Cybercrooks behind the resilient Pushdo botnet are bombarding legitimate small websites with bogus traffic in order to camouflage requests to the zombie network's command and control servers.
A varying cast of around 300 genuine (mostly smaller) websites is at the receiving end of queries from zombie drones infected with the latest variant of Pushdo, according to Brett Stone-Gross, a senior security researcher at Dell SecureWorks' Counter Threat Unit (CTU).
"In some cases these fake web server requests are overwhelming small web sites," Stone-Gross explained.
Previous variants of the malware that appeared around two years ago spewed bogus requests buried in SSL traffic to large websites (CIA, PayPal etc) - for reasons that have never been adequately explained. The latest behaviour is arguably even more noxious because it might easily be to blame for exhausting the bandwidth limits of small websites.
"The purpose of these fake HTTP requests is to make Pushdo's command and control (C2) traffic, which also uses HTTP, blend in with legitimate traffic," a blog post by Dell SecureWorks on the latest Pushdo botnet trickery explains.
The Pushdo botnet first appeared around 2007 and has been associated with the spam-pushing Cutwail botnet throughout that time. Stone-Gross reckons both Pushdo and Cutwail are the work of the same well-resourced eastern European gang. Pushdo creates a backdoor onto infected machines through which bot-herders can push Cutwail. The same approach has also been used to distribute variants of the infamous ZeuS banking Trojan and other similar nasties.
Despite four takedowns over five years Pushdo continues to flourish.
"The group behind the botnet is well funded and every takedown is only temporary," Stone-Gross said. The botmasters behind Pushdo have recently begun sending spam to eastern Europe and Russia: but not the phishing emails or malware that users in the west are bombarded with.
The latest variant of Pushdo is using a combination of spam and drive-by downloads to spread. The population of infected drones has varied from 112,000 to 200,000 bots in recent months, according to monitoring stats from Dell SecureWorks. ®
Fake HTTP GET/POST requests generated by the latest variant of Pushdo variant can be spotted by the anomalous requests that follow the format "http : //<domain>/?xclzve_[30 random characters]". Website owners affected by Pushdo may filter the requests using a web server rule dropping traffic that matches this pattern.