Malware stays around on infected PCs far longer than previously thought, according to the latest research from Trend Micro.

Previous estimates suggested that a compromised machine remains infected for approximately six weeks. Based on an analysis of around 100 million compromised IPs, Trend Micro concludes that many infected IPs are infected (or repeatedly infected) for more than two years, with a median infection length of 300 days. Four in five compromised machines are infected for more than a month.

A graph from Trend Micro suggests that if systems aren't disinfected quickly then infection tends to linger around indefinitely, possibly until the point users exchange compromised boxes for new machines:



Trend's study also looked at the botnet landscape. Three strains of botnet agent - Koobface, Zeus/Zbot and Ilomo/Clampi - are causing the most damage in terms of identity theft.

The Koobface botnet, for example, has co-opted around 51,000 machines into its ranks. Koobface uses between five and six command and control centers (C&C) to control these zombie clients at any one time. If a particular control domain is taken down by a particular provider, then botnet herders behind the malware establishes a new command outpost elsewhere. Between the middle of March and mid-August 2009, Trend Micro recorded around 46 Koobface control domains.

Trend's stats don't cover dormant infections by the Conficker worm. Stats from the Anti-Conficker Working Group suggest the malware is still resident on more than five million compromised machines: http://www.confickerworkinggroup.org/wiki/pmwiki.php/ANY/InfectionTracking

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