A Trojan is masquerading as Microsoft Security Essentials (MSE) in an attempt to trick people into downloading fake security products, warns security firm F-Secure.
The malware is distributed by drive-by-download attacks, according to Mikko Hyppönen, chief research officer at F-Secure.
The bogus MSE tries to scare victims into purchasing equally fake security software to fix fictitious infections, he said in a blog post.
The fake MSE found by F-Secure looks very much like a similar bogus version reported by Microsoft in February.
All successful anti-malware seems to be mimicked at some point, and with 30 million users worldwide, clearly MSE is being very successful, said Cliff Evans, head of security and privacy at Microsoft UK.
"Rogue Security Software has become one of the most common methods used to swindle money from victims," he told Computer Weekly.
"Our advice is for organisations and individuals to maintain an up-to-date anti-malware solution from a trusted supplier," he said.