Virus writers are targeting Skype users with a new Trojan that poses as the latest version of the popular VoIP software.

Net security firm MessageLabs has detected and blocked more than 800 copies of a new variant of the MyTob (AKA Fanbot) Trojan, which is being distributed by email. References and nicknames contained in the code lead MessageLabs to conclude that the malware was probably created by a well known Chinese black hat hacker and not the original author of the malware strain.

Maksym Schipka, a senior antivirus researcher at MessageLabs, said the malware is the first he's aware of that mentions Skype and as such represents a new theme for social engineering attacks.

The malware arrives in an attachment in messages posing as the latest (v1.4) release of Skype. Legitimate downloads of the software only came out last week, so the attack is timely. If users open the infected payload on a vulnerable Windows machine they will find their PCs transformed into zombie clients (theoretically at least) under the control of computer hackers.

Schipka said that compromised machines fail to connect to IRC servers so they are not much use to the bad guys, right now. He added that the release of a variant of MyTob by someone other than the original author showed the source code was available in at least "some circles" of the computer underground. ®