TROJ_UPATRE, the most common malware threat distributed via spam, is known for downloading encrypted Gameover ZeuS onto affected systems. This ZeuS variant, in turn, is known for its use of peer-to-peer connections to its command-and-contol (C&C) servers. This behavior has been known about since October 2013.
We have observed that these specific ZeuS variants are now employing non-binary files. The UPATRE downloader is also responsible for decrypting these malicious files. This is done to bypass security features and avoid detection and removal from the infected systems.
Previously, ZBOT malware can be detected via its header with ZZP0 even though it is initially encrypted by UPATRE. However, in our recent findings, it is found that ZeuS dropped this header and now uses random headers and changed its file extension, thus making it arduous to be detected in the network.
Here is one particular example of these random headers:
Figure 1. Random headers
The decryption of these files starts from the 5th byte onwards using a key found in the TROJ_UPATRE downloader. The sample shown below uses 1b23fed8h as the XOR key. After performing the XOR routine, the file shows compressed version of the executable file. The XOR key used for decryption varies in each sample.
Figure 2. Portion to be decrypted highlighted
To get the actual executable file, the UPATRE variant uses the RtlDecompressBuffer function on the decrypted portion of the file. You end up with the original executable, as seen below.
Figure 3. Original executable
UPATRE is continuously developing not only in terms of effective social engineering lures such as the abuse of Dropbox links to lead to ZBOT, NECURS, and just recently, Cryptolocker. This improvement can also be seen in the use of XOR key to decrypt the downloaded file.
We can say that the cybercriminals behind UPATRE are aware that their tactic of encrypted downloaded file is already detected by security solutions. As such, they continually modify their algorithm to circumvent efforts to detect and mitigate the risk posed by UPATRE. We’re monitoring this threat for any further developments.
As a downloader, the main function of UPATRE is to deliver the main payload: Gameover ZeuS. In the past, the Pony loader and Cutwail spam botnet was used to download GoZ malware. Just recently, Trend Micro researchers took part in the efforts of FBI to takedown activities of Gameover ZeuS. While these efforts significantly affected the operations of Gameover ZeuS, we’re continuously monitoring and investigating these threats (ZeuS, UPATRE) so as to proactively protect our users.
With additional insights from Lord Alfred Remorin
The SHA1 hashes of the files with this behavior we’ve seen are: