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A new variant of the Pushdo Trojan 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 botnets, computers infected with Pushdo attempt to communicate with their command-and-control server for instructions. The twist here is the botmasters have customized the malware so 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.

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“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 said. “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 end up flooded to the point they go offline. Dell’s CTU team also found Pushdo malware generates a specific HTTP request, which starts with a “?xclzve_” prefix, and can help determine which HTTP requests are legitimate.

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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, Stone-Gross said. Researchers have found compromised machines across the globe, though more cases than not are in Asia, he said.

Once machines have Pushdo, the botnet delivers malicious emails with links to websites that foist banking Trojans, such as Zeus, Torpig and Bugat. Sometimes, the messages look like credit card statements or they contain an attachment disguised as an order confirmation.

The botnet’s master have also struck deals with rogue online pharmacies, in which they drive traffic to these shady companies through links.

“Pushdo is generally spread by drive-by download attacks, which means if you visit a website or you click on a link and your machine is vulnerable to the threat, it can automatically download the malware without you seeing anything,” Stone-Gross said. “A user would not see anything obvious. The only thing they might notice is that their computer runs a bit slower. Pushdo also installs a rootkit and is able to hide other malware, which makes it harder for anti-virus programs to detect.”

Stone-Gross recommended keeping web browsers and software up to date, in particular Adobe Flash, Reader and Java. Most exploits take advantage of vulnerabilities in these commonly used products to infect computers.

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