Mozilla's Firefox 2.0 is vulnerable to attackers armed with the Windows animated cursor exploit, despite contrary reports, a researcher claims.

Alexander Sotirov, vulnerability researcher at Determina, discovered the ANI flaw and notified Microsoft last December. Sotirov yesterday posted a demonstration of an ANI exploit that hijacks a PC when Firefox users are conned into visiting a malicious site.

"It turns out that Firefox uses the same vulnerable Windows component to process .ani files, which can be exploited in a way similar to Internet Explorer," Sotirov said during the demo.

He showed how both IE7 and Firefox 2.0, when run on Vista, can be hijacked by the proof-of-concept ANI exploit he created in December, which he also shared with Microsoft's security team. When the attack was run against IE 7, the ANI exploit gave access to all files on the system.

"However, we cannot alter any system files" because of IE's protected mode, which is enabled by default in Vista, said Sotirov.

Vista's version of IE7 runs in a low-privilege mode - called "protected mode" by Microsoft - that blocks disk write access to all but a temporary files folder.

An identical attack against Firefox 2.0, however, gave Sotirov total access to the PC's drive. "Since Firefox does not have a low-privilege mode similar to the protected mode in IE, we'll be able to overwrite files as well," he said.

Third-party security vendors have claimed that Firefox 2.0 is not vulnerable. In a threat alert to its DeepSight customers, Symantec flatly stated "Mozilla Firefox is not vulnerable to the vulnerability."

Not so, said Sotirov. "The reason for the confusion over Firefox is that an exploit that works against it has not become public. So in a sense, since there are no attacks in the wild [that work in Firefox], it is safer. But people should also consider that the bad guys will figure out how to exploit Firefox."

Mozilla did not respond to requests for comment about Firefox's risk of exploitation, or confirmation that it is vulnerable.

So far this year, Mozilla has issued 10 Firefox patches.