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Wednesday (9 January) marks the fifth anniversary of the SoBig-A virus, an item of malware experts reckon marked the transition to money rather than mischief as the main drive behind malware creation.

While it wasn't until August 2003 that a variant of the malware (SoBig-F) caused disruption on a massive scale, the first iteration of the virus in January began what became a steadily evolution in cybercrime. SoBig "irrevocably changed" the malware landscape by heralding the introduction of the botnet phenomenon, according to email security firm MessageLabs.

The SoBig worm in its various guises commonly appeared as an attachment to electronic messages with subject lines such as "Re: Approved", "Thank you!", or "Re: That movie". The body of email messages containing text such as "See the attached file for details" designed to tempt prospective marks into infectious .pif or screensaver files. The rudimentary social engineering technique was then new but didn't really take off until a sequel of the worm released some months after the first variant of the malware hit the net.

The driving force behind all variants of SoBig was to distribute self-replicating Trojans which created botnets of compromised zombie PCs, useful for the distribution of spam or other nefarious purposes. Following the introduction of anti-spam legislation in 2003, botnets provided the anonymity that spammers required and the increasing penetration of always-on home broadband networks made them increasingly effective as a distribution tool. The increased ineffectiveness of older techniques - such as open mail relays - in a face of evolving spam filtering technologies helped further fuel the transition to a new cybercrime economy based on trade in compromised PCs and hacker tools.

Five years on, the malware landscape has become even more sophisticated. Recent months have seen the evolution of the Storm worm Trojan and other sophisticated "professionally developed" botnet clients, such as Nugache, a new malware strain that can be controlled without use of a command and control server.