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An unpatched Yahoo Messenger vulnerability allows attackers to change people’s status messages and possibly perform other unauthorized actions like sending malicious links to a large number of users.

Security researchers from antivirus vendor BitDefender found the vulnerability while investigating a customer’s report about unusual Yahoo Messenger behavior.

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The flaw appears to be in the application’s file transfer application programming interface (API) and allows attackers to send malformed requests that result in the execution of commands without any interaction from victims.

“An attacker can write a script in less than 50 lines of code to malform the message sent via the YIM protocol to the victim,” said Bogdan Botezatu, an e-threats analysis & communication specialist at BitDefender.

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“Status changing appears to be only one of the things the attacker can abuse. We’re currently investigating what other things they may achieve,” he said.

Victims are unlikely to realize their status messages have changed and if they use version 11.5 of Yahoo Messenger, which supports tabbed conversations, they might not even spot the rogue requests, Botezatu said.

Attackers can leverage the vulnerability to earn money through affiliate marketing schemes by driving traffic to certain websites or to spam malicious links that point to drive-by download pages.

Drive-by download attacks exploit unpatched vulnerabilities in browser plug-ins like Java, Flash Player, or Adobe Reader, and are currently one of the primary methods of distributing malware.

Links included in status messages usually have a high click-through rate because they go out to the victim’s friends. This means that URLs spammed in this way will get click throughs from most of their contacts, the BitDefender researchers said.

According to the antivirus vendor, Yahoo is now aware of the vulnerability through the appropriate channels.

Until there is a patch for this vulnerability is fixed, users can protect themselves by configuring Yahoo Messenger to ignore anyone who is not in their Messenger list. This option will not prevent attacks from their current contacts.

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