Symantec Reports New Zero-Day Word Bug

Attackers are using a critical, unpatched vulnerability in Microsoft Word -- the fourth known unfixed flaw in the popular word processor -- to hijack computers, Symantec warned Friday.

The bug, which affects Word 2000, lets hackers execute their own code with the same privileges as the user. In most cases, Windows users run in administrator mode, which allows full access to the PC and its files. If a user opens a malformed Word file, which would typically be delivered as a file attachment to a spoofed e-mail, the Mdropper.m Trojan creates a backdoor that allows later remote access by the attacker.

Danish vulnerability tracker Secunia rated the new vulnerability as "Extremely critical," the highest warning in its 1 through 5 scale.

Although only Word 2000 can lead to a complete compromise of the PC, Word 2002 and Word 2003 users attacked by the exploit will find that all the computer's processor cycles are consumed by the malware, effectively creating a denial-of-service attack. Rebooting the computer is the only way to regain control of the machine.

The new flaw is different from three other unpatched Word bugs that appeared in December 2006. Microsoft has let the vulnerabilities slide in the last two monthly security updates.

Symantec recommended that users not open Word attachments "unless they are expected and come from a known and trusted source."