Hackers can wield malicious Windows Media Player files to exploit any unpatched Internet Explorer (IE) vulnerability on a PC -- even if the user relies on Firefox, Opera or some other Web browser, a UK security researcher said yesterday.

Microsoft is investigating, a spokesman said Wednesday.

Petko Petkov, a penetration tester who released proof-of-concept code last week for a flaw in Apple's QuickTime, said Tuesday that Microsoft's media software also harbors critical bugs that could be used to hijack PCs. On his blog, Petkov posted several exploits targeting a vulnerability in the "HTMLView value" XML tag that's used in several support Windows Media Player file formats, including .asx.

"HTMLView will display a page of our choice within the stand-alone Windows Media Player," Petkov said. "I repeat, the page will be opened within the Media Player surroundings, not a stand-alone browser. This is very interesting behavior."

On a fully-patched PC running Windows XP SP2 with either IE6 or IE7, Petkov was able to force Media Player to open a malicious URL. "Let me translate this for you," he said. "It means that even if you are running Firefox and you think that you are secure, by simply opening a media file, you expose yourself to all IE vulnerabilities."

Petkov embedded malicious script code into the pages he created, then injected into Media Player in the multiple exploit samples he posted on his blog yesterday. In his proofs of concept, the script simply launches a pop-up window that tells the user his machine has been compromised.

Version 11 of Media Player throws up a confirmation dialog box that may make some users think twice about allowing a malicious file to access the program, as does Media Player 10. Earlier editions, however, including Version 9, blithely allow the exploit without warning the user.

"Attackers are in a very good position to abuse the technology," said Petkov.

Late last night, Symantec warned customers of its DeepSight threat network about Petkov's findings, including the Windows Media Player problem. "[These] provide an indirect vector to execute malicious script code, which in some cases may be more difficult to filter," Symantec said in a security bulletin.

Petkov's domain, including his blog and the posted exploits, was offline Wednesday morning, but a cached version of the entry's text was available from Google.

Microsoft is looking into Petkov's claims, but a spokesman downplayed the threat. "We're currently unaware of any attacks trying to use the claimed vulnerability, or of customer impact," he said Wednesday via e-mail. "We will take steps to determine how customers can protect themselves should we confirm the vulnerability."

The next scheduled security update from Microsoft is slated for Oct. 9.

Windows Media Player has been pinned with one other critical vulnerability so far this year (Microsoft patched it in August) and three during 2006. The new Media Player 11, which is bundled with Windows Vista, debuted October 2006 and has been patched only once.