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Who says you can’t teach old malware new tricks? Recently, we reported on how ZBOT had made a comeback of sorts in 2013; this was followed by media reports that it was now spreading via Facebook. Now, we have spotted a new ZBOT variant that can spread on its own.

This particular ZBOT variant arrives through a malicious PDF file disguised as a sales invoice document. If the user opens this file using Adobe Reader, it triggers an exploit which causes the following pop-up window to appear:

Figure 1. The error message upon execution of the malicious PDF file

While this is going on, the malicious ZBOT variant – WORM_ZBOT.GJ – is dropped onto the system and run. It is here that several differences start to appear.

First of all, WORM_ZBOT.GJ has an autoupdate routine: it can download and run an updated copy of itself. Secondly, however, it can spread onto other systems via removable drives, like USB thumb drives.

It does thus by searching for removable drives and then creating a hidden folder with a copy of itself inside this folder, and a shortcut pointing to the hidden ZBOT copy.

Figure 2. WORM_ZBOT.GJ Infection Chain

Figure 3. Portion of WORM_ZBOT.GJ code creating copy of itself

This kind of propagation by ZBOT is unusual, to say the least. ZBOT malware is usually distributed by exploit kits and/or malicious attachments. This kind of behavior from ZBOT is not something we’d usually expect to see; it could mean an increase in ZBOT infections moving forward. This unusual behavior, however, is in line with our 2013 predictions, which noted that we believed that older threats would continue to be seen in 2013, although they would have refinements to make them more effective. In the past, some ZBOT variants have used unusual means – like file infectors - to spread as well.

These threats are some of the newest changes to the crimeware landscape; we earlier documented this particular threat in the paper The Crimeware Evolution. Neither propagating via removable drives nor auto-updating are particularly novel; many malware threats have used these behaviors in the past. Most notably, the Conficker/DOWNAD threat used both of these quite effectively; to this day it is a significant threat. It’s still in the top 10 malware in the Americas and Carribean in 2012, despite it being several years old.

We protect Trend Micro users by detecting WORM_ZBOT.GJ, as well as as blocking websites related to this threat. We will provide additional information if necessary to protect our users.

With analysis from Threat Response Engineers Joie Salvio and Alvin Bacani