Would-be intruders already have attempted to compromise PCs at a Japanese government entity by exploiting the flaw, Vincent Weafer, the senior director at Symantec Security Response, said in an interview. In response, Symantec has raised its ThreatCon to Level 2, which means an outbreak is expected.
"What we're seeing is a continuation of the targeted threat using zero-day vulnerabilities," Weafer said. (Zero-day flaws are ones for which no patch exists.) "We got it from a single large customer inside Japan. We have not seen anyone else get it."
Microsoft is readyingthat repairs this vulnerability, a company representative said in an e-mailed statement. The fix is scheduled to be released as part of the June 13 security updates, or sooner, if warranted, the representative said.
The malicious software arrives as a Microsoft Word file attachment to an e-mail message. When the document is opened by the user, the vulnerability is triggered. In the Japanese case, the Word document actually displayed some text related to a treaty with China, but while the text was displayed, a backdoor was installed on the system, Weafer said. Backdoor software allows intruders to enter computers surreptitiously.
"The backdoor in turn pings an IP address located in Asia. It just pings to say it is available, but then, of course, you have a backdoor on your system," he said.
The vulnerability was confirmed in Word 2003, Symantec said. The malicious file caused Word 2000 to crash, but did not run the malicious payload, it added.
Exploitation of the security hole so far is only known as part of a single, targeted attack, Symantec said. "However, with the disclosure of this previously unknown vulnerability, new attackers may begin to exploit it in a widespread manner," the Cupertino, Calif., security company said in an advisory sent to customers.
The targeted attack can bypass spam filters, and Symantec's antivirus software doesn't yet detect the particular Word file as malicious, Weafer said. "We are looking at the vulnerability itself, in terms of generic blocking," he said, adding that the security software does detect the backdoor and the installer of the backdoor.
Microsoft and Symantec urge caution in the opening of Word documents received as an unexpected e-mail attachment.