This advisory is available online at:
http://www.securiteam.com/securitynews/5XP0B0U9PE.html


  Misuse of Macromedia Flash Ads clickTAG Option May Lead to Privacy
Breach
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SUMMARY

"Over 497 million Internet users now use Macromedia Flash Player to 
seamlessly view content created with Macromedia Flash, the solution for 
developing rich Internet content and applications."

A vulnerability discovered in Macromedia Flash ad user tracking field 
allows a remote user to perform Cross-Site-Scripting attacks and
retrieve 
session information. 



DETAILS

About the 'clickTAG' option:

Macromedia flash supplies user-tracking field to swf (flash movies) ads:
"The clickTAG is the tracking code assigned by the ad serving network to

an individual ad. The clickTAG allows the network to register where the
ad 
was displayed when it was clicked on. This click through data is
reported 
to the ad serving servers so advertisers may determine the effectiveness

of their campaign. 

The code below will allow ad serving networks to dynamically assign a 
clickTAG to their ad. 

In this example, a getURL action is being assigned to a button that will

navigate the browser to ["clickTAG"]. The "getURL(clickTAG)" statement 
appends the variable data passed in via the OBJECT EMBED tag and
navigates 
the browser to that location. It is the tracking code assigned by the ad

serving network, which allows them to register a user's click on that 
advertisement. 

<EMBED src="ad_banner_example.swf?clickTAG= 
http://adnetwork.com/tracking?http://www.destinationURL.com" >   ..."

The information was taken from Macromedia designer's guide:
http://www.macromedia.com/resources/richmedia/tracking/designers_guide/ 

Vulnerability details:

Vulnerability in the clickTAG field enables a remote user to run
malicious 
javascript code in the context of the remote web site, and therefore 
retrieve session information and possibly other sensitive information.
For example in the following script:
http://www.example.com/victim.swf?clickTag=XXXX
("XXXX" = arbitrary script or tag)

Replacing "XXXX" with a script to steal cookies will enable an attacker
to 
perform session hijacking if the session is saved in the cookie, or to 
gain the private information present in ad tracking cookies.

Solution:

"A new player version is NOT required. Macromedia Flash advertisements 
that accept clickTAGs need to validate that the clickTAG URL begins with

"http:". This helps ensure the clickTAG does not contain malicious
code."
Quote from the official Macromedia security advisory.

We recommend that all user input should be filtered for malicious code
and 
characters and never trusted "as-is".

Vendor status:
We would like to thank Macromedia for its prompt response and
cooperation 
for solving this issue.
Macromedia quickly acted to notify possibly affected sites and has 
released an official security announcement, which can be found at:
 
http://www.macromedia.com/support/flash/ts/documents/clicktag_security.h
tm.

Macromedia has also revised the Designer's Guide and added this note:
"Note: The ActionScript in this Flash advertisement is verifying that
the 
clickTAG URL begins with "http:". This is an important security measure.

If you do not take this precaution, a malicious HTML page could source 
your SWF and pass a clickTAG URL that begins with "javascript:" or
another 
scripting pseudo-protocol. If your ActionScript code were to call getURL

with a maliciously crafted JavaScript URL, it would be possible for the 
site serving the malicious HTML page to obtain the contents of your HTTP

cookies or perform other actions on your site's behalf."


ADDITIONAL INFORMATION

The vulnerability was reported by Scan Security Wire  
<http://www.scan-web.com>.


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