The "activation" of Windows machines infected with the latest variant of the Conficker worm has allowed security watchers to come up with a far more accurate estimate of how many machines are infected.
Early versions of Conficker called home to 250 different domain names every day to check for updates. Since Wednesday, machines infected by with the latest version of the worm (Conficker-C) began using a sample of 500 out of pre-programmed 50,000 domains a day to search for upgrades.
The unknown virus writers who created the worm are yet to publish any such update, but the call-back behaviour has allowed anti-virus firms to come up with an estimate of how many machines are infected by Conficker-C for the first time.
Vietnamese antivirus firm Bkis reckons 1.3m machines are infected with Conficker-C. A breakdown of infections by country, compiled by Bkis, can be found here. The combined number of computers infected by Conficker A and B is 2.2m, according to Bkis.
That total of around 3.5m is in line with a detailed technical analysis by Conficker which puts the size of the Conficker botnet at between three and four million strong.
Estimates of the number of machines ever infected by Conficker vary from ten to 15 million, but these figures ignored disinfections and other factors. It's more meaningful to talk of the current number of zombie drones rather than the number ever infected, because it gives a much better idea of the potential for harm.
As predicted by security watchers beforehand, Conficker's 1 April "activation" passed by without anything happening, much like previous malware trigger dates associated with nuisances such as the Michelangelo virus (1992), CIH (1999), SoBig (2003), and MyDoom (2004). The unknown authors of Conficker didn't publish updates on 1 April, but since infected machines check for updates on a daily basis it would be wrong to think that the threat has passed. Updates using the P2P mechanism built into the worm are also possible.
F-Secure has compiled an updated, informative FAQ on Conficker here, which cover important questions such as how to tell if you're infected and much more besides. Conficker blocks access to security sites, a factor exploited by Joe Stewart of the Conficker Working Group to create a simple test here. ®