A zero-day flaw in the ubiquitous Microsoft Word software program is being used in an active exploit by sophisticated hackers in China and Taiwan, according to warnings from anti-virus researchers.
The exploit arrives as an ordinary Microsoft Word document attachment to an e-mail. However, when the document is launched by the user the vulnerability is triggered to drop a backdoor with rootkit features to mask itself from anti-virus scanners.
The SANS ISC (Internet Storm Center) said in a diary entry that it received reports of the exploit from an unnamed organization that was targeted. "The e-mail was written to look like an internal e-mail, including signature. It was addressed by name to the intended victim and not detected by the anti-virus software," said Chris Carboni, an ISC incident handler tracking the attack.
When the .doc attachment is opened, it exploits a previously unknown vulnerability in Microsoft Word and infects a fully patched Windows system. The exploit functioned as a dropper, extracting and launching a Trojan that immediately overwrites the original Word document with a "clean," uninfected copy.
"As a result of the exploit, Word crashes, informs the user of a problem, and offers to attempt to re-open the file. If the user agrees, the new 'clean' file is opened without incident," the ISC explained.
Microsoft has been notified and is working with security researchers to investigate the bug.
Symantec's DeepSight team said the exploit successfully executes shellcode when it is processed by Microsoft Word 2003. The malicious file caused Microsoft Word 2000 to crash, but shellcode execution did not occur.
As a temporary mitigation method, Symantec is recommending that Microsoft Word document e-mail attachments be blocked at the network perimeter. "Furthermore, extreme caution should be exercised while processing Microsoft Word attachments received as an unexpected e-mail Attachment," company officials said.
