In this article, ANY.RUN delves into the inner workings of PrivateLoader, a notorious malicious loader family. We will explore the encryption and decryption processes utilized by this malware, particularly focusing on its ability to protect itself using VMProtect, as well as its decryption of loaded libraries. Let’s dive in!

PrivateLoader analysis Introduction

PrivateLoader is a malicious loader family, written in C++ and first discovered in early 2021.

It is known for distributing a wide range of malware, from simple information stealers to complex rootkits and spyware, utilizing payloads.

The distribution of this type of malware is managed by the Pay-Per-Install (PPI) service, a popular tool within the cybercriminal ecosystem that generates revenue by adding payloads to malware.

• The code itself involves the decryption of loaded libraries.
• At present, there are two versions of PrivateLoader available: one protected by VMProtect, and a regular version.
• Every day, between 2 and 4 samples of this malware are uploaded.

Static Analysis of the Source File

SHA256: 27c1ed01c767f504642801a7e7a7de8d87dbc87dee88fbc5f6adb99f069afde4

Using the Detect It Easy utility, we can see that the analyzed executable file is compiled in C++. There is no information about the packer, which could mean it was not possible to identify it.

Fig. 1 – PrivateLoader’s sample data

The next step is to search for unencrypted strings using the strings command:

strings –encoding=l loader.exe

Fig. 2 – Interesting strings detected in the executable file

Analyzing the discovered strings allows us to identify several interesting elements:

• A user-agent, which is likely used to masquerade as a legitimate browser application
• URL addresses for determining the current IP and geolocation

PrivateLoader dynamic analysis with ANY.RUN

We analyzed the sample in ANY.RUN interactive malware sandbox.

Here’s a link to the task: https://app.any.run/tasks/3e359dc7-934b-4ae1-89bf-ad33e346ed60/?utm_source=md&utm_medium=article&utm_campaign=private&utm_content=task

The process tree generated by the executable file appears as follows:

Fig 3. – Process tree

Analyzing the process tree leads to the following conclusions:

1. The main PrivateLoader process creates a child process named “FhuC750omh76YtB1xgR7diEy.exe”, whose executable file is located in the user’s “Pictures” directory (T1564 – Hide Artifacts):

C:\Users\admin\Pictures\Minor Policy

2. The created child process is added to the startup using Task Scheduler (T1053.005 – Scheduled Task/Job: Scheduled Task):

schtasks /create /f /RU “admin” /tr “”C:\Program Files (x86)\ClipManagerP0\ClipManager_Svc.exe”” /tn “LOLPA4DESK HR” /sc HOURLY /rl HIGHEST

The executable file of the child process was downloaded from the Internet (T1105 – Ingress Tool Transfer). We will not go into the detailed analysis of it.

Fig 4. – downloaded payload

Analyzing the HTTP requests, we can observe connections and data exchanges with the C2 server (T1071.001 – Application Layer Protocol):

Fig. 5 – С2 addresses

The content sent (as well as received) in POST requests consists of BASE64-encoded strings (T1132.001 – Data Encoding: Standard Encoding). Decoding these strings does not yield any readable results:

data=-kSYhy9HPjD5Jhn9y6Evty4XFfJ3JgIwrSzln5bGnLfKDmbXix2ebDEXy6Ty3Bb8Hz2GB8w0Y2SL2JeBSZ4G80iHAkSS7JJyeiPwZOpWJONOFzEBarRHP-ljR9hkvX_TJhqr1nNqQpYUB2lQ9i7NmmHeL_QSx8hUka_C3jOxi02ml5FyDDruXM_IWwPXvAGxtT8TV-i9wLtfd0mF1O369GUAEeI45sF1pKeyDfssmqE=

Moving forward to the indicators, we can see that the malware steals user credentials from browsers (T1552.001 Credentials In Files):

Fig. 6 – Stealing data

Technical Analysis of PrivateLoader

For the technical analysis, the following tasks were set:

1. Locate the C2 server within the code
2. Identify the encryption algorithms for the C2 server and, if possible, for strings as well.
3. Automate the decryption of the C2 server and strings

The analysis of the executable file revealed that string encryption is done using the XOR algorithm (T1027 – Obfuscated Files or Information). Initially, the data and key are loaded into the stack, and then decrypted using the SIMD instruction “PXOR” and the “XMM” register. The result of the XOR operation is also stored in the stack.

The three stages of C2 server decryption are shown below.

1. Loading encrypted data into the stack:

Fig. 7 – Data

• Loading the encryption key into the stack:

Fig. 8 – Key

• Decrypting the C2 server using the “PXOR” instruction and saving the results in the stack:

Fig. 9 – Decrypting

During the analysis process, it was also found that the method similar to C2 decryption is used to decrypt the following:

• Used API functions (T1027.007 – Obfuscated Files or Information: Dynamic API Resolution)
• Payloads
• URLs and more.

Some of the analyzed samples are protected by VMProtect. The search for string decryption is complicated by the fact that the decryption data is located in one function, while the XOR and key are in another. Moreover, the key is always the same.

Fig. 10 – Decript VMprotect sample

Example of automating C2 server decryption of PrivateLoader

To automate the extraction of data and configuration, we can use the Triton framework. It will emulate code blocks that contain all the necessary encrypted information

You can find an example of a script for emulating a specific block in our GitHub repository. The output of the script will be the decrypted C2 server.

Fig 11. – Script output

Therefore, by emulating all the code blocks that contain encrypted data, we can obtain a set of strings with the necessary information, including the C2 server.

Extracting the PrivateLoader configuration

In our service, you can view the configuration, which is extracted automatically:

Fig. 12 – PrivateLoader configuration and strings

The decrypted data includes C2 addresses and strings. The strings contain information such as: used libraries and their functions, registry keys, paths to crypto wallets and browsers, etc.

Conclusion

In this article, we discussed encryption in PrivateLoader.

Its main feature is the XOR of all strings it interacts with (C2, URLs, DLLs). Also, some samples are protected by VMProtect, which makes the code a bit more complex due to the use of many functions.

MITRE (ARMATTACK)

IOCs

Extracted URLs

• http://23[.]254[.]227[.]214/api/tracemap[.]php
• http://23[.]254[.]227[.]205/api/tracemap[.]php
• http://23[.]254[.]227[.]202/api/tracemap[.]php
• http://208[.]67[.]104[.]60/api/tracemap[.]php
• http://208[.]67[.]104[.]60/api/firegate[.]php
• http://163[.]123[.]143[.]4/download/YT_Client[.]exe

Dropped executable file

DNS requests

• iplogger.org
• ipinfo.io
• Iplis.ru

Connections (IP)

• “23[.]254.227.214”
• “23[.]254.227.202”
• “23[.]254.227.205”
•  “208[.]67.104.60”