Beware a malicious Firefox browser add-on that infects PCs and makes them scan websites for known exploitable vulnerabilities.

The Firefox plugin, which has infected more than 12,500 systems to date, ties into what its developers have dubbed the "Advanced Power" botnet, according to security reporter Brian Krebs, who first reported on the attack campaign.

The add-on has been in circulation since at least May 31, 2013, according to malware analysis service Malwr. When the malware was first spotted, only two out of 47 antivirus engines were recognizing it as malicious code, according to Virus Total. By August 2013, however, 29 out of 45 antivirus scanning engines were flagging the code as malicious.

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