A serious security flaw reported this week in WinAmp is still unpatched, contrary to the vendor's assurances, according to the researcher who discovered the vulnerability. What's more, exploit code taking advantage of the flaw has begun circulating on the Internet, making attacks simpler to carry out, said security experts.

"It appears that the 'patched' version 5.05 does not fix the buffer overflow issue that we notified Nullsoft about," said Brett Moore, chief technical officer of Security-Assessment.com, in an email to the Bugtraq security mailing list on Wednesday. "We have sent Nullsoft a copy of this email, and hope that they can remedy this problem quickly." Nullsoft did not immediately respond to Techworld's request for comment.

The bug, a boundary error in the "IN_CDDA.dll" file, is the latest in a string of serious vulnerabilities in WinAmp, including an August flaw in the handling of "skin" files which attackers began to exploit before it had been discovered by researchers. The new bug, the skin file flaw and an April flaw in the handling of ".xm" files could all be exploited by luring an affected user to a website containing a specific type of file, which would then be automatically downloaded and executed.

This week's bug can be exploited in a number of ways, the most dangerous being via an ".m3u" playlist file, according to Moore. "When hosted on a website, these files will be automatically downloaded and opened in winamp without any user interaction," he wrote in Security-Assessment.com's advisory. "This is enough to cause the overflow that would allow a malicious playlist to overwrite EIP and execute arbitrary code." Exploitation could allow an attacker to take over a system.

Moore recommended that in the absence of a patch, users disassociate the filename extensions for playlists - .cda and .m3u - from WinAmp.

Moore noted that WinAmp had released the patched WinAmp last week, without initially notifying Security-Assessment.com. When Moore asked about the new WinAmp, the vendor responded: "The problem was fixed in the latest release of WinAmp. Version 5.06 went live on the site last Thurday."

On Thursday, Danish security firm Secunia escalated the flaw to "extremely critical", its most serious rating, saying that exploit code has become available. IT managers might want to be particularly wary of attacks as the US' four-day Thanksgiving weekend began on Thursday, traditionally an active time for programming enthusiasts and bored teenaged hackers.