Hackers are working around the clock to exploit a critical Windows vulnerability that was patched on Tuesday, security researchers warn - and the only version of Windows not at risk is the unfinished Windows XP SP3.

Fortunately, these initial sorties have been unsuccessful, Symantec said in a brief warning to customers of its DeepSight threat service. "The DeepSight honeynet has observed in-the-wild exploit attempts targeting a graphics device interface [GDI] vulnerability patched by Microsoft on April 8, 2008," said Symantec in its alert.

On Tuesday, Microsoft patched two bugs, both named as 'critical', in the Windows GDI, one of the core components of the operating system. According to Microsoft, every current version of Windows, including the very newest, Vista Service Pack 1 (SP1) and Server 2008, is open to attack.

The vulnerabilities can be triggered by malformed WMF (Windows Metafile) or EMF (Enhanced Metafile) image files, Microsoft noted in its accompanying advisory.

On Tuesday analysts named the GDI bugs as the most dangerous of the 10 disclosed and patched by Microsoft that day. They noted similarities between the two new vulnerabilities and others revealed in late 2005, which were extensively exploited by attackers for months afterward.

Amol Sarwate, manager of Qualys's vulnerability research lab, said at the time that he expected attackers to quickly begin leveraging the bug. "Users who simply view an image online or in email could be compromised," he said.

Yesterday, Symantec said it had spotted three different websites hosting malicious WMF/EMF image files that were targeting one of the two GDI bugs. However, those images weren't able to exploit the flaw. "Analysis of the images has shown that although [they] appear to be malicious, they do not contain enough data in the associated image property to sufficiently trigger the vulnerability," read Symantec's warning. "We are still investigating the issue as to why this may be the case."

The security company urged users to apply the GDI patches pronto if they have not done so already. "These attack attempts highlight the severity of this issue and it is only a matter of time before new images that successfully trigger the issue are observed in the wild," Symantec concluded.

Ironically, the only version of Windows not vulnerable to attack is XP SP3, the still-not-released final update to the aged operating system. Hidden in the MS08-021 security bulletin was the sentence: "Windows XP SP3 is not affected by this vulnerability."

Windows XP SP3's release date remains a mystery. Although Microsoft has not budged from its "first half of 2008" public statements, others have speculated that the service pack will wrap up later this month. One website, which correctly predicted release dates for Vista SP1, has pegged XP SP3's roll-out as coming in the second half of April.

Microsoft's GDI patches can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.

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