Both Internet Security Systems, of Reston, Va., and Copenhagen-based Secunia warned that systems running the Windows editions of AOL Instant Messenger (AIM) can be loaded with malicious code if they're enticed to specially-crafted Web sites.
The problem stems from AIM's "Away" function, which displays a user's current online status to other contacts. The flaw could be exploited by attackers to create a buffer overflow on the machine, then once the system's compromised, feed it other code or a Trojan horse, which could give the hacker later access to the PC.
The most likely attack vector would be a link embedded in an instant message that leads users to a hacker's Web site.
Secunia broke the new first, and called the vulnerability "critical." Internet Security Systems (ISS), which had notified AOL of the vulnerability last month but had held off publicly announcing the AIM hole, followed suit.
The current version of AIM for Windows, 5.5, is vulnerable, and earlier editions are suspect, said ISS.
For the moment, the only fix is a workaround posted on ISS' Web site, but the company also included a statement it claimed came from AOL that said a new beta version would be rolled out Monday to plug the problem.
As of late Monday afternoon, AIM's Web site had not been updated with a new beta version of the instant messaging client. AOL was unavailable for comment.