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Posted Jun 20, 2008
Authored by Jouko Pynnonen | Site klikki.fi

The Facebook fb:silverlight FBML tag suffers from a persistent cross site scripting vulnerability.

tags | advisory, xss
SHA-256 | 12ce66213e05ec6e311c300a52ea6a73436286c058c21c6733c9f54c32e0d897


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Facebook is a free-access social networking website with over 100
million active users. Facebook allows anyone to develop web
applications to be used on the site with the Facebook Platform. The
Platform includes a markup language called FBML and a sandboxed,
specialized flavor of JavaScript called FBJS.

A persistent XSS vulnerability was found in way fb:silverlight FBML
tag is processed. The tag is used to embed Microsoft Silverlight
content in Facebook pages. JavaScript can be injected in the tag so
that it runs when the page in question is viewed.

An attacker exploiting the vulnerability could take control of the
victim user's web browser to do any operations on Facebook he/she
could do, including read or manipulate personal information stored on
Facebok, access Facebook's internal e-mail messages, send such
messages impersonating the user, friend list manipulation,
installation of Facebook applications, posting messages on "walls" or
other public forums, etc.

The method would allow creating a JavaScript worm or virus. In this
scenario, users viewing an infected profile, wall post, internal
e-mail message, or application page would get infected without other
user interaction. The script could get automatically, quietly injected
in their profile and spread on to any visitors' profiles.

Facebook has fixed the problem this week. However, four other
vulnerabilities of roughly similar impact remain at the time of


The fb:silverlight FBML tag looks like the following:

<fb:silverlight silverlightsrc="http://src.site/silverlight"
width="400" height="300" />

The Facebook Platform processes this into a piece of HTML code
containing an embed or object tag, plus a piece of JavaScript
initializing the Silverlight object, resembling the following:

var parentElement = document.getElementById("silverlightControlHost");
createSilverlightControl("http://src.site/silverlight", "400", "300");

The framework encodes the parameters correctly when it comes to the
HTML part, but a trailing backslash in the widh or height paramer
causes syntactical problems in the JavaScript part. Thus, the
following FBML tag:

<fb:silverlight silverlightsrc="a" width="\"
height=",any_javascript_code_here);//" />

would produce the following JavaScript on the resulting web page:

var parentElement=document.getElementById("silverlightControlHost");
createSilverlightControl("a", "\", ",any_javascript_code_here);//");

An attacker can then place arbitrary JavaScript code in the function
argument e.g. using eval() or an inline function.


In total five vulnerabilities of roughly the same impact were found
during the few days after June 13th, 2008. Facebook development team
was notified of the first one on that day and the others when they
were discovered. The vulnerability described here appears to have been
fixed during the following days. The others seem to remain at the

Facebook's response was an e-mail stating "We are aware of the
problems that you described and hope to resolve them as soon as
possible." It is unclear if Facebook has been aware of all the issues
prior to my reports as my queries concerning this haven't been


The vulnerabilities were found and investigated by Jouko Pynnonen, Finland.

Jouko Pynnonen <jouko@iki.fi>

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