The problem lies within the WordPad Text Converter for Word 97 files, Microsoft said in an advisory.
The systems affected include Windows 2000 Service Pack 4, Windows XP Service Pack 2, Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2, Microsoft said. XP Service Pack 3 and the Vista operating systems are not affected.
The company said it has seen limited, targeted attacks. If exploited, a hacker could gain the same rights on a PC as a local user and could remotely execute code.
Microsoft is investigating the problem. Microsoft typically releases patches on the second Tuesday of the month. If Microsoft sticks to its schedule, the earliest a patch could be released would be Jan. 13. However, Microsoft has deviated from its patching cycle when a vulnerability is considered particularly dangerous.
The vulnerability can’t be exploited by simply opening an e-mail, Microsoft said. The victim would have to open an attachment containing a malicious file designed to exploit the problem.
Microsoft said Word 97 documents are opened by default with Office Word if a user has that application installed. Word is not affected by the problem, but attackers could try to rename the malicious file with a Windows Write (.wri) extension, which would be cause WordPad to try to open it. The company advised that “.wri” attachments could be blocked at the network gateway, reducing the risk a user would open a harmful file.
Microsoft released on Tuesday eight patches covering 28 vulnerabilities within applications including Internet Explorer, Sharepoint, Office, Windows Media Player and its Vista OS. Six of those patches were classified as “critical.”