Yahoo has issued a security update for a flaw in Messenger's ActiveX control.

The vulnerability is located in YMailAttach Active X control, which is provided by ymmapi.dll, according to an advisory released by US-CERT (the United States Computer Emergency Readiness Team).

An attacker can exploit the vulnerability to execute arbitrary code onto an affected PC, causing web browsers to crash. To infect a PC, a malicious user would have to convince a victim - most likely by using social engineering - to view a specially crafted HTML email message or attachment, according to US-CERT.

Yahoo urged users who have a Windows version of Messenger obtained before Nov. 2 to update. No exploit code has been released for the flaw, according to a Yahoo advisory.

Messenger users will be prompted to update every time they sign on, according to the Sunnyvale, Calif. web giant.

US-CERT also issued a workaround for the flaw, advising users to disable ActiveX controls in the Internet Zone.

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