The Monti ransomware, known for its dual Windows and Linux-based variants, garnered attention in June 2022 due to its uncanny resemblance to the notorious Conti ransomware. This not only included a shared name but also replicated tactics and techniques employed by Conti. Operating under the moniker “Monti,” the group intentionally adopted Conti’s methods, incorporating a significant portion of their tools and even utilizing leaked Conti source code.
Having continuously targeted various companies since its discovery, the Monti group recently recommenced its malicious activities, focusing on legal and governmental sectors. Simultaneously, a fresh Linux-based variant of Monti (Ransom.Linux.MONTI.THGOCBC) has surfaced, displaying substantial deviations from its predecessors, particularly in its choice of encryptor and additional behaviors. It’s noteworthy that only three security vendors flagged the sample as malicious on VirusTotal as of the current writing.
Comparing the newly emerged Linux variant to its older counterpart using BinDiff, a similarity rate of 29% was revealed, in stark contrast to the 99% similarity rate shared among older variants and Conti ransomware. An analysis of the new Linux variant uncovered distinct changes in its command line arguments, incorporating new parameters and omitting others from its predecessor.
Notably, this version employs the -type=soft parameter, indicative of a strategy to reduce immediate detection risks while executing activities, as opposed to the earlier –type=hard parameter. The Monti group’s manipulation of key files such as /etc/motd and index.html replaces their contents with ransom notes, notifying victims of successful server infiltration. Among the additions in this new variant is an “infection marker” consisting of appended bytes “MONTI” followed by an additional 256 bytes linked to the encryption key.
Further investigation revealed the malware’s file size-based encryption process, differing from the earlier variant’s use of a –size argument. This new variant solely relies on the file size for encryption determination, with the ransomware encrypting files differently based on their size. Notably, files exceeding 4.19MB employ a Shift Right operation to calculate total encryption size, while files smaller than 1.048MB have all their content encrypted. The new version continues the trend of appending the .monti extension to encrypted files and distributing its ransom note readme.txt throughout directories.
An interesting discovery during sample analysis was a decryption code, suggesting testing of functionality by the threat actor. However, this code remains ineffective without the private key known only to the malware author, having no connection to the malware’s routine.
In conclusion, while evidence suggests that Monti’s operators integrated portions of Conti’s source code as the foundation for this new variant, significant modifications have been made, especially to the encryption algorithm. This strategic shift enhances the malware’s ability to evade detection, posing a greater challenge for identification and mitigation.
To protect against such ransomware attacks, organizations are strongly advised to adopt robust defense strategies encompassing data safeguarding protocols, along with comprehensive backup and recovery procedures, ensuring data security and potential restoration in the face of encryption or deletion threats.
