Thursday, April 27, 2023

LimeRAT Malware Analysis: Extracting the Config

Apr 27, 2023The Hacker NewsMalware Analysis / Cyber Threat

Remote Access Trojans (RATs) have taken the third leading position in ANY. RUN's Q1 2023 report on the most prevalent malware types, making it highly probable that your organization may face this threat.

Though LimeRAT might not be the most well-known RAT family, its versatility is what sets it apart. Capable of carrying out a broad spectrum of malicious activities, it excels not only in data exfiltration, but also in creating DDoS botnets and facilitating crypto mining. Its compact footprint allows it to elude endpoint detection systems, making it a stealthy adversary. Interestingly, LimeRAT shares similarities with njRAT, which ANY.RUN ranks as the third most popular malware family in terms of uploads during Q1 2023.

ANY.RUN researchers have recently conducted an in-depth analysis of a LimeRAT sample and successfully extracted its configuration. In this article, we'll provide a brief overview of that analysis.

Collected artifacts

SHA1 14836dd608efb4a0c552a4f370e5aafb340e2a5d
SHA256 6d08ed6acac230f41d9d6fe2a26245eeaf08c84bc7a66fddc764d82d6786d334
MD5 d36f15bef276fd447e91af6ee9e38b28
SSDEEP 3072:DDiv2GSyn88sH888wQ2wmVgMk/211h36vEcIyNTY4WZd/w1UwIwEoTqPMinXHx+i:XOayy
IOC Description
20[.]199.13.167:8080 LimeRAT's Command and Control server
IOC Description
https://pastebin[.]com/raw/sxNJt2ek PasteBin used by LimeRAT to hide its original Command and Control server
Tactic Technique Description
TA0005: Defense Evasion T1027: Obfuscated Files or Information Malware is using obfuscator to strip its method names, class names, etc.
TA0005: Defense Evasion T1027: Obfuscated Files or Information Malware uses Base64 algorithm to encode and decode data
TA0005: Defense Evasion T1027: Obfuscated Files or Information Malware uses AES algorithm to encrypt and decrypt data

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Breaking down LimeRAT's decryption algorithm

We'll share a condensed version of the article here. For a complete walkthrough and the extended analysis, head over to ANY. RUN's blog if you're interested in learning more about the workflow they employed.

Since the sample under review was written in .NET, researchers utilized DnSpy to examine the code. Immediately, it was obvious that obfuscation techniques were being employed:

LimeRAT Malware
Sample overview in DnSpy; note that use of obfuscation techniques

Closer examination of the code revealed a class resembling the malware configuration. Within this class, was a field containing a string that was both base64 encoded and encrypted.

LimeRAT Malware
Possibly, malware configuration class

Continuing the code inspection, ANY.RUN researchers pinpointed a function responsible for decrypting the string. By employing the "Read by" filter in DnSpy, they tracked down methods where the string was being read, which led to a total of two methods. The first method proved unfruitful, but the second one looked interesting:

LimeRAT Malware
The second x-ref is more interesting. It seems that it uses our string in WebClient.DownloadString method

This method turned out to be responsible for decryption. By closely examining it, it was possible to reconstruct the process by which LimeRAT decrypts its configuration:

  1. Instances of the RijndaelManaged and MD5CryptoServiceProvider classes are instantiated. As per MSDN, RijndaelManaged is an outdated implementation of the AES encryption algorithm (MITRE T1027), while MD5CryptoServiceProvider computes MD5 hashes.
  2. A 32-byte array, initialized with zeros, is generated to store the AES key.
  3. The key is created by first calculating the MD5 hash of a distinct string within the configuration class (in our analysis, the string is "20[.]199.13.167").
  4. The initial 15 bytes, followed by the first 16 bytes of the calculated hash, are copied into the previously established array. The final element of the array remains zero.
  5. The derived key is assigned to the key property of the RijndaelManaged instance, while the Mode property is configured as CipherMode.ECB.
  6. Ultimately, the primary string undergoes decoding via the Base64 algorithm and decryption using the AES256-ECB algorithm.

Decrypting the string revealed a link to a PasteBin note: https://pastebin[.]com/raw/sxNJt2ek. Within this note, was LimeRAT's Command and Control (C2) server:

LimeRAT Malware
LimeRATs C2 discovered with decrypted data

To wrap up

We hope you found this brief overview of our LimeRAT configuration decryption process insightful. For a more comprehensive examination, head over to the full article on ANY.RUN's blog, to get additional context on the steps and check the decryption process using CyberChef.

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from The Hacker News

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