A vulnerability in the way Windows handles animated cursors puts users at risk of being pwnd, and several nefarious websites are already trying to exploit the flaw, according to the SANS Internet Storm Center.

The flaw is present on virtually the entire line of Windows OSes, including Vista, which has been held up as Redmond's poster child for safe computing.

According to McAfee, Windows users browsing malicious sites using IE versions 6 or 7 risk having arbitrary code run on their machines. Those using Firefox are not vulnerable. Microsoft said in an advisory that it is investigating reports of the flaw.

"Upon viewing a web page, previewing or reading a specially crafted message, or opening a specially crafted email attachment the attacker could cause the affected system to execute code," Microsoft warns. Files that can exploit the vulnerability are not limited to those with the .ani extension that come with most programs that animate the cursor on Windows machines. Some exploits in the wild are reported to be embedded in jpeg files, SANS says in an advisory.

Microsoft said those using IE 7 on Vista are safe from the vulnerability because of a protected mode, which restricts where the browser can write files. ®