Warning issued over Windows Media Player 7 bug

Exploit that gets under the skin

Veteran bug hunter Georgi Guninski has identified a potentially serious security vulnerability affecting Microsoft Windows Media Player 7 "skins", which control the look and feel of the application.

The bug is said by Guninski to allow hackers to read "local files and browsing directories which in turn allows executing arbitrary programs" on victim's machines. Properly exploited it could allow an attacker to obtain complete control of a user's computer, he warned. Users of other browsers are not affected by the vulnerability whose affects are believed to be limited to a particular machine.

The vulnerability exploits the fact that "skins", downloadable files which change the appearance of a program's user interface, are stored at a particular location on a user's hard disk, C:/Program files/Windows Media Player/Skins/skin.wmz.

A HTML document can be constructed which could lead a user into downloading an arbitrary file, which might contain executable Java code disguised as a Windows Media Player skin file, to this known location.

An applet tag in the remote HTML document can then execute the 'skin.wmz' file as Java code.

Deri Jones, security services director at NTA Monitor, said: "This is an example of a type of attack which gets you whilst you're browsing the web. The root cause is bugs with Internet Explorer which allow the downloading or uploading of files onto a machine."

Microsoft said users can protect themselves from the vulnerability by setting Internet Explorers' Internet options for security zones so that any unsigned Java content will not run on a PC. A more complete fix may follow.

In November a very similar problem was reported to affect Windows Media Player. Microsoft has issued a patch to correct the vulnerability, which meant a custom skin file could potentially include script, which would execute if Windows Media Player was run and that skin was selected. The same patch also corrected a buffer overflow vulnerability that affects the code which parses Active Stream Redirector (.ASX) streaming media files. ®

Related links

Guninski's demonstration of the vulnerability

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