Turning the Pushdo Bot Into the Push-oh-no-you-don’t Bot

Options to put a stop to the latest mutation of the Pushdo trojan

The Pushdo bot is a malevolent little beast that is nothing new to Infosec professionals. What might be new, however, is that it recently changed its code and now creates junk SSL connections. Lots of them.

I mean you are likely seeing an unexpected increase in traffic by several million hits spread out across several hundred thousand IP addresses. No you didn't read that wrong that is millions of hits and hundreds of thousands of IP addresses. This might be a big deal if you're used to only getting a few hundred or thousands of hits a day or you don't have unlimited bandwidth. -- ShadowServer 01/29/2010 

Pushdo is usually classified as a "downloader" trojan - meaning its true purpose is to download and install additional malicious software. (SecureWorks, Analysis of a Modern Malware Distribution System) That’s something you definitely don’t want to let loose inside your network, right? So the trick is to recognize its new behavior, somehow, and kick it in the derriere before it can do any real damage or consume resources or leave little bot droppings that might clog up the network pipes.

Luckily, Pushdo has a recognizable pattern: it sends malformed SSL HELLO requests after the TCP connection is established. This means we have several options for dealing with this new variant.