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The U.S. Department of Energy
Computer Incident Advisory Capability
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INFORMATION BULLETIN
Flash Authoring Tool Vulnerability
[US-CERT Vulnerability Note VU#249337]
January 3, 2008 22:00 GMT Number S-101
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PROBLEM: A number of authoring tools for Flash content may generate
files that contain cross-site scripting vulnerabilities.
PLATFORM: Flash Authoring Tools
DAMAGE: May be vulnerable to cross-site scripting vulnerabilities.
SOLUTION: Upgrade to the appropriate version.
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VULNERABILITY The risk is MEDIUM. May be vulnerable to cross-site scripting
ASSESSMENT: vulnerabilities.
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LINKS:
CIAC BULLETIN: http://www.ciac.org/ciac/bulletins/s-101.shtml
ORIGINAL BULLETIN: http://www.kb.cert.org/vuls/id/249337
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[***** Start US-CERT Vulnerability Note VU#249337 *****]
Vulnerability Note VU#249337
Flash authoring tools create Flash files that contain cross-site scripting vulnerabilities
Overview
A number of authoring tools for Flash content may generate files that contain
cross-site scripting vulnerabilities. Any site hosting Flash generated by an
affected tool could be vulnerable to cross-site scripting.
I. Description
ActionScript is a scripting language based on ECMAScript (also referred to as
JavaScript) used primarily for the development of websites and software using
Adobe Flash. The resulting Flash content is typically published in the form of
SWF files embedded in web pages. ActionScript within a Flash file creates dynamic
content on the web and interacts with web browsers in a manner similar to JavaScript,
VBScript, and other client-side scripting languages. As with traditional script
content in HTML pages, improperly validated user-controlled input in Flash files
can execute arbitrary ActionScript and JavaScript in the context of the domain
hosting the affected Flash file. One specific type of vulnerability depends on the
behavior of a special ActionScript protocol called asfunction. The asfunction
protocol is used for URLs in HTML text fields and causes a link to invoke an
ActionScript function in a Flash file instead of opening a URL. An attacker could
call all public and static functions by supplying a string parameter to asfunction.
There exist other types of vulnerabilities as well.
Applications that generate Flash files (e.g., "save as SWF", "export to SWF",
etc.) by using vulnerable templates or by including static Flash content like a
controller may automatically insert generic and vulnerable ActionScript into
saved files. As a result, all Flash files generated by these affected applications
create cross-site scripting vulnerabilities in the domains hosting these files.
Furthermore, exploitation of these vulnerabilities would be consistent across all
sites using a particular product and vulnerable sites could be identified through
a web search.
II. Impact
Any website hosting Flash files generated by an affected product is vulnerable
to cross-site scripting in the context of the domain hosting the vulnerable file.
Secondary impacts include the ability to spoof or modify web content, access
website information such as cookies, or retrieve data from an encrypted HTTPS
connection. For a more detailed description of the impact of cross-site scripting
vulnerabilities, please see CERT Advisory CA-2000-02.
III. Solution
Upgrade or apply an update or patch
Updates for affected authoring tools have been released to address this issue.
Please see the Systems Affected section of this document for more information.
Note that this section is not complete, vendors that create vulnerable Flash
authoring tools may not be listed.
Vulnerable Flash files must be regenerated after the affected authoring tools
have been updated.
Adobe has included mitigations in Flash Player to block attacks against some
cross-site scripting vulnerabilities in Flash files (APSB07-20).
Workarounds
Until patches can be applied and affected Flash files regenerated, website
owners may wish to remove automatically generated Flash files or at least
serve automatically generated Flash files from "safe" domains, such as numbered
IP addresses.
Developers of applications that save to SWF are encouraged to perform proper
input validation on all user definable variables used in URL loading
ActionScript functions and the htmlText variable. Specifically,
Whitelist protocol handlers to allow only "http:" and "https:" in all URL
loading functions.
Whitelist characters in user definable input when used in getURL(). Do not
rely on escape(). When feasible, only allow alphanumeric characters.
Whitelist and/or HTML entity encode user input in htmlText.
Load SWF files from relative URLs. The relative URL should not contain "..".
Systems Affected
Vendor Status Date Updated
Adobe Vulnerable 2-Jan-2008
Autodemo Vulnerable 13-Dec-2007
InfoSoft Global Vulnerable 13-Dec-2007
TechSmith Corporation Vulnerable 13-Dec-2007
References
http://www.wisec.it/sectou.php?id=464dd35c8c5ad
http://eyeonsecurity.org/papers/flash-xss.pdf
http://docs.google.com/View?docid=ajfxntc4dmsq_14dt57ssdw
http://www.adobe.com/support/security/advisories/apsa07-06.html
http://www.adobe.com/devnet/flashplayer/articles/fplayer9_security.html
http://www.adobe.com/devnet/flashplayer/articles/secure_swf_apps.html
http://code.google.com/p/flash-validators/
https://www.owasp.org/index.php/Category:SWFIntruder
http://www.mhprofessional.com/product.php?isbn=0071494618
Credit
Thanks to Rich Cannings of the Google Security Team for reporting this
vulnerability. Stefano Di Paola of Minded Security originally published
information about the general problem of cross-site scripting in Flash
applications.
This document was written by Chad Dougherty.
Other Information
Date Public 05/18/2007
Date First Published 01/02/2008 03:16:35 PM
Date Last Updated 01/02/2008
CERT Advisory
CVE Name
Metric 5.40
Document Revision 48
[***** End US-CERT Vulnerability Note VU#249337 *****]
_______________________________________________________________________________
CIAC wishes to acknowledge the contributions of US-CERT for the
information contained in this bulletin.
_______________________________________________________________________________
CIAC, the Computer Incident Advisory Capability, is the computer
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