The company said the update to the widely used virus removal program that's free for Windows users should make it harder for the many criminals who use Zeus to keep running their software on computers that don't have antivirus software installed - often an easy target up until now.
According to a September 2009 study by security vendor Trusteer, 45 percent of Zeus-infected machines have either no antivirus software or an out-of-date product. On the other hand, Zeus has been effective at avoiding the type of detection that Microsoft is now adding to its MSRT. According to that same report, 55 percent of Zeus infections were on machines that did have working antivirus programs installed.
In a series of raids starting September 28, authorities in the UK, along with the US and Ukraine arrested more than 100 members of the largest-known Zeus gang, but there are still probably dozens of smaller gangs in operation. Zeus is very easy to obtain online, and it has been adapted by many different criminals since it first popped up four years ago.
"Underground forums are teeming with questions ranging from the very basics about configuring the malware to people boasting about the size of their botnets," said Matt McCormack, a Microsoft spokesman, in a blog. "Even the botnet controllers are themselves quite varied, from apparent hobbyists to those that likely have more nefarious intent."
The software is best known for stealing online banking credentials, but recently security experts have started to worry that it could be used to steal corporate secrets as well.
Microsoft's decision to add MSRT protection has had a big effect on some malicious programs. It's credited with pretty much knocking the Storm Worm offline in 2007, for example.
Microsoft clearly hopes to have a similar effect on Zeus, also known as Zbot. "We find ourselves knocking on Zbot's door this month, and we're glad we are," McCormack said.
See also: Zeus botnet hits 100,000 UK computers