the HTTPS site to access the cookie for its own
3) High-level results: We obtained our results by ren- functionality and any sharing with the HTTP site should be
dering each of the 100,000 seed links using WebAnalyzer, done explicitly.
saving all interposition logs for offline analysis. This way, We found a large number of sites (16.2%) using HttpOnly
we were able to obtain data for 89,222 of the 100,000 sites. cookies, which is an encouraging sign — many sites appear
There are several reasons why no data was produced for the to be tightening up their cookie usage to better resist XSS
rest of sites. First, some sites could not be accessed at the attacks.
time of our analysis due to failed DNS lookups, “404 Not 2) Cookies and XMLHttpRequest: Our measurements
Found” errors, and other similar access problems. Second, show that the issues arising from undesirable interplay of
some sites timed out within our chosen threshold interval of XMLHttpRequest and HttpOnly cookies (Section II-C2)
2 minutes, due to their slow or continuous rendering. We can possibly be eliminated, since very few sites (0.30%)
decided to drop any such sites from our analysis. Finally, manipulate cookie headers in XMLHttpRequest responses.
some sites did not contain any JavaScript code, and as a 3) DOM and Display: Section II-C3 argued that the
result they did not trigger our event filters. Nonetheless, we descendant navigation policy is at conflict with SOP for
believe that we have been able to analyze a sufficiently large DOM. We observe iframe navigations on 7.7% of sites and
set of sites with a reasonable success ratio, and our data set all of them are child navigation (regardless of the origin).
and the scope of measurement is much larger than that used The absence of descendant navigation in the top 100,000
by earlier related studies [24]. sites indicates a potentially very low cost to remove it.
Total instances Unique sites
Measurement Criteria
(count) Count Percentage
document.cookie (read) 5656310 72587 81.36%
document.cookie (write) 2313359 68230 76.47%
document.cookie domain usage (read) 2032522 59631 66.83%
document.cookie domain usage (write) 1226800 41327 46.32%
Secure cookies over HTTP 259 62 0.07%
Non-secure cookies over HTTPS 15589 4893 5.48%
Use of “HttpOnly” cookies 33180 14474 16.22%
Frequency of duplicate cookies 159755 4955 5.55%
Use of XMLHttpRequest 19717 4631 5.2%
Cookie read in response of XMLHttpRequest 1261 265 0.30%
Cross-origin descendant navigation (reading descendant’s location) 6043 61 0.07%
Cross-origin descendant navigation (changing descendant’s location) 0 0 0.00%
Child navigation (parent navigating direct child) 22572 6874 7.7%
document.domain (read) 1253274 63602 71.29%
document.domain (write) 8640 1693 1.90%
Use of cookies after change of effective domain 295960 1569 1.76%
Use of XMLHttpRequest after change of effective domain 225 87 0.10%
Use of postMessage after change of effective domain 0 0 0.00%
Use of localStorage after change of effective domain 42 10 0.01%
Use of local storage 1227 169 0.19%
Use of session storage 0 0 0.00%
Use of fragment identifier for communication 5192 3386 3.80%
Use of postMessage 6523 845 0.95%
Use of postMessage (with no specified target) 0 0 0.00%
Use of XDomainRequest 527 125 0.14%
Presence of JavaScript within CSS 224266 4508 5.05%
Table IV
U SAGE OF VARIOUS BROWSER FEATURES ON POPULAR WEB SITES (F EBRUARY 2010). A NALYSIS INCLUDES 89,222 SITES .
Sites containing at least one