A vulnerability has been reported in Yahoo! Messenger, which can be exploited by malicious people to conduct Cross-Site Scripting attacks.

The vulnerability is caused due to missing input validation when generating an error reply containing the name of an invalid IMVironment.

This can be exploited via a malicious web page to execute arbitrary HTML or script code on a user's system in the context of Yahoo! Messenger by including it in a specially crafted URI using the "ymsgr:" URI handler.

Successful exploitation may e.g. disclose a user's Yahoo ID and encoded password but requires that the IMVironment feature is enabled (default setting).

The vulnerability has been reported in versions 5.5 (Build 1249) and 5.6 (Build 1355). Prior versions may also be affected.

Solution
Yahoo! has reportedly fixed this in their IMVironment server and in an updated version (5.6 build 1356).
Further details available in Customer Area

Provided and/or discovered by
Chet Simpson

Deep Links
Links available in Customer Area