Trouble came in pairs for instant messenger users this week with the disclosure of unpatched flaws in chat software from both AOL and Yahoo!

The Yahoo! vulnerability is by far the worst of the two, not least because exploit code was posted on the milw0rm full disclosure mailing list on Wednesday.

A security bug in Yahoo!'s IM client reportedly makes it possible for crackers to download arbitrary files - potentially malware - onto the PCs of prospective marks. Whether it's possible to get this code to run depends on a user's Internet Explorer security settings, a factor that mitigates against the potency of the attack.

This is the fifth time in less than three months that the Yahoo! IM software has been the subject of a critical security vulnerability. Previous examples are listed by security notification firm Secunia here.

Security firm nCircle describes the bug as the ninth zero-day threat against Messenger this year, PC World reports.

By comparison, the AOL instant message bug is a walk in the park. Hackers might be able to send script in specifically malformed messages to other users. Marks would either need to already be friends with the hackers or accept arbitrary messages for the attack to be effective, Secunia reports. ®

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