At least two attacks against an “extremely critical” vulnerability in Yahoo Messenger have been published.
Shortly after eEye Digital Security notified Yahoo that its Messenger IM client was vulnerable to attack this week, another researcher found two flawed ActiveX controls and publicised exploit code that can be used to hijack Windows machines.
eEye's advisory was vague about flaws but the researcher spelt out details of the vulnerability on a Full-Disclosure mailing list.
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 rated the Messenger bugs as "extremely critical" - its highest-possible threat ranking.
Until Yahoo provides a patch, eEye said the only work-around defence 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, 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.