In a series of posts on his Security Fix blog, Brian Krebs provides a good explanation of how the Srizbi botnet was able to come back to life after being killed (we thought!) earlier this month.

“The botnet Srizbi was knocked offline Nov. 11 along with Web-hosting firm McColo, which Internet security experts say hosted machines that controlled the flow of 75 percent of the world’s spam. One security firm, FireEye, thought it had found a way to prevent the botnet from coming back online by registering domain names it thought Srizbi was likely to target. But when that approach became too costly for the firm, they had to abandon their efforts.”

In a example of good distributed programming design, the botnet had a backup plan if its control servers were taken down.

“The malware contained a mathematical algorithm that generates a random but unique Web site domain name that the bots would be instructed to check for new instructions and software updates from its authors. Shortly after McColo was taken offline, researchers at FireEye said they deciphered the instructions that told computers infected with Srizbi which domains to seek out. FireEye researchers thought this presented a unique opportunity: If they could figure out what those rescue domains would be going forward, anyone could register or otherwise set aside those domains to prevent the Srizbi authors from regaining control over their massive herd of infected machines.”

Unfortunately, FireEye did not have the resources to carry out its plan and was forced to abandon it, but not before seeking help from other companies and organizations with deeper pockets.

“A week ago, FireEye researcher Lanstein said they were looking for someone else to register the domain names that the Srizbi bots might try to contact to revive themselves. He said they approached other companies such as VeriSign Inc. and Microsoft Corp. After FireEye abandoned its efforts, some other members of the computer security community said they reached out for help from the United States Computer Emergency Readiness Team, or US-CERT, a partnership between the Department of Homeland Security and the private sector to combat cypersecurity threats.

File this one under opportunity, lost.