mailing list archives
Facebook fb:silverlight persistent XSS
From: "Jouko Pynnonen" <jouko () iki fi>
Date: Fri, 20 Jun 2008 02:36:52 +0300
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,
A persistent XSS vulnerability was found in way fb:silverlight FBML
tag is processed. The tag is used to embed Microsoft Silverlight
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.
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:
width="400" height="300" />
The Facebook Platform processes this into a piece of HTML code
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
following FBML tag:
<fb:silverlight silverlightsrc="a" width="\"
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>
Full-Disclosure - We believe in it.
Hosted and sponsored by Secunia - http://secunia.com/
- Facebook fb:silverlight persistent XSS Jouko Pynnonen (Jun 19)