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DOM Based Cross Site Scripting
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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 is a kind of XSS that does not match this description,
at least not in some of its 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.
DETAILS
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 there which are different than the
ones applicable for standard XSS.
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.
To read more about the subject please visit:
<http://www.webappsec.org/projects/articles/071105.shtml>
http://www.webappsec.org/projects/articles/071105.shtml
ADDITIONAL INFORMATION
The information has been provided by <mailto:contact@webappsec.org>
Robert Auger.
The article has been written by Amit Klein
The original article can be found at:
<http://www.webappsec.org/projects/articles/071105.shtml>
http://www.webappsec.org/projects/articles/071105.shtml
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