AOL has acknowledged a potentially serious security vulnerability affecting users of its popular AOL Instant Messenger (AIM) software. It has promised a fix within days. In the meantime, the media giant is advising concerned punters to try a beta version of its forthcoming update.

The vulnerability in current version of the IM client stems from a bug in the 'Away' function of AIM which opens the door to stack-based buffer overflow attacks. The 'Away' functions is used to let a users let people know when they are away from their desk. A bug is AIM's URL handler means that the function misbehaves when it receives very long messages. Computer crackers might exploit this behaviour to inject malicious code into vulnerable system, security firm iDefense warns.

The vulnerability has been confirmed in version 5.5.3595. Other versions may also be affected.

Exploit of the vulnerability requires that an AIM user click on a malicious URL supplied in an instant message or embedded in a Web page. The spread of mass mailing worm tells us it not hard to coax people into such risky behaviour, so a software update can't come too soon.

For now there are two options: update to AOL's beta software or block exploitation by removing the following key from the registry: 'HKEY_CLASSES_ROOT\aim'. Avoid the latter option unless you're comfortable with using Window's Registry Editor software, the misuse of which can render a system inoperable. ®

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