Microsoft has issued an update for Windows. This fixes two vulnerabilities, allowing malicious websites to compromise a vulnerable system.

1) A problem in the handling of a specially crafted showHelp URL allows execution of arbitrary code in the Local Security Zone.

This issue may be related to:
SA10523

2) A boundary error within HTML Help ("hh.exe") can be exploited to cause a heap-based buffer overflow via a malicious ".CHM" file containing an overly large value in a certain length field.

Reportedly, all versions of Microsoft Windows running Internet Explorer 5.5 SP2 and 6 SP1 are affected.

Microsoft will not be releasing updates for Windows 98, 98SE, and ME.

Solution
Apply patches.
Further details available to Secunia VIM customers

Provided and/or discovered by
2) Brett Moore of Security-Assessment.com

Changelog
Further details available to Secunia VIM customers

Original Advisory
MS04-023 (KB840315)
http://www.microsoft.com/technet/security/bulletin/ms04-023.mspx

Brett Moore:
http://www.security-assessment.com/Papers/CHM_File_Heap_Overflow.pdf

Other references
Further details available to Secunia VIM customers

Deep Links
Links available to Secunia VIM customers