[VulnWatch] Misuse of Macromedia Flash Ads clickTAG Option May Lead to Privacy Breach

From: Aviram Jenik (aviram@beyondsecurity.com)
Date: 04/14/03

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    From: "Aviram Jenik" <aviram@beyondsecurity.com>
To: <vulnwatch@vulnwatch.org>
Date: Mon, 14 Apr 2003 08:30:56 +0200

    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
    ------------------------------------------------------------------------

    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>.

    ====================
    ====================

    DISCLAIMER:
    The information in this bulletin is provided "AS IS" without warranty of
    any kind.
    In no event shall we be liable for any damages whatsoever including
    direct, indirect, incidental, consequential, loss of business profits or
    special damages.

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