A noted ActiveX researcher Monday revealed a bug in RealNetworks' RealPlayer that could be exploited by attackers to hijack Windows machines running nternet Explorer.

Elazar Broad, who has uncovered other ActiveX control vulnerabilities in MySpace , Facebook and Yahoo software in the last two months, posted findings to the Full Disclosure security mailing list on Monday that fingered RealPlayer as flawed.

"It is possible to modify heap blocks after they are freed and overwrite certain registers, possibly allowing code execution," Broad said in his message to the mailing list. He also posted proof-of-concept attack code, and said he is trying to come with a working exploit.

Danish vulnerability tracker Secunia rated the RealPlayer bug as "highly critical," its second-highest ranking, and said that the flawed ActiveX control -- the "rmoc3260.dll" file is the culprit -- can be exploited by the usual method of tricking users into visiting malicious or compromised Web sites.

Secunia confirmed the vulnerability, and added that at minimum, the newest build of RealPlayer 11 is buggy. Other, earlier versions may be, too.

The last time RealPlayer was knocked about by hackers was October 2007, when exploits leveraged another bug in its ActiveX control; the attackers were serious enough for some organizations, including a NASA research facility, to tell their users to discard Internet Explorer.

Because the bug is in an ActiveX control, only IE users are at risk. ActiveX, though widely used by Microsoft to add functionality to its browser, has been plagued with a huge number of vulnerabilities. According to Symantec, 89 per cent of the more than 230 browser plug-in bugs tallied in the first half of 2007 were ActiveX flaws.

Some security professionals, in fact, have called for users to ditch ActiveX . Last month, US-CERT did just that: "US-CERT encourages users to disable ActiveX controls as described in the Securing Your Web Browser document," the organization advised.

There is no patch for the RealPlayer vulnerability, but technically-astute users can edit the Windows registry to set the "kill bit" for the flawed control. Another alternative, said the SANS Institute's Internet Storm Center, is to switch browsers; Firefox and Opera, for instance, don't rely on ActiveX.

RealNetworks did not reply to a request for comment.