The crooks behind the Pushdo botnet agent have developed variants of the malware that are more resistant to take-down attempts or hijacking by rival hackers.
Dell SecureWorks and Damballa warned (PDF) on Wednesday that the latest variant of Pushdo comes packed with a fallback mechanism for cases where zombie clients are unable to contact the main command-and-control server for whatever reason.
The malware starts by using a Domain Generation Algorithm (DGA) to come up with a list of 1,380 unique domains to poll on any particular day. Bot-herders can thus restore control of compromised hosts by leaving updated malware and instructions available for download at any of these domains.
However after the first DGA involved was exposed, security researchers began to work hard at developing countermeasures that block communication to the generated .COM domains. But it seems the nimble cybercrooks behind Pushdo were alive to that possibility and have already adapted, according to Aviv Raff, CTO of Seculert.
"The group behind Pushdo probably figured out that they are being investigated by the security vendors, because it didn’t take them too long to adapt to this new reality and change their Domain Generation Algorithm," Raff explains in a blog post.
"This new DGA now generates .KZ domains instead of .COM domains. Not only that but there are now at least two new variants of Pushdo that are being pushed to victims from several different hijacked websites."
This latest development is likely to kick off a further round of cat-and-mouse games between Pushdo's cybercrooks and security researchers.
Pushdo has been used to distribute other malware such as ZeuS and SpyEye, as well as conduct spam/phishing campaigns with its Cutwail module. Despite four takedowns in five years of Pushdo command-and-control servers, the botnet (believed to be run by a single Eastern European hacker group) endures.
The malware is responsible between 175,000 and 500,000 active bots on any given day. The botnet is typically used to deliver malicious emails with links to websites that foist banking Trojans upon unsuspecting victims. Sometimes, the messages are made to look like credit card statements or they contain an attachment disguised as an order confirmation.
As well as applying new secondary recovery techniques, the unknown crooks behind Pushdo have begun masking command and controller traffic using a fake JPEG image file, said the researchers. They have also made greater use of encryption.
A blog post by Damballa giving more background on Pushdo and how the latest variants were uncovered can be found here. David Dagon of the Georgia Institute of Technology worked together with three researchers from Damballa and one from Dell SecureWorks Counter Threat Unit in researching the latest form of the malware. ®