A vulnerability has been discovered in Microsoft Windows, which can be exploited by malicious people to compromise a vulnerable system.

The vulnerability is caused due to an input validation error within the Windows shell (shell32.dll) when handling specially crafted URIs with registered URI handlers (e.g. "mailto", "news", "nntp", "snews", "telnet", and "http"). This can be exploited to execute arbitrary commands when a user of certain applications (e.g. Firefox) visits a malicious website or clicks on a link with a specially crafted URI containing a "%" character and ending with a certain extension (e.g. ".bat" or ".cmd").

Examples:
mailto:test%../../../../windows/system32/calc.exe".cmd
nntp:../../../../../Windows/system32/telnet.exe" "secunia.com 80%.bat

Successful exploitation using currently known vectors requires that Internet Explorer 7 is installed on the system, though the core problem exists on Windows system with other versions of Internet Explorer as well.

The following applications have been identified as attack vectors on a fully patched Windows XP SP2 and Windows Server 2003 SP2 system:
* Firefox version 2.0.0.5
* Netscape Navigator version 9.0b2
* mIRC version 6.3
* Adobe Reader/Acrobat version 8.1 and prior (when opening PDF files)
* Outlook Express 6 (e.g. when following specially crafted links in VCards)
* Outlook 2000 (e.g. when following specially crafted links in VCards)

Other versions and applications may also be affected.

Solution
Apply patches.
Further details available to Secunia VIM customers

Provided and/or discovered by
Reported as a Firefox browser vulnerability by:
* Billy (BK) Rios
* Nathan McFeters, Ernst & Young's Advanced Security Center

Firefox not escaping quotes originally discussed by:
* Jesper Johansson

Identification of core Windows problem and additional research by Secunia Research.

Changelog
Further details available to Secunia VIM customers

Original Advisory
Microsoft (KB943521):
http://www.microsoft.com/technet/security/advisory/943521.mspx
http://blogs.technet.com/msrc/archive/2007/10/10/msrc-blog-additional-details-and-background-on-security-advisory-943521.aspx

Billy (BK) Rios:
http://xs-sniper.com/blog/2007/07/24/remote-command-execution-in-firefox-2005/

Other references
Further details available to Secunia VIM customers

Alternate/detailed remediation
Further details available in Customer Area

Deep Links
Links available to Secunia VIM customers