Yahoo Messenger Zero-Day Exploits on the Loose

Shortly after eEye Digital Security notified Yahoo Inc. Wednesday that the portal's Messenger IM client was vulnerable to attack, a researcher fingered two ActiveX controls as flawed and posted exploit code that can be used to hijack Windows machines.

Although eEye's advisory was vague about details -- it said Messenger's Webcam ActiveX control was at fault -- the researcher laid all bare on the full-disclosure list.

The researcher, who went by the name "Danny," cited "45 minutes of fuzzing!" in a post Wednesday about the flaw. In a follow-up today, Danny published a second exploit. "This affects the viewer ywcvwr.dll with yahoo messenger," he said.

Aliso Viejo, Calif.-based eEye called the Yahoo Messenger bugs serious. "ActiveX remote code execution vulnerabilities have very high impacts since the source of the malicious payload can be any site," the security vendor said. "An even more critical problem is generated when clients are administrators on their local hosts, which would run the malicious payload with administrator credentials."

Most Windows XP users run in administrator mode.

Danish vulnerability tracker Secunia ApS rated the Messenger bugs as "extremely critical" -- its highest-possible threat ranking.

Until Yahoo provides a patch, eEye said the only work-around defense is to set the kill-bit for the two Yahoo ActiveX controls. However, because that involves manually editing the Windows registry, it's not a tactic most users will feel comfortable doing. Microsoft Corp., which in the past has recommended kill-bitting to temporarily protect users against vulnerabilities in Internet Explorer and its other software, has offered a set of technical instructions on setting kill bits.

Yahoo has not yet posted a fix for the flaws to its security update page. The last Messenger bug, also because of a vulnerable ActiveX control, was fixed in April.