New Pushdo variant infects more than 100k computers

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A new variant of the revived Pushdo trojan has infected more than 100,000 computers since the beginning of August, and it's using a new technique to trip up researchers trying to study the botnet.

As is the case with most botnet scenarios, computers that are infected with Pushdo attempt to communicate with their command-and-control server for instructions. The twist here is that the botmasters have customized the malware so that it simultaneously delivers HTTP requests to some 300 lesser known, but legitimate, websites, which mixes in with traffic meant for the command-and-control hub, said Brett Stone-Gross, a senior security researcher at Dell SecureWorks Counter Threat Unit.

"The purpose of the HTTP requests to legitimate sites is to make it harder to identify C2 (command-and-control) traffic, [which] also uses HTTP," Stone-Gross told SCMagazine.com on Wednesday. "As a result, security researchers have to sift through all of the HTTP requests in order to locate the C2, which is important for detection and mitigation efforts."

In some cases, the sites receiving the bogus HTTP traffic are flooded to the point that they are knocked offline. Dell's CTU team also found that Pushdo malware generates a specific HTTP request, which starts with a “?xclzve_” prefix, and can be used to help determine which HTTP requests are legitimate.

Previous versions of Pushdo have taken a similar tactic, but they sent traffic to very popular sites, like Yahoo, Twitter and the FBI. Targeting smaller sites enables the botnet to fly under the radar, Stone-Gross said.

Researchers at Dell SecureWorks have been tracking the new Pushdo threat since late July, according to a blog post on Friday by Stone-Gross. He told SCMagazine.com that compromised machines have been detected across the globe, though many cases have been concentrated in Asia. 

Once machines are infected with Pushdo, the botnet is used to deliver malicious emails with links to websites that foist banking trojans, such as Zeus, Torpig and Bugat. Sometimes, the messages are made to look like credit card statements or they contain an attachment disguised as an order confirmation.

The botnet's purveyors also have been known to strike up deals with rogue online pharmacies, in which they are paid to drive traffic to these shady companies through links.

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