DOM Based Cross Site Scripting
or
XSS of the Third Kind
A look at an overlooked flavor of XSS
Amit Klein, July 2005
Version: 0.2.8
Last modified: 4th of July, 2005
Summary
=======
We all know what Cross Site Scripting (XSS) is, right? It's that
vulnerability wherein one sends malicious data (typically HTML stuff
with Javascript code in it) that is echoed back later by the
application in an HTML context of some sort, and the Javascript code
gets executed. Well, wrong. There's a kind of XSS which does not
match this description, at least not in some fundamental properties.
The XSS attacks described above are either "non-persistent"/
"reflected" (i.e. the malicious data is embedded in the page that is
returned to the browser immediately following the request) or
"persistent"/"stored" (in which case the malicious data is returned
at some later time). But there's also a third kind of XSS attacks -
the ones that do not rely on sending the malicious data to the
server in the first place! While this seems almost contradictory to
the definition or to common sense, there are, in fact, two well
described examples for such attacks. This technical note discusses
the third kind of XSS, dubbed "DOM Based XSS". No claim is made to
novelty in the attacks themselves, of course, but rather, the
innovation in this write-up is about noticing that these belong to a
different flavor, and that flavor is interesting and important.
Application developers and owners need to understand DOM Based XSS,
as it represents a threat to the web application, which has
different preconditions than standard XSS. As such, there are many
web applications on the Internet that are vulnerable to DOM Based
XSS, yet when tested for (standard) XSS, are demonstrated to be "not
vulnerable". Developers and site maintainers (and auditors) need to
familiarize themselves with techniques to detect DOM Based XSS
vulnerabilities, as well as with techniques to defend against them,
both therewhich are different than the ones applicable for standard
XSS.
Introduction
============
The reader is assumed to possess basic knowledge of XSS ([1], [2],
[3], [4], [8]). XSS is typically categorized into "non-persistent"
and "persistent" ([3], "reflected" and "stored" accordingly, as
defined in [4]). "Non-persistent" means that the malicious
(Javascript) payload is echoed by the server in an immediate
response to an HTTP request from the victim. "Persistent" means that
the payload is stored by the system, and may later be embedded by
the vulnerable system in an HTML page provided to a victim. As
mentioned in the summary, this categorization assumes that a
fundamental property of XSS is having the malicious payload move
from the browser to the server and back to the same (in non-
persistent XSS) or any (in persistent XSS) browser. This paper
points out that this is a misconception. While there are not many
counterexamples in the wild, the mere existence of XSS attacks which
do not rely on the payload embedded by the server in some response
page, is of importance as it has a significant impact on detection
and protection methods. This is discussed in the document.
Example and Discussion
======================
Before describing the basic scenario, it is important to stress that
the techniques outlined here were already demonstrated in public
(e.g. [5], [6] and [7]). As such, it is not claimed that the below
are new techniques (although perhaps some of the evasion techniques
are).
The prerequisite is for the vulnerable site to have an HTML page
that uses data from the document.location or document.URL or
document.referrer (or any various other objects which the attacker
can influence) in an insecure manner.
NOTE for readers unfamiliar with those Javascript objects: when
Javascript is executed at the browser, the browser provides the
Javascript code with several objects that represent the DOM
(Document Object Model). The document object is chief among those
objects, and it represents most of the page's properties, as
experienced by the browser. This document object contains many
sub-objects, such as location, URL and referrer. These are
populated by the browser according to the browser's point of view
(this is significant, as we'll see later with the fragments). So,
document.URL and document.location are populated with the URL of
the page, as the browser understands it. Notice that these objects
are not extracted of the HTML body - they do not appear in the
page data. The document object does contain a body object that is
a representation of the parsed HTML.
It is not uncommon to find an application HTML page containing
Javascript code that parses the URL line (by accessing
document.URL or document.location) and performs some client side
logic according to it. The below is an example to such logic.
In analogy to the example in [2] (and as an essentially identical
scenario to the one in [7]), consider, for example, the following
HTML page (let's say this is the content of
http://www.vulnerable.site/welcome.html):
Welcome!
Hi
Welcome to our system
...
Normally, this HTML page would be used for welcoming the user, e.g.:
http://www.vulnerable.site/welcome.html?name=Joe
However, a request such as:
http://www.vulnerable.site/welcome.html?name=
would result in an XSS condition. Let's see why: the victim's
browser receives this link, sends an HTTP request to
www.vulnerable.site, and receives the above (static!) HTML page. The
victim's browser then starts parsing this HTML into DOM. The DOM
contains an object called document, which contains a property called
URL, and this property is populated with the URL of the current
page, as part of DOM creation. When the parser arrives to the
Javascript code, it executes it and it modifies the raw HTML of the
page. In this case, the code references document.URL, and so, a part
of this string is embedded at parsing time in the HTML, which is
then immediately parsed and the Javascript code found (alert(...))
is executed in the context of the same page, hence the XSS
condition.
Notes:
1. The malicious payload was not embedded in the raw HTML page at
any time (unlike the other flavors of XSS).
2. This exploit only works if the browser does not modify the URL
characters. Mozilla automatically encodes < and > (into %3C and %3E,
respectively) in the document.URL when the URL is not directly typed
at the address bar, and therefore it is not vulnerable to the attack
as shown in the example. It is vulnerable to attacks if < and > are
not needed (in raw form). Microsoft Internet Explorer 6.0 does not
encode < and >, and is therefore vulnerable to the attack as-is.
Of course, embedding in the HTML directly is just one attack mount
point, there are various scenarios that do not require < and >, and
therefore Mozilla in general is not immune from this attack.
Evading standard detection and prevention technologies
======================================================
In the above example, it may be argued that still, the payload did
arrive to the server (in the query part of the HTTP request), and so
it can be detected just like any other XSS attack. But even that can
be taken care of. Consider the following attack:
http://www.vulnerable.site/welcome.html#name=
A more strict security policy would require that the name parameter
be sent (to avoid the above tricks with names and number sign). We
can therefore send this:
http://www.vulnerable.site/welcome.html?notname=
Even if the payload is inspected by the server, protection can be
guaranteed only if the request in its fullness is denied, or if the
response is replaced with some error text. Consider [5] and [6]
again, if the Authorization header is simply removed by an
intermediate protection system, it has no effect as long as the
original page is returned. Likewise, any attempt to sanitize the
data on the server, either by removing the offending characters or
by encoding them, is ineffective against this attack.
In the case of document.referrer, the payload is sent to the server
through the Referer header. However, if the user's browser, or an
intermediate device eliminates this header, then there's no trace of
the attack - it may go completely unnoticed.
To generalize, traditional methods of:
- HTML encoding output data at the server side
- Removing/encoding offending input data at the server side
Do not work well against DOM Based XSS.
Regarding automatic vulnerability assessment by way of fault
injection (sometimes called fuzzing) won't work, since products that
use this technology typically evaluate the results according to
whether the injected data is present in the response page or not
(rather than execute the client side code in a browser context and
observe the runtime effects). However, if a product is able to
statically analyze a Javascript found in a page, then it may point
out suspicious patterns (see below). And of course, if the product
can execute the Javascript (and correctly populating the DOM
objects), or simulate such execution, then it can detect this
attack.
Manual vulnerability assessment using a browser would work because
the browser would execute the client side (Javascript) code. Of
course, a vulnerability assessment product may adopt this kind of
technology and execute client side code to inspect the runtime
effects.
Effective defenses
==================
1. Avoiding client side document rewriting, redirection, or other
sensitive actions, using client side data. Most of these effects can
be achieved by using dynamic pages (server side).
2. Analyzing and hardening the client side (Javascript) code.
Reference to DOM objects that may be influenced by the user
(attacker) should be inspected, including (but not limited to):
- document.URL
- document.URLUnencoded
- document.location (and many of its properties)
- document.referrer
- window.location (and many of its properties)
Note that a document object property or a window object property may
be referenced syntactically in many ways - explicitly
(e.g. window.location), implicitly (e.g. location), or via obtaining
a handle to a window and using it
(e.g. handle_to_some_window.location).
Special attention should be given to scenarios wherein the DOM is
modified, either explicitly or potentially, either via raw access to
the HTML or via access to the DOM itself, e.g. (by no means an
exhaustive list, there are probably various browser extensions):
- Write raw HTML, e.g.:
* document.write(...)
* document.writeln(...)
* document.body.innerHtml=...
- Directly modifying the DOM (including DHTML events), e.g.:
* document.forms[0].action=... (and various other collections)
* document.attachEvent(...)
* document.create...(...)
* document.execCommand(...)
* document.body. ... (accessing the DOM through the body object)
* window.attachEvent(...)
- Replacing the document URL, e.g.:
* document.location=... (and assigning to location's href, host
and hostname)
* document.location.hostname=...
* document.location.replace(...)
* document.location.assign(...)
* document.URL=...
* window.navigate(...)
- Opening/modifying a window, e.g.:
* document.open(...)
* window.open(...)
* window.location.href=... (and assigning to location's href, host
and hostname)
- Directly executing script, e.g.:
* eval(...)
* window.execScript(...)
* window.setInterval(...)
* window.setTimeout(...)
To continue the above example, an effective defense can be replacing
the original script part with the following code, which verifies
that the string written to the HTML page consists of alphanumeric
characters only:
Such functionality can (and perhaps should) be provided through a
generic library for sanitation of data (i.e. a set of Javascript
functions that perform input validation and/or sanitation). The
downside is that the security logic is exposed to the attackers - it
is embedded in the HTML code. This makes it easier to understand and
to attack it. While in the above example, the situation is very
simple, in more complex scenarios wherein the security checks are
less than perfect, this may come to play.
3. Employing a very strict IPS policy in which, for example, page
welcome.html is expected to receive a one only parameter named
"name", whose content is inspected, and any irregularity (including
excessive parameters or no parameters) results in not serving the
original page, likewise with any other violation (such as an
Authorization header or Referer header containing problematic data),
the original content must not be served. And in some cases, even
this cannot guarantee that an attack will be thwarted.
A note about redirection vulnerabilities
========================================
The above discussion is on XSS, yet in many cases, merely using a
client side script to (insecurely) redirect the browser to another
location is considered vulnerability in itself. In such cases, the
above techniques and observations still apply.
Conclusion
==========
While most XSS attacks described in public do indeed depend on the
server physically embedding user data into the response HTML pages,
there are XSS attacks that do not rely on server side embedding of
the data. This has material significance when discussing ways to
detect and prevent XSS. To date, almost all detection and prevention
techniques discussed in public assume that XSS implies that the
server receives malicious user input and embeds it in an HTML page.
Since this assumption doesn't hold (or only very partially holds)
for the XSS attacks described in this paper, many of the techniques
fail to detect and prevent this kind of attacks.
The XSS attacks that rely on server side embedding of user data are
categorized into "non-persistent" (or "reflected") and "persistent"
(or "stored"). It is thus suggested that the third kind of XSS, the
one that does not rely on server side embedding, be named "DOM Based
XSS".
Here is a comparison between standard XSS and DOM Based XSS:
+------------------------------------------------------------------+
| | Standard | XSS DOM Based XSS |
|-----------------+------------------------+-----------------------|
| Root cause |Insecure embedding of | Insecure reference |
| |client input in HTML | and use (in a client |
| |outbound page | side code) of DOM |
| | | objects that are not |
| | | fully controlled by |
| | | the server provided |
| | | page |
|-----------------+------------------------+-----------------------|
| Owner | Web developer (CGI) | Web developer (HTML) |
|-----------------+------------------------+-----------------------|
| Page nature | Dynamic only | Typically static |
| | (CGI script) | (HTML), but not |
| | | necessarily. |
|-----------------+------------------------+-----------------------|
| Vulnerability | * Manual Fault | * Manual Fault |
| | Detection injection | Injection |
| | * Automatic Fault | * Code Review (can be |
| | Injection | done remotely!) |
| | * Code Review (need | |
| | access to the page | |
| | source) | |
|-----------------+------------------------+-----------------------|
| Attack | * Web server logs | If evasion techniques |
| detection | * Online attack | are applicable and |
| | detection tools | used - no server side |
| | (IDS, IPS, web | detection is possible |
| | application | |
| | firewalls) | |
|-----------------+------------------------+-----------------------|
| Effective | * Data validation at | * Data validation at |
| defense | the server side | the client side (in |
| | * Attack prevention | Javascript) |
| | utilities/tools | * Alternative server |
| | (IPS, application | side logic |
| | firewalls) | |
+------------------------------------------------------------------+
References
==========
Note: the URLs below are up to date at the time of writing (July
4th, 2005). Some of these materials are live documents, and as such
may be updated to reflect the insights of this paper.
[1] "CERT Advisory CA-2000-02 - Malicious HTML Tags Embedded in
Client Web Requests", CERT, February 2nd, 2000
http://www.cert.org/advisories/CA-2000-02.html
[2] "Cross Site Scripting Explained", Amit Klein, June 2002
http://crypto.stanford.edu/cs155/CSS.pdf
[3] "Cross-Site Scripting", Web Application Security Consortium,
February 23rd, 2004
http://www.webappsec.org/projects/threat/classes/cross-site_scripting.shtml
[4] "Cross Site Scripting (XSS) Flaws", The OWASP Foundation,
updated 2004
http://www.owasp.org/documentation/topten/a4.html
[5] "Thor Larholm security advisory TL#001 (IIS allows universal
CrossSiteScripting)", Thor Larholm, April 10th, 2002
http://www.cgisecurity.com/archive/webservers/iis_xss_4_5_and_5.1.txt
(see also Microsoft Security Bulletin MS02-018
http://www.microsoft.com/technet/security/bulletin/MS02-018.mspx)
[6] "ISA Server Error Page Cross Site Scripting", Brett Moore, July
16th, 2003
http://www.security-assessment.com/Advisories/ISA%20XSS%20Advisory.pdf
(see also Microsoft Security Bulletin MS03-028
http://www.microsoft.com/technet/security/bulletin/MS03-028.mspx and
a more elaborate description in "Microsoft ISA Server HTTP error
handler XSS", Thor Larholm, July 16th, 2003
http://www.securityfocus.com/archive/1/329273)
[7] "Bugzilla Bug 272620 - XSS vulnerability in internal error
messages", reported by Michael Krax, December 23rd, 2004
https://bugzilla.mozilla.org/show_bug.cgi?id=272620
[8] "The Cross Site Scripting FAQ", Robert Auger, May 2002 (revised
August 2003)
http://www.cgisecurity.com/articles/xss-faq.shtml
About the author
================
Amit Klein is a renowned web application security researcher. Mr.
Klein has written many research papers on various web application
technologies--from HTTP to XML, SOAP and web services--and covered
many topics--HTTP request smuggling, insecure indexing, blind XPath
injection, HTTP response splitting, securing .NET web applications,
cross site scripting, cookie poisoning and more. His works have been
published in Dr. Dobb's Journal, SC Magazine, ISSA journal, and IT
Audit journal; have been presented at SANS and CERT conferences; and
are used and referenced in many academic syllabi.
Mr. Klein is WASC (Web Application Security Consortium) member.