Code that exploits a critical Windows vulnerability disclosed and patched last week has been posted to a public mailing list, raising the risk of an attack, security vendors said Wednesday.

The exploit code, which appeared on the Bugtraq list Tuesday, targets a flaw in Windows' Vector Markup Language rendering that was patched Jan. 9 in Microsoft's monthly security update. VML, an extension of XML that defines Web images in vector graphics format, had been patched before this month; in September 2006, for example, Microsoft issued a fix outside its normally scheduled cycle.

When Microsoft patched the VML bug in its MS07-004 security bulletin, it acknowledged that exploits were ongoing, even though attack code had not yet been released. Within hours of MS07-004's release, exploit code was made available to partners of Immunity, a Florida-based penetration testing vendor. It took a week, however, for code to go public.

According to Symantec's DeepSight threat management service, the posted exploit doesn't execute properly when run against Windows XP SP2 or Windows 2000 SP4 (English versions), but simply crashes Internet Explorer. "Additionally, Internet Explorer 6 running with the default security settings on XP SP2 will prompt the user regarding active content, further reducing the threat," the DeepSight warning read Wednesday.

Even the code's creator admitted poor results, saying that although the attack worked against the Korean edition of Windows XP SP2 and IE 6, it did so only about 20% of the time.

The VML flaw was considered the top threat by most security professionals when Microsoft issued its January patches, in large part because users were vulnerable when they visited a specially crafted Web site. Internet Explorer 7, Microsoft's newest browser, also is at risk, reported Microsoft.

Symantec and others advised users to patch immediately. "We urge customers to apply the associated patches as soon as possible, because more stable variants of this exploit might emerge," said Symantec's alert.

Users unable to patch can protect themselves by disabling the flawed Vgx.dll file using the directions Microsoft has provided:

-- Click Start, click Run

-- Enter: "%SystemRoot%\System32\regsvr32.exe" -u "%CommonProgramFiles%\Microsoft Shared\VGX\vgx.dll"

-- Click OK.