Software Engineering Institute

The Microsoft Windows help viewer (winhlp32.exe) provides application assistance to users through a special type of file (.hlp). The help file format allows for compression of frequently used phrases. When this feature is employed, information about phrases, including size and number, is stored in a table in the help file. An unsafe use of information supplied during decoding of the phrase table could result in an incorrect memory allocation. This error results in a heap memory overflow that can be exploited by a malformed help file that specifies an invalid size in the phrase table.

Note that this vulnerability may only affect versions of Microsoft Windows with certain language packs installed.  The full scope of affected systems is unclear at the current time.

A remote attacker may be able to execute arbitrary code on a vulnerable system through a maliciously crafted help file (.hlp). The malicious file could be introduced to a victim user through a web site or an email message. The intruder-supplied code would be run with the privileges of the user who opened the malicious help file.

The CERT/CC is currently unaware of a practical solution to this problem.

Workarounds

As always, best practices dictate careful handling of all files received from unknown sources. Users should not open help files from unknown or potentially malicious sources.