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Users could face a "highly critical" risk if they have both IE and Firefox version 2.0, or later, loaded on their computer. The trouble begins when browsing a malicious site while using IE and it registers a "firefoxurl://" URI (uniform resource identifier) handler, which allows the browser to interact with specific resources on the Web. As a result, users may find their systems remotely compromised.

Earlier Tuesday, security researcher Thor Larholm, who discovered the IE flaw, and security research giant Symantec put much of the blame on IE, while Secunia's Thomas Kristensen, chief technology officer, attributed the problem to Firefox versions 2.0 or later.

"It's a little bit of both," said Oliver Friedrichs, director of Symantec's Security Response Center. "You have two very complex applications that are not playing well together and leading to a security issue. The components themselves are secure as stand-alone products but not together."

"Firefox is the current attack vector, but Internet Explorer is to blame for not escaping...characters when passing on the input to the command line," said Larholm, in response to a reader's comments. "I agree that Firefox could have registered its URL handler with pure DDE (dynamic data exchange, the protocol for information exchange) instead and thereby have avoided the possibility of a command-line argument injection, but IE should still be able to safely launch external applications."

Friedrichs noted that while Firefox, which released version 2 in October, has gained in popularity, most Firefox users will also have IE loaded on their computers, since it comes with the Windows operating system.

The number of people who may be at risk could be substantial, he added.

Meanwhile, Kristensen of Secunia said: "A new URI handler was registered on Windows systems to allow Web sites to force launching Firefox if the 'firefoxurl://' URI was called, like ftp://, http://, or similar would call other applications."

But because of the way the URI handler was registered by Firefox, it causes any parameter--which activates a program to perform a particular task--to be passed from Microsoft's Internet Explorer, or another application, to Firefox, when firefoxurl:// is activated.