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# Same-Origin Policy: Evaluation in Modern Browsers

**Same-Origin Policy: Evaluation in Modern Browsers** - Jörg Schwenk, Marcus Niemietz, Christian Mainka, usenix.org.

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Same-Origin Policy: Evaluation
                      in Modern Browsers
Jörg Schwenk, Marcus Niemietz, and Christian Mainka, Horst Görtz Institute for IT Security,
             Chair for Network and Data Security, Ruhr-University Bochum
     https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/schwenk




              This paper is included in the Proceedings of the
                     26th USENIX Security Symposium
                        August 16–18, 2017 • Vancouver, BC, Canada
                                      ISBN 978-1-931971-40-9




                                                   Open access to the Proceedings of the
                                                    26th USENIX Security Symposium
                                                         is sponsored by USENIX
                  Same-Origin Policy: Evaluation in Modern Browsers

                      Jörg Schwenk, Marcus Niemietz, and Christian Mainka
              Horst Görtz Institute for IT Security, Chair for Network and Data Security
                                        Ruhr-University Bochum



Abstract                                                     (e.g., [15, 16, 5]). Therefore, recurrent browser bugs en-
                                                             abling SOP bypasses are not surprising.
The term Same-Origin Policy (SOP) is used to denote a            SOP rules can roughly be classified according to the
complex set of rules which governs the interaction of dif-   problem areas which they were designed to solve (cf. Ta-
ferent Web Origins within a web application. A subset of     ble 1). It is impossible to cover all these subsets in a sin-
these SOP rules controls the interaction between the host    gle research paper and even may be impossible to find a
document and an embedded document, and this subset           “unifying formula” which covers all subsets.1 However,
is the target of our research (SOP-DOM). In contrast to      it is possible to cover single subsets, as previous work on
other important concepts like Web Origins (RFC 6454)         HTTP cookies has shown [12]. Thus, we restricted our
or the Document Object Model (DOM), there is no for-         attention to the following research questions:
mal specification of the SOP-DOM.
   In an empirical study, we ran 544 different test cases      I How is SOP for DOM access (SOP-DOM) imple-
on each of the 10 major web browsers. We show that in            mented in modern browsers?
addition to Web Origins, access rights granted by SOP-
DOM depend on at least three attributes: the type of the       I Which parts of the HTML markup influences SOP-
embedding element (EE), the sandbox, and CORS at-                DOM?
tributes. We also show that due to the lack of a formal
                                                               I How does the detected behavior match known ac-
specification, different browser behaviors could be de-
                                                                 cess control policies?
tected in approximately 23% of our test cases. The is-
sues discovered in Internet Explorer and Edge are also          More precisely, we concentrate on a subset of SOP
acknowledged by Microsoft (MSRC Case 32703). We              rules according to the following criteria:
discuss our findings in terms of read, write, and execute
rights in different access control models.                     I Web Origins. We use RFC 6454 as a foundation.
                                                               I Browser Interactions. We concentrate on the inter-
1   Introduction                                                 action of web objects once they have been loaded.

The Same-Origin Policy (SOP) is perhaps the most im-         It is a difficult task to select a test set for SOP-DOM that
portant security mechanism for protecting web applica-       has constantly evolved over nearly two decades. The
tions, and receives high attention from developers and       SOP-DOM has been adapted several times to include
browser vendors.                                             new features (e.g., CORS) and to prevent new attacks.
                                                             15 out of 142 HTML elements have a URL attribute and
                                                             may thus have a different Web Origin [17]. Additionally,
Complex Set of SOP Rules. Today there is no for-
                                                             sandbox and CORS attributes also modify SOP-DOM.
mal definition of the SOP itself. Web Origins as de-
scribed in RFC 6454 are the basis for the SOP, but they
do not formally define the SOP. Documentation pro-           The Need for Testing. Amongst web security re-
vided by standardization bodies [1] or browser vendors       searchers, SOP-DOM is partially common knowledge,
[2] is still incomplete. Our evaluation of related work      but not thoroughly documented. Although this means
has shown that the SOP does not have a consistent de-           1 For example, the SOP rules for DOM access and HTTP cookies

scription – both in the academic and non-academic world      are inconsistent, because their concept of “origin” differs.




USENIX Association                                                               26th USENIX Security Symposium             713
 SOP Subset          Description                                                                                           Related Work
 DOM        access   This subset describes if JavaScript code loaded into one “execution context” may access               [1], [2], [3],
 (this paper)        web objects in another “execution context”. This includes modifications of the standard               [4], [5] , [6]
                     behavior by changing the Web Origin, for example, using document.domain.
 Local     storage   This subset defines which locally stored web object ([name,value] pairs) may be accessed              [7], [8]
 and       session   from a JavaScript execution context.
 storage
 XMLHttpRequest This subset imposes restrictions on cross-origin HTTP network access. It contains many                     [9], [7],   [8],
                   ad-hoc rules and its main concepts have been standardized in CORS.                                      [10]
 Pseudo-           Browsers may use Pseudo-protocols like about:, javascript: and data: to de-                             [8], [10]
 protocols         note locally generated content. A complex set of rules applies for the definition of Web
                   Origins here.
 Plugins           Many plugins like Java, Flash, Silverlight, PDF come with their own variants of a SOP.                  [11], [8]
 Window/Tab        Cross-window communication functions and properties: window.opener, open()                              [8], [10]
                   and showModalDialogue().
 HTTP Cookies      This subset, with an extension of the Web Origin concept (path), defines to which URLs                  [12], [13], [14]
                   HTTP cookies will be sent. This defines their accessibility in the DOM for non-httpOnly
                   cookies.

                                         Table 1: Different subsets of SOP rules.


that most researches are familiar with many edge cases in                           Host Document (HD)
SOP-DOM, especially those relating to attacks and coun-                                      Web Origin HD
termeasures, it is likely that some of those edge cases will                                                           Embedding
not be covered in this paper. Additionally, each individ-                                                             Element (EE)
ual researcher will be unaware of other edge cases, which                                                           Web Origin ED
may include novel vulnerabilities. For example, it is well                                         SOP               Embedded
known that JavaScript code from a different web origin                                                             Document (ED)
                                                                                                  read?
has full read and write access to the host document; nev-                        Subject:                               Web
                                                                                JavaScript        write?
                                                                                                                       Object
ertheless, recently Lekies et al. [5] pointed our that there
is also read access from the host document to JavaScript                                          read?
                                                                        Web                                          Subject:
                                                                                                  write?
code, which may constitute a privacy problem.                          Object                                       JavaScript
                                                                                         allow script execution?
   Additionally, HTML5 has brought greater diversity to
seemingly well-known HTML elements. For instance,                                                                  {ee,sandbox,cors}

the term “authority” used in RFC 6454 [18] may not be
sufficient any more if we compare the power of SVG im-            Figure 1: Setup for our test cases for SOP DOM access.
ages [19] with the following quote from RFC 6454: “an             The embedding element (EE) itself belongs to the host
image is passive content and, therefore, carries no au-           document (HD).
thority, meaning the image has no access to the objects
and resources available to its origin”. Our evaluation
shows that this statement is true for all image types if          imposed by CSS code as write access on certain DOM
they are embedded via <img>. This statement does not              elements.
hold if SVG images are embedded via <iframe> or <
object>. Novel standards like Cross-Origin Resource                  We define “comprehensive” by meaning the coverage
Sharing (CORS, [9]) also influence access rights granted          of all interesting edge cases. We thus do not cover all 15
by the SOP. To be able to keep the implementation of              elements with URI attributes but only a selected subset
the SOP consistent through all these extensions, a formal         according to importance and interesting properties. In-
model is needed.                                                  stead, we include “URL-attribute-like” constructions in
                                                                  the <canvas> element. We also do not restrict the test
                                                                  set to full DOM read or write access (which easily could
Our Approach. The aim of this paper is to develop                 have been automated to cover more test cases) but in-
a comprehensive testing framework for SOP-DOM (see                stead, also concentrate on the more interesting cases of
Figure 1). The SOP restricts access of active content like        partial read and write access.
JavaScript on other components of a web page. We also                Our tests thus cover only a representative sample of
apply it to CSS code by interpreting the style changes            SOP-DOM, but this sample was chosen to cover each



714   26th USENIX Security Symposium                                                                               USENIX Association
known edge case of SOP-DOM. To cover these edge                       More than 23% of the test cases revealed differ-
cases, many of the 544 test cases were designed man-                  ent SOP-DOM access rights implemented in at least
ually. We use these representative test results to dis-               one of the tested browsers. Our ABAC model pro-
cuss if classical access control models like DAC, RBAC                vides a systematic way to describe these differ-
and ABAC are applicable to SOP-DOM. We reformulate                    ences.
access restrictions in terms of read, write, and execute
rights granted to an embedded document (ED) contained            I We prove that a better understanding of SOP-DOM
in the HD and vice versa. We thus highlight the impor-             is useful by describing a novel CSS-based login or-
tance of the EE in defining the access rules of the SOP.           acle attack for IE and Edge, which we found using
                                                                   the ABAC rules for cross-origin access to CSS.
Testbed. We show the applicability of our test method-
                                                                 I We critically discuss the applicability of standard
ology for SOP implementations in current web browsers
                                                                   access control models like DAC, RBAC, and ABAC
by providing a testbed at www.your-sop.com, where
                                                                   to SOP-DOM.
proof-of-concept HTML, JavaScript, and CSS code is
given for each test case. Our tool consists of more than
10,000 lines of code covering 544 test cases with five       2       Foundations
types of ED and ten types of EE. The tests are created
in a semi-automatic manner. For each EE to be tested,        Document Object Model (DOM). DOM is the stan-
we automatically load the ED with possible CORS/sand-        dardized application programming interface (API) for
box attributes successively. We did not choose a fully-      scripts running in a browser to interact with the HTML
automatic test creation because this would lead to an        document. It defines the “environment” in which a script
overwhelming number of errors. Combining each EE             operates. The first standard (DOM Level 1) was pub-
with all possible attributes would lead to errors; for       lished in 1998 and the latest published version is DOM
example, neither <img data=".."> nor <object                 Level 3 (2004). The DOM standard is now a “living stan-
  src="..."> are semantically correct. In addition,          dard” since it has to be adapted to each new HTML5 fea-
there is no universal access from HD to ED and vice versa;   ture, resulting the DOM Level 4 to remain in the “work
for example, accessing the SVG ED can be achieved with       in progress” stage.2
a dedicated getSVGDocument() method.                            A browser’s DOM includes more objects and proper-
                                                             ties than just the pure HTML markup, as shown in Fig-
Limitations. We describe a subset of the SOP for             ure 2. These objects can be accessed through a variety
the interaction of web objects that are loaded into the      of different methods. For example, the iFrame element
browser. Zalweski describes other contexts such as           can be accessed through predefined selector methods
cookie, local storage, Flash, XMLHttpRequest, Java, Sil-     like document.getElementByID("ID1"). The
verlight, and Gears [8]. For each of them a different SOP    DOM structure does not necessarily match the markup
is used. For example, Zheng et al. [12] have analyzed        structure. Although the <iframe> element from Fig-
the SOP for HTTP cookies in-depth; here the SOP takes        ure 2 is a child element of the HTML document, there is
the path contained in an URI into account, which is an       no property document.frames[0]; instead, there is
extension of the Web Origin concept. An in-depth dis-        only window.frames[0].
cussion of the limitations of our approach can be found
in Section 5.
                                                                                                                    window
                                                                            e.g.,	
  main	
  HTML	
  document	
                     e.g.,	
  iFrame	
  

                                                                  doctype                   document
Contributions. We make the following contributions:                HTML	
  5	
                <html>	
  

                                                                                         head                body
 I We systematically test edge cases of the SOP that                                    <head>	
         <body>	
  
                                                                                                                                                           window.
                                                                                                                                                          frames[0]
   have not been previously documented like the influ-                                                            <iframe	
  src="URL2"	
  
                                                                                                                       id="ID1">	
                         id=ID1
   ence of the embedding element, and the CORS and                                 <script	
  src="URL1">	
  

   sandbox attributes.                                           <img	
  src="URL3"	
  name="bear">	
                             document                        doctype
                                                                                                                                    <html>	
                       XHTML	
  
                                                                  img.src=URL3	
  
 I We provide a testbed where the SOP implementa-                                   <link	
  src="URL4">	
  
                                                                                                                             head                     body
                                                                                                                            <head>	
                 <body>	
  
   tion of a browser can be automatically tested and
   visualized.
                                                                             Figure 2: Small extract from the DOM.
 I We used this testbed to extensively evaluate our
   model in 544 test cases on 10 modern browsers.                2 https://dom.spec.whatwg.org/




USENIX Association                                                                              26th USENIX Security Symposium                                            715
       To access and modify the DOM, JavaScript code can                    images, iFrames) that can be loaded without CORS
    be used. Each JavaScript script runs in a specific DOM                  and without XMLHttpRequest. However, in HTML5
    execution context. Consider Listing 1 as an example.                    some elements (e.g., <img>) may have crossorigin
    If this small HTML file is opened in a web browser,                     attributes which invoke CORS and subsequently modify
    first the <iframe> element will be parsed. After that                   the SOP access controls.
    the iFrame’s source code from Listing 2 will be loaded
    and the alert function contained therein will be exe-
    cuted. The <script> element will then be parsed and                     3     Methodology
    the (second) alert function will be executed.
                                                                            3.1    SOP-DOM Attributes
1 <html><head><title>a.html</title></head>
2 <body><iframe src="b.html" />                                             The Same-Origin Policy for DOM Access (SOP-DOM)
3 <script>alert(document.location)</script>                                 controls the access of a subject – typically JavaScript
4 </body></html>
                                                                            code – to a web object (e.g., an HTML form). The sub-
          Listing 1: Code of http://a.org/a.html                            ject may be located directly in the HD or in an ED. The
                                                                            element that loads the ED is called the EE (cf. Figure 1).
       The two alert pop-up windows, triggered by                           Both HD and ED have a Web Origin. The Web Origin
    the two script elements, will display different                         of ED is defined by src or similar attributes of EE (e.g,
    URLs because they are acting in different DOMs.                         dynsrc, lowsrc, and srcset).
    The alert window called in Listing 1 will display                          SOP-DOM is often described as a boolean switch
    the URL http://a.org/a.html, whereas the                                which either allows interaction between HD and ED in
    alert window in Listing 2 will display the URL                          the same-origin case or blocks access in case of different
    http://a.org/b.html.                                                    web origins (e.g., Karlof et al. [15]). In reality, SOP-
1 <html><head><title>b.html</title></head>                                  DOM is more complex; some EEs like <img> block
2 <body><script>alert(document.location)</                                  almost all access even in the same-origin case, some
      script>                                                               EEs like <script> allow full read and write access
3 </body></html>
                                                                            (in one direction) even in the case of different origins,
          Listing 2: Code of http://a.org/b.html                            and some EEs like <iframe> (in the cross-origin case)
                                                                            only grant partial access. Furthermore, access decisions
                                                                            may be influenced by additional attributes like CORS or
    Cross-Origin Resource Sharing (CORS). Using                             sandbox.
    XMLHttpRequest, a web page may send arbitrary HTTP                         In our investigations, we have used five values as our
    requests to any webserver. This is different from just                  test attributes, two of which contribute to the definition
    opening an URL or submitting an HTML form since with                    of Web Origin. These attributes are summarized in Ta-
    XMLHttpRequest the web page has full control over all                   ble 2.
    HTTP headers. To restrict such potentially dangerous
    queries, XMLHttpRequest is restricted by default to the
    domain from which the calling document was loaded                       Notation. In this paper and in our testbed, we use HD
    (same-domain). To enable controlled cross-domain                        and ED to denote that HD and ED share the same Web
    requests, the CORS standard [9] was developed. It                       Origin, and HD and ED if the origin differs. If cross-
    works as follows: a) in a preflight request,3 the browser               origin and same-origin behavior are identical we, use HD
    sends an origin header (Origin: http://a.com)                           and ED to save space.
    to the target web service requesting CORS privileges.
    b) the target server may now answer with an error                       Coverage and Restrictions. The SOP-DOM is very
    message (access denied) or with a CORS header,                          complex, because with each newly considered attribute,
    such     as    Access-Control-Allow-Origin:                             the number of test cases may grow by a factor propor-
    http://a.com, to grant the access. Instead of a                         tional to the number of possible attribute values. Thus,
    domain name, the CORS header may contain a wildcard                     it should be clear that it is nearly impossible to test and
    (*) to grants access from arbitrary domains.4 Although                  describe the whole SOP-DOM in one research paper.
    CORS is designed to relax the Same-Origin Policy                           Since Web Origins are well understood and have
    (SOP) in a secure manner, there are many cross-origin                   been covered in numerous other publications, we have
    resources used in the web (e.g., scripts, stylesheets,                  only covered two different origins with the same
      3 The preflight request can be skipped in simple cases                protocol (HTTP) and two different domains with dif-
      4 This additionally denies the use credentials such as cookies in a   ferent domain values. Our focus is on ee, where
    CORS request.                                                           we considered HTML elements with URI attributes and



    716    26th USENIX Security Symposium                                                                        USENIX Association
 Attribute      Description                S/O/E   HD/ED          Full Read and Full Write Access. Supposing that
 protocol       protocol of URL,           S,O     HD+ED          JavaScript code has DOM read access, it typically
                value of                                          also has write access using some DOM methods (e.g.,
                location.                                         innerHTML). We have tested this by first writing into a
                protocol                                          particular DOM property, and then by reading the same
 domain         domain/hostname of         S,O     HD+ED          property to verify whether it contains the newly written
                URL, value of
                                                                  value. For full DOM access, we successfully verified that
                location.
                hostname
                                                                  any DOM property which can be read, can also be writ-
 ee             type of EE                 S,O      ED            ten. In our proof-of-concept implementation, a script
 cors           value of the CORS at-       O       ED            contained in the ED tries to read DOM properties from
                tribute of the ee, i.e.,                          HD and vice versa. To test full DOM access, we inter
                ee.crossOrigin                                    alia use the code depicted in Listings 3 and 4.
 sandbox        value of sandbox           S,O,E    ED        1  <html>
                                                              2  <head>HD from HD.org</head>
Table 2: SOP-DOM Attributes. S denotes subject at-             3 <body>

tributes, O object attributes, and E denotes attributes        4 <script>

which may also be set independent of the markup (e.g.,         5 ED=document.getElementById("EE").

through a HTTP security policy like Content Security                 contentDocument;
                                                               6 HD2ED=ED.getElementById("ID2");
Policy (CSP)).
                                                               7 read_success = (HD2ED.textContent == "

                                                                     Text in ED");
                                                               8 </script>
properties. By systematically analyzing the provided
                                                               9 <element id="ID1">Text in HD</element>
list of the W3C [20] and the WHATWG [21], we                  10 <EE id="EE" src="ED.org/ED.mime"></EE>
picked the representative HTML elements <script>,             11 </body>
<img>, <canvas>, <link>, <iframe>, <object                    12 </html>
>, <embed>, and <link>. We have also examined
                                                                  Listing 3: Host document (HD) verifying full read
CORS (the value of the crossorigin attribute) and
                                                                  access.
sandbox, as a proof-of-concept, to show that these at-
tributes do have an influence on the SOP-DOM. More
limitations of our approach are discussed in Section 5.       1  <html>
                                                              2  <head>ED from ED.org</head>
                                                               3 <body>

3.2    Access Control Test Cases                               4 <ED><element id="ID2">Text in ED</element

                                                                     ></ED>
Web Object Structure. Web objects may have an in-              5 <script>

ternal DOM structure, as it is the case with iFrames or        6 var ED2HD;

                                                               7 ED2HD=parent.getElementById("ID1");
SVG images. In this case, we can use standard DOM
                                                               8 read_success = (ED2HD.textContent == "
selector methods to test for read and write access.
                                                                     Text in HD");
   Other web objects do not have a DOM structure (e.g.,
                                                               9 </script>
JPEG and PNG images). In this case, we define the type        10 </body>
of access for each such web object separately (e.g., single   11 </html>
pixel access for JPEG) and use adapted code examples.
                                                                  Listing 4: Embedded Document (ED) for verifying full
                                                                  read access.
Distinguishing Full and Partial Access. In case that
the object has an internal DOM structure, we define
full access if we can access arbitrary parts of the DOM           Partial Access. Many partial access rules have been
by standard selectors like getElementbyID(). We                   added to browser implementations over the years in order
define partial access as only being able to read, or              to implement new features, or to defend against new at-
only being able to write some specific properties (e.g.,          tacks. The best-known examples are certainly the DOM
window.top.location).                                             properties of an iFrame’s top frame that are used to build
   If the web object does not have an internal DOM, we            JavaScript framebusters to defend against UI Redress-
always specify exactly what we can read or write. To              ing [22].
name one example, single pixels in images or the source              Partial access cannot be tested systematically. Instead,
code of scripts.                                                  we relied on our knowledge from pentesting, blog posts



USENIX Association                                                                26th USENIX Security Symposium         717
of security researchers, and – in some cases – on intu-        <iframe> (21; 261; 1,406). To name an example, the
ition. Please note that our goal was not to give a full list   script-element was the third most common element
of partial access rules, but only to document the variety      listed on 453 out of 500 domains with a total of 12,625
of such rules.                                                 findings. The elements <object> or <embed> are not
                                                               listed under the TOP-30 elements.
Partial Read: Examples. An example for partial                    Our testbed executes all tests on a single website so
read (and write) access is the pixel-based manipula-           that tests can be easily repeated with different browsers.
tion of images with the help of CANVAS (e.g., via              It uses one of the previously mentioned EEs and loads
context.getImageData).                                         an external ED via its dedicated attributes. For ex-
   Lekies et al. [5] underlined that every script executed     ample, the <img> elements uses the src attribute;
within the same web document is able to read global vari-      however, the <object> elements uses the data at-
ables created by another script. However, local variables      tribute. If the element supports CORS, we created a
inside a function cannot be accessed unless their values       test as follows; we used the three attribute cases, (1.) no
are not explicitly returned by the function. This illus-       crossorigin attribute is set, (2.) crossorigin
trates clearly that we have partial read access.               ="use-credentials", and (3.) crossorigin=
   As an edge case example for partial read access, CSS        "anonymous". For each attribute, we created a
in combination with browser features like plain HTML           test that receives an HTTP response header Access
and inactive SVG files can be used to extract some values      -Control-Cross-Origin (1.) set to a specific
from the SOP-DOM [23].                                         domain your-sop.com or other-domain.org,
                                                               (2.) set to the wildcard *, (3.) or not set at all. In ad-
                                                               dition, the HTTP response header Use-Credentials
Partial Write: Examples. Partially writable are prop-
                                                               is once set for each test to (1.) to yes, to (2.) no, (3.) and
erties like parent.location.path and parent.
                                                               not set. The immense number of combinations lead to a
location.hash. In the past location.hash was
                                                               significant number of test cases if CORS is supported.
used to share data cross-origin. Nowadays, this feature
                                                                  Each test loads an external resource (ED), first from
can be replaced by using PostMessage or CORS and
                                                               the same domain (your-sop.com), and then from a
write access to parent.location can be restricted
                                                               different one (other-domain.org). When retrieved
in iFrames by using the sandbox attribute.
                                                               through any browser, the SOP decisions of the currently
                                                               used browser are presented in different overview tables.
Execute. Current sandboxing concepts consider block-           Since the exact method to access specific objects from
ing JavaScript execution but not CSS execution. To be          ED to HD – and vice versa – differs with each test, its
consistent with this view, we say that an EE grants ex-        source code can be inspected by hovering on the result
ecute rights to an ED when JavaScript code contained           field in the table on the testbed website (cf. Figure 3).
in the ED can be executed. For example, when EE=<
iframe sandbox>, then the execution of JavaScript
is blocked. We verified this by using script execution to
send a PostMessageAPI message to HD.


4     Evaluation
We implemented a testbed as a web application which
automatically evaluates the SOP implementation of the
currently used browsers. Additionally, it displays the re-
sults of 10 tested browsers from six different vendors and
highlights the differences between them. Our testbed is
publicly available at www.your-sop.com.

4.1    Experiment Setup
We evaluated the following elements with src attributes
and determined their Alexa 500 rank through an analysis
of the Alexa Top–500 start pages. The results are (rank;        Figure 3: Screenshot of our your-sop.com testbed.
domains; occurrences): <script> (3; 460; 12,625), <
link> (8; 453; 5,197), <img> (11; 439; 24,015), and              Using the testbed, we evaluated the SOP of ten differ-



718    26th USENIX Security Symposium                                                                 USENIX Association
ent browsers, including Google Chrome, Mozilla Fire-              For the read/write access from the HD (subject) to the
fox, Internet Explorer, Edge and Safari. We added a fea-       ED (JavaScript, object), this is less well-known. It is
ture to export all test results in a JSON file. We then        clear that we cannot change the content of the external
used this feature to add a comparison table of different       file (write), but we can overwrite functions defined in
browser behaviors. It displays all test cases and SOP de-      this external file, and thus change the functionality of the
cisions of all browsers at once or can only highlight the      loaded code. We are able to read variable values and the
differences. Figure 4 shows a small part of the compari-       source code of defined functions7 . However, there are
son of different SOP implementations.                          some exceptions: we cannot read var cnt = 2+5;
                                                               but we can read the cnt’s value 7. We can also read
                                                               the complete line of code if it is contained in a function
4.2    Results
                                                               (cf. [5]). Thus, we have partial read/write access from
In the following, we describe the general outcome of our       the HD to the ED.
testbed. The results are structured by the type of the em-
bedding element (EE).                                          Style Sheets. External CSS code can be loaded via the
                                                               embedding element <link>. In the case where the CSS
Images. An <img> element acts like a sandboxed                 code is loaded from the same origin, we can read the
iFrame; read and write access is blocked in both direc-        complete source code. If the CSS file is loaded cross-
tions, even in the same-origin case. Script execution is       origin, we can only read the source code if proper CORS
blocked in the ED; even if the ED is an SVG containing         values are set. An exception is MS IE/Edge, which al-
some JavaScript code, the script is not executed. This         lows read access in every case (see Section 4.3 for de-
behavior holds for both the same-origin and cross-origin       tails).
case.                                                             Write access for CSS code is defined by the ability of
   If we use <canvas> as the embedding element EE5 ,           CSS to change the visual display of a web object. Since
we can get read access to pixels in JPG, PNG and SVG           this is the desired behavior, write access from the ED to
images if loaded from the same origin. This allows             the HD is independent of the web origin.
reading out the color of each pixel and it may be crit-
ical in some security contexts like JPG- or PNG-based          Frames. For <iframe> (without sandox attribute)
CAPTCHAs. Here, an attacker could use CANVAS to                we have full read/write access in both directions in the
automatically read out the displayed token.6                   same-origin case, and partial read/write access in the
   SVG files are basically XML-based vector graphics.          cross-origin case.
Please note, that unlike <img>, the <svg> element does            The cross-origin case from ED (subject) to HD (ob-
not support a src attribute to load an external SVG file.      ject) is of special interest; we have partial read/write
If embedded into a website with <img> or <canvas>,             access. Some properties that can be read are: top
they behave as if they were bitmaps; thus, we can only         .length (number of frames/iFrames in HD), top.
read pixels. It is also possible to include SVGs in EEs        closed (boolean value if HD is closed), top.opener
like <iframe>, <object>, and <embed>. Then the                 (reference to opener HD in the event of a popup). Al-
DOM of the SVG is mounted into the HD and we can               though this is a very limited read access, we have a side-
access it fully, and additionally read all SVG vector in-      channel allowing us to read some cross-origin informa-
structions.                                                    tion. Especially the first property is noteworthy; it allows
                                                               to get the number of frames/iFrames that are contained in
Scripts. Cross-origin loaded JavaScript code via <             the HD. We also have partial write access in this case; for
script src="..."> is a well-known special case in              example, to the top.location property (a property
the SOP; it is treated as if it had been loaded from the       that we can only write, but are unable to read).
same origin. Technically, a script loaded by the src at-          Similar results hold for the other direction (subject HD
tribute is appended to the document.scripts array              to object ED) in the cross origin case. In this case, the
in the HD’s DOM, independent of the domain on which            properties are accessed via the window.frames[] ar-
the script is hosted. In the <script> case, no access          ray (instead of top).
restrictions are imposed by the SOP: we have full read-
/write access from the ED to the HD, and execution rights
                                                               Sandboxed Frames. The origins of the SOP-DOM lie
from HD to ED.
                                                               in the necessity of a clear separation of two HTML doc-
  5 See the example on https://developer.mozilla.org/
                                                               uments, shown by several attacks over the last ten years
en-US/docs/Web/API/Canvas_API/Tutorial/Pixel_
manipulation_with_canvas                                          7 For example,
                                                                               by using Object.getOwnPropertyNames(
  6 http://ejohn.org/blog/ocr-and-neural-nets-in-javascript/   window), we can read all properties defined in the window object




USENIX Association                                                                 26th USENIX Security Symposium         719
                           Figure 4: Evaluation result by comparing 10 different browsers.


[24, 25, 26]. However, a complete separation between         cross-origin, FF, IE, and Edge do not allow read ac-
two HTML documents is often not possible; for example,       cess in the following CORS cases of <canvas> with
to allow UI redressing countermeasures with JavaScript       SVG and PNG: Access-Control-Allow-Origin
frame-busters [22].                                          : your-sop.com (ED sets the domain of HD) and
   To allow a better separation between the iFrame ED        Use-Credentials: true. Irrespective of CORS,
and the HD, sandboxed iframes were introduced [27].          <canvas> and SVG have 44 differences that are based
We limited our evaluation to the attribute values            on a denied access in IE 11.9
that directly affect our read, write, and execute re-           Second, over 12% of the test cases show differences
sults: allow-scripts, allow-same-origin,                     between Safari 9 and the other browsers by looking on <
allow-top-navigation.                                        object> and <embed> elements that load SVG files.
   The sandbox attribute is a special case that is dis-      Safari 9 does not show an SVG if it is loaded by code
cussed in Section 7.                                         like <object data="image.svg"></object>.
                                                             Therefore, JavaScript code contained in the SVG file
Recommendations for Browser Vendors. From the                cannot be executed. It needs an additional type attribute
perspective of a browser vendor, it is interesting to know   with the value image/svg+xml such that JavaScript
how the results of our tool can be used to identify bugs     execution is allowed. Since Safari 10.1 Apple has
and therefore potential vulnerabilities. In our analysis,    changed their implementation and both elements behave
we have automatically compared each SOP-DOM differ-          similar to the other browsers. The attribute type="
ence with the behavior of all other browsers. In case that   image/svg+xml" is no more required.
at least one browser grants SOP-DOM access that the             Third, over 51% of the test cases show different behav-
other browsers restrict, a browser vendor should have a      iors because of <link>. Nearly all the cases have dif-
closer look on this test case. We recommend to adjust        ferent CORS implementations. CORS thus shows that a
the SOP-DOM behavior to the majority of other browser        relatively new and complex technology leads to different
behaviors for reasons of clarity. For each test, our web-    interpretations of “well-known” web concepts like SOP.
site recommends a result, which is based on the major-          Similarly to Chromium’s testbed that have been ap-
ity of all ten tested browsers (see Figure 4). Because       plied to other browsers to find bugs, our testbed could
our testbed includes browsers of different vendors (e.g.,    be used and extended by browser vendors and security
Apple, Google, Mozilla, Microsoft), we believe that this     researchers to identify browser differences leading to ex-
might be a representative SOP-DOM result.                    ploits.10


4.3   Different Browser Behaviors                            4.4     Cross-Origin Login Oracle Attack.
We implemented 544 test cases and 129 of these cases
                                                             We have detected one browser difference due to IE/Edge,
differ across ten tested browsers (23.71%).8 We identi-
                                                             which does not need CORS. In this case, IE/Edge allows
fied three subsets of different browser behaviors.
                                                             us to read CSS rules cross-origin while other browsers
   First, more than 35% of the identified differences
                                                             do not allow such access.
could be attributed to <canvas> and PNG/SVG. In
contrast to the other seven browser tests that allow par-        9 We have communicated these differences to Microsoft and it seems

tial read access with the help CORS from HD to ED            that they have fixed them in the newest browser versions.
                                                               10 https://github.com/thomaspatzke/
  8 http://www.your-sop.com/stats.php                        BrowserCrasher




720   26th USENIX Security Symposium                                                                     USENIX Association
   By using the difference that was detected in case of      we excluded from our research and give a rationale for
<link>, we show that dynamically generated CSS files         these decisions.
can be abused to attack the user’s privacy. In case of CSS
code from different origins, IE/Edge behaves differently
                                                             Link. One technical limitation of our evaluation frame-
from GC and FF; it does not set DOM properties like
                                                             work is that we used the <link> element only to
cssRules to null. Therefore, an attacker is always
                                                             load CSS. We did not consider, for example, HTML
allowed to read the CSS code regardless of its origin.
                                                             imports via <link rel="import"href="data.
This allows us to build a novel login oracle:
                                                             html">. An interesting novel technology that is
    I Suppose a webserver delivers different CSS files,      highly under development are Service Workers [29].
      depending on whether the user is logged in or not.     They can, for example, be loaded using <link rel=
                                                             "serviceworker"href="worker.js">. How-
    I The attacker’s website consists of the EE <link>       ever, it is currently “an experimental technology” ac-
      loading the victim’s CSS code (ED).                    cording to Mozilla [30], although they are used by many
                                                             websites (e.g., Google and Twitter). Our evaluation does
    I Though HD has another origin than ED, the at-          not cover Web Workers [31]. This technology allows
      tacker’s JavaScript code in HD automatically reads     running a JavaScript in different context; for example,
      all CSS rules. By comparing the CSS code with          there is no window object reference. For this reason,
      CSS code of a logged out user, the attacker can de-    we excluded it.
      termine the logged in state.

   We verified our login oracle with the startpage service   SVG. We only covered <svg> as an ED which directly
start.me (ED); an attacker is clearly able to decide         embeds the JavaScript code for testing read/write access.
whether a user is logged in or not. This attack is sim-      It is also possible, to use <svg> as a HD; for example,
ilar to [5]. We have informed the website administra-        an external JavaScript can be loaded by using <svg><
tors about this vulnerability. Microsoft (Research Center,   script xlink:href=".."></svg>. Our testbed
MSRC) acknowledged this bug (Case 32703) and the fix         always uses an HTML document as HD.
will be incorporated into a future version of IE/Edge.
                                                             JavaScript. We only cover a small, but hopefully rep-
5     Limitations                                            resentative, set of DOM properties. Our testbed only
                                                             covers the location property, but sub-properties such
Even if we restrict our attention to SOP-DOM, the Same-      as location.hash or location.path were not
Origin Policy has a very large scope. We have 15 HTML        analyzed. The same holds for the window.name prop-
elements with src attributes, and several more with a        erty, which is well-known to be writable across origins.
similar functionality (e.g. <canvas>). There are six            A design decision for our testbed was to be able to
different sandbox attributes, and they (e.g., the CORS at-   easily execute all test simultaneously. Therefore, only
tribute) may be influenced by HTTP-based security poli-      one index.html is capable to run all 544 tests with only
cies like CSP. There are many different ways how to em-      one click by the user. For this reason, we excluded pop-
bed a document of a given MIME type into a webpage           ups and the corresponding window.opener property.
(e.g., SVG via <img> or <iframe>), and there are
many different MIME types with and without a DOM
                                                             Other Mime Types. Our testbed is limited to HTML,
structure to consider. There are pseudoprotocols like
                                                             JavaScript, CSS, and SVG. For example, it would be
data: and about:, which have different Web Origin
                                                             interesting to investigate PDF, which can also include
definitions. There is also a large number of DOM prop-
                                                             JavaScript code. There are many more active MIME
erties which could be tested for partial access.
                                                             types, such as Flash or ActiveX, which should be ad-
   Covering all interactions within this scope would re-
                                                             dressed in further research.
sult in an exponential number of test cases, which can-
not be covered in one research paper. For example, Zal-
weski [28] lists four classes of common URL schemes          Pseudoprotocols. We excluded pseudo protocols (e.g.,
(e.g., document-fetching and third-party) consisting of      about:, chrome:) and Data and JavaScript-URIs
different subclasses (e.g., browser specific schemes like    from our tests, because in a (possibly outdated) overview,
vbscript, firefoxurl, and cf). Moreover, it is               Zalewski [28] already pointed out that there are different
possible to register self-defined handlers for particular    Web Origin assignments in different browser implemen-
schemes via registerProtocolHandler. In this                 tations. However, extending the testbed to selected pseu-
section, we therefore discuss several technologies that      doprotocols is future work.



USENIX Association                                                          26th USENIX Security Symposium         721
6     Related Work                                            be bypassed by adding whitespace characters to IP ad-
                                                              dress strings.11 In 2016, Ormandy [40] showed that Co-
Different SOP Contexts. Jackson and Barth [32] dis-           modo’s browser Chromodo disables, at least partially, the
cussed different SOP contexts, and showed vulnerabil-         SOP and thus Chromodo “actually disables all web secu-
ities introduced by the interaction of these contexts.        rity”. There are also SOP bypasses via Java applets [41],
Zheng et al. [12] describe in detail the SOP for HTTP         Adobe Reader [42], Adobe Flash [11], and inter alia Mi-
cookies. They also presented bypasses based on sub-           crosoft Silverlight [10].
domains. Session integrity problems resulting from the
cookie context SOP are discussed by Bortz et al. [13].
Karlof et al. [15] and Masone et al. [14] describe refined    Formal approaches to Web Security. Yang et al. [6]
origins for the cookie SOP: they replaced the domain          propose to describe the SOP in terms of Information
name with a server’s X.509 certificate and public keys.       Flow Control. Akhawe et al. [43] have a much broader
Thus, they are able to use different cookies for different    scope and describes the backbone of a formal model for
servers on the same domain. Singh et al. [7] analyzed         the Web itself.
in-coherencies in web browser access control policies by
showing that there are different definitions of Web Ori-      Other Approaches. Crites et al. [44] proposed the ab-
gins; there are web-origins for DOM objects, localStor-       straction and access control model OMash, as a replace-
age, and XMLHttpRequest, as well as other definitions         ment of SOP. Barth et al. [45] proposed a browser exten-
for cookies (domain, path) and the clipboard (user).          sion system for protecting browsers from extension vul-
                                                              nerabilities. They reused the SOP to isolate extensions
SOP Enhancements. Wang et al. [33] proposed their             from attacks, which needs inter alia access to browser
secure browser Gazelle with a multi-principal OS ar-          internals and web page data. Chen et al. [46] described
chitecture and showed how to implement extended ac-           an opt-in app isolation mechanism that acts like the user
cess control policies. Chen et al. [34] analyzed browser      is executing different browsers. Even if the attacker is
domain-isolation bugs and attacks. They proposed              able to act in the same origin, the users credentials might
“script accenting” as a defense mechanism so that frames      only be available in a logged-in state which is isolated.
cannot communicate if they have different accents.            Stamm et al. [47] proposed CSP, which is implemented
                                                              in all modern browsers. In CSP, code injection attacks
SOP Bypasses. Ways to bypass SOP restrictions are             are mitigated through restrictions imposed on code ori-
regularly published in the academic and non-academic          gins (whitelisting of allowed origins), and through aban-
areas. Jackson et al. [35] and Johns et al. [36] dis-         doning inline code. Jackson and Wang [48] introduced
cuss DNS rebinding attacks (which manipulate Web Ori-         Subspace as a cross-domain communication primitive al-
gins and thus disable the SOP) and proposed mitiga-           lowing communication across domains.
tion techniques. Oren and Keromytis [16] used Hybrid
Broadcast-Broadband Televisio (HbbTV) to bypass the
                                                              7     Access Control Policies
SOP. In contrast to websites, HbbTV data does not have
a origin. This characteristic allows an attacker to inject
                                                              Since SOP-DOM restricts access of subjects (mainly
malicious code of his choice into any website, which are
                                                              JavaScript code) to web objects, we think that an appro-
loaded via the HbbTV data stream. Lekies et al. [5] are
                                                              priate formal model could be found amongst the class of
using dynamically generated JavaScript files to attack the
                                                              access control policies. Access control policies restrict
privacy of a victim. Singh et al. [7] describe major access
                                                              the access of subjects from a set S (humans, machines
control flaws in browsers. Complicated side-channels
                                                              or code) to objects from a set O. In the following, we
have been abused to read DOM properties in [23].
                                                              discuss how well the three main classes fit our findings.
   Various non-academic publications describe ways to
                                                                 SOP-DOM is a global access control policy regulating
bypass the SOP. Jain [37] states that Safari v6.0.2 does
                                                              access between websites throughout the Internet; how-
not have SOP restrictions in case the file protocol
                                                              ever, decisions through the SOP-DOM can only be made
is used. In 2010, Stone [38] showed that UI redress-
                                                              on that which is locally available. This data includes
ing can be used to bypass the SOP. Even if the SOP
                                                              the web origins of the different subjects and objects, the
is restricting access on the script level, copy-and-paste
                                                              HTML markup (elements and attributes), and more re-
as well as drag-and-drop actions are not restricted. In
                                                              cently, security policies communicated through HTTP
2012, Heyes [39] showed that the location of a window
                                                              headers like CORS, CSP, X-Frame-Options, and others.
can be accessed cross-origin in FF; however, this should
not be allowed. Three years later, Bentkowski demon-              11 https://www.mozilla.org/en-US/security/

strated with CVE-2015-7188 that FF’s ≤42 SOP can              advisories/mfsa2015-122/




722    26th USENIX Security Symposium                                                              USENIX Association
   In SOP-DOM, the set O of objects may contain any            7.2    Role-Based Access Control (RBAC)
element or property of the local DOM of the web page.
                                                               RBAC is often used in distributed environments as an ab-
Typically, access rights granted to two objects o1 and o2
                                                               straction to improve the manageability of access control
should only differ if the Web Origins of these two objects
                                                               rules. By means of example, the role system adminis-
differ. The set S of subjects could be defined as S = O;
                                                               trator may be assigned to different subjects over time or
however, this would only result in numerous “inactive”
                                                               even periodically, and this role has many important ac-
subjects which do not need any access rights since they
                                                               cess rights. Instead of assigning, revoking, and reassign-
never access any other objects (e.g., text nodes). We
                                                               ing these access rights periodically to individual subjects,
therefore restrict the set S to “active” objects, where the
                                                               the access rights are assigned to the role “system admin-
definition of “active” still awaits a mathematically pre-
                                                               istrator”, and this single role is assigned, revoked and
cise definition. We include all script objects in S and all
                                                               reassigned over time.
CSS code; however, since the discovery of scriptless at-
tacks [23], there may be a need to extend this definition.
                                                               Definition 2 In RBAC, subjects are assigned to roles
                                                               from a set R, and access rights are assigned to roles:
                                                               P1 ⊆ S × R, P2 ⊆ R × O, and s has access to o if there
7.1    Discretionary Access Control (DAC)                      exists a role r such that (s, r) ∈ P1 and (r, o) ∈ P2 .
DAC access control is well-known from operating sys-
                                                                  In typical RBAC installations, access rights to individ-
tems (OSs); each user has a login name and the OS de-
                                                               ual resources are assigned manually by the system ad-
cides if this particular user has access to a certain re-
                                                               ministrator. This is problematic for SOP-DOM, since
source (e.g., a data file or network printer). Each resource
                                                               access policies must be created automatically. We dis-
also has a unique name; therefore, S and O contain the
                                                               cuss the following variant of RBAC where roles are as-
names of users and resources. Another example is email
                                                               signed to both subjects and objects, and access decisions
encryption in which read access is granted on the basis
                                                               are based on both roles only.
of the RFC 822 email addresses of the recipients.
                                                               Definition 3 In enhanced RBAC (eRBAC), subjects are
Definition 1 In DAC, access rights are directly assigned       assigned subject roles from a set RS , objects are assigned
to subjects: the policy set P is a subset of S × O, and        object roles from a set RO , i.e. PS ⊆ S × RS , PO ⊆ O × RO .
subject s has access to object o if (s, o) ∈ P.                Access rights are assigned between roles: P ⊆ RS × RO .
                                                               So subject s has access to object o if there exists roles
                                                               rs ∈ RS and ro ∈ RO such that (s, rs) ∈ PS , (o, ro) ∈ PO
    In the WWW, each subject from S and each object
                                                               and (rs, ro) ∈ P.
from O can be assigned a unique name, which is the URL
at which it can be found. Thus, this part would fit in the        Since we have identified the important influence of the
DAC model. However, there is no global “web operating          embedding element EE on the access decisions in SOP-
system” which keeps track of all possible pairs in S × O.      DOM, we may use EE to assign a “role” to subjects and
Instead SOP-DOM uses only a part of this name in its           objects. So in SOP-DOM, PS and PO would be computed
access decisions, namely the Web Origin.                       locally from the HTML markup and additional security
    Some sources trivialize RFC 6454 in the sense that         policies, and P would be the global SOP-DOM rules im-
they state that read and write access are only possible        plemented in each browser.
if the Web Origins of the subject and object are identical.       For example, to specify that both external and inline
If this was true, it would be a perfect fit for DAC and a      scripts have full cross-origin read and write access rsco
                                                                                                                       rw
very simple global DAC policy could be formulated as           we may formulate:
follows:
                                                                       (s, rsco
                                                                             rw ) ∈ PS ⇐⇒ EE(s) = <script>
                                                                                                                        (1)
         (s, o) ∈ P ⇐⇒ origin(s) = origin(o).                                               ∨ EE(s) = HD.
                                                                  Access to objects is again mainly defined by the em-
   This however is simply incorrect, since in many cases       bedding element. An image embedded via <img> is,
(s, o) ∈ P even if origin(s) 6= origin(o), for example, in     for example, inaccessible at all, whereas the same image
case a script s was embedded via a <script> element,           embedded via <canvas> is partially readable. So we
or if s is contained in a sandboxed iFrame with top-level      could define a role roso
                                                                                     r with the following equation:
frame access.
   Unfortunately, the elegant DAC-based definition of
SOP-DOM via web origins does not fit.                             (o, roso
                                                                        r ) ∈ PO ⇐⇒ EE(o) ∈
                                                                                          / { < img >, ...}             (2)



USENIX Association                                                             26th USENIX Security Symposium          723
   Web origins could be taken into account in P by stat-                       an additional variable dd and assign the value
ing that for all values X, (rsso       so
                                 X , roX ) ∈ P (subject role                   of document.domain to it. All these vari-
has same-origin access to object role), (rsco        co
                                               X , roX ) ∈ P                   ables are both subject and object variables (cf.
(subject role has cross-origin access to object role), and                     Section 7), and are present for both HD and ED
(rsco    so
   X , roX ) ∈ P (if subject role has cross-origin access to                   (cf. Table 2).
object role, then it also has same-origin access).                          I The assignment of random Web Origins to
   This shows that eRBAC seems to be a feasible model,                        sandboxed iFrames can be specified by stat-
however, the rules to assign roles to subjects and objects                    ing that origin(o) = $RAND if sandbox(o) =
could become quite complicated because in addition to                         T RUE.
the EE, we have identified at least two attribute values
(cors and sandbox) which may influence the assignment                Embedding Element. The important role of the embed-
of such roles. This complexity will be increased if we                  ding element EE is modeled as a variable ee, appli-
extend the scope to HTTP security policies such as CSP                  cable to both subject and object, but set only for the
and pseudo-URIs like data:, which are not covered by                    embedded document ED. The value of ee is set to
our current analysis.                                                   the type of the embedding element. It modifies both
                                                                        same-origin and cross-origin access decisions sig-
7.3    Attribute-Based Access Control                                   nificantly.

Attribute-Based Access Control (ABAC) [49] is a flex-                Additional Attributes. Similar to the ee attribute, the
ible access control mechanism used in, for example,                      cors and sandbox attributes are only defined for
XACML [50]. It may also be used to implement RBAC:                       the embedded document ED. For cors, our tests
roles can be modeled as role attributes assigned to both                 revealed that this attribute modifies access rights to
subject and object. The policy decision in ABAC may                      a web object and therefore, it is only an object at-
depend on other subject, object and environment at-                      tribute.
tributes as well.
                                                                     Attributes not fixed by the HTML source code. The
Definition 4 Let Ai = {NULL, value1i , ..., valueki i } be the            ABAC model also defines environment attributes,
set of different values of attribute i. Let S A = A1 × ... ×              which may not depend on subject or object alone
 Al , OA = Al+1 × ... × Am and E A = Am+1 × ... × An                      but rather on the execution environment. The only
 be the cartesian products of all subject, object and en-                 attribute we could qualify to be in E A during our
 vironment attribute values. Let R be the set of all ac-                  tests is sandbox, since it may be set interactively
 cess rights. Then an ABAC policy P is defined as                         by using a suitable directive of Content Security
 P ⊆ S A × OA × E A × R.                                                  Policy.
    Now let s~a be the array of subject attributes of subject
 s, o~a the array of object attributes of object o, and e~a          Extended Web Origin. The ABAC model for SOP-
 the actual array of environment attributes. Then subject            DOM can be presented as the set P but this does not
 s has access r ∈ R to object o if the array ~a, formed              give any insights into the structure of SOP-DOM. How-
 by concatenating s~a, o~a, e~a, and r, is contained in P:           ever, four of the seven variables can be combined into a
~a ∈ P.                                                              very elegant description of an extended Web Origin. This
                                                                     shows that the ABAC model can also be used to simplify
   ABAC could be suitable for SOP-DOM because we                     the description of SOP-DOM.
can model any parameter that influences the access deci-
sions as an attribute. This allows to give a unified treat-      1 Read(protocol,domain,port,dd);
ment to some well-known concepts.                                2 if dd=NULL or (dd is not a
                                                                      superdomain)
Extended Web Origins. Both subject and object have               3 then wo:=(protocol,domain,port)

    attributes from which their Web Origin can be com-           4 else wo:=(protocol,dd,NULL)

    puted. In the classical definition of Web Origins                    Listing 5: Computation of extended Web Origin.
    in RFC6454 these are protocol (location.
    protocol), domain (location.hostname)                               Listing 5 shows how an extended web origin is com-
    and port (location.port).                                        puted from the four given ABAC variables. Please note
                                                                     that the else branch of this algorithm has been veri-
        I We can, for example, extend this definition to             fied by our testbed but different descriptions exist in the
          take the legacy document.domain decla-                     literature. In contrast to previous descriptions of the in-
          ration into account (see below). We define                 teraction of Web Origins and the document.domain



724    26th USENIX Security Symposium                                                                     USENIX Association
declaration, the novel ABAC based concept of extended          sandboxed iFrames, for example, a random Web Origin
Web Origin is both simpler ans less error-prone.               should be generated according to the specification. This
                                                               is however only possible if other EEs imposing similar
7.4    Summary                                                 restrictions (e.g., the <img> element) also use random
                                                               Web Origins. This remains to be tested.
The requirements on an access control model for SOP-
DOM can be formulated as follows: the general rules
of SOP-DOM must be expressible without reference to            References
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