The trend for exploiting vulnerabilities around the same time as Microsoft's monthly Patch Tuesday update continues.

Hackers have crafted a new Word exploit based on a memory corruption vulnerability addressed by Microsoft on Tuesday. The flaw was already exploited prior to the release of the fix, according to Microsoft.

Until this week the critical flaw had only been exploited in targeted attacks, but it's now getting wider play.

Symantec reports that the vulnerability is now being exploited in more widespread attacks. Malformed Word documents doing the rounds contain shell code and three pieces of malware. The malware package is unusual in that it was created using the Word for Macintosh format instead of the standard Windows (OLE) format.

Despite the formating, the malicious Word documents are actually targeted at infecting Windows PCs, according to a preliminary analysis by Symantec.

Detection against the malware - dubbed the Mdropper-Z Trojan - has been added to Symantec's security software. It also detected the separate malware files contained in the payload of the maliciously-constructed word files as Trojan-Dropper, Backdoor-Trojan, and Hacktool-Rootkit.

The use of older file formats in the exploit could backfire on hackers. "The good news is that the default configuration in Microsoft Office 2007 and Office 2003, Service Pack 3 will not allow you to open some older Office file formats, including Office for Macintosh documents," Symantec notes. ®

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