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Trojan Vundo/Virtumonde turns a good file into a Trojan-Dropper

VirusList posted about new variant Trojan Vundo/Virtumonde. Vundo Authors are now using file infection so Virtumonde checks which files run at Windows startup and tries to infect them. Effectively this means that Virtumonde turns the original host file into a Trojan-Dropper.

Dropper code is prepended to the original host file, with a copy of Virtumonde being appended to the same file. When the infected file is launched it drops the original host file to %temp% and the Virtumonde file to the system directory.

Although Virtumonde is using an infection marker to prevent re-infecting the same file over and over again, this doesn’t always work. There are samples of already infected files being re-infected and the host file then won’t run. However, re-infection doesn’t prevent Virtumonde itself from running.