PyCL Ransomware Test Campaign Spotted in the Wild

A new ransomware family being dropped by the RIG exploit kit (EK) appears to be in the testing phase and could surface as a major threat, security researchers warn.

Dubbed PyCL, the ransomware variant was seen being distributed via the EITest compromise chain into the RIG EK, one of the largest malware distributors at the moment. The malware is written in Python, with the script called cl.py, which determined BleepingComputer’s Lawrence Abrams to name the threat PyCL.

The ransomware was first dropped by EITest-RIG on Sunday, in a campaign that abused hacked websites to redirect visitors to the EK and attempted to exploit vulnerabilities on their computers for further compromise. However, PyCL was dropped for a single day, and the security researcher sees it as an indicator that this was only a test run.

The new threat is distributed as an NSIS installer that contains a Python package designed to encrypt the user’s files, and a tutorial on how to pay the ransom. Apparently, the malware communicates with the command and control (C&C) server during each stage of the encryption process, to provide debugging/status information to the developer.

David Martínez, one of the researchers who discovered the malware (alongside Kafeine, MalwareHunterteam, BroadAnalysis), found a file called user.txt in the installer and discovered that a string in it is being sent to the C&C during every request. According to Abrams, this suggests the PyCL is part of a Ransomware as a Service (RaaS), where the username is the affiliate identifier.

The ransomware was first observed checking if it has administrative privileges and deleting the shadow volume copies on the computer if it does. Next, the malware sends the victim’s Windows version to the C&C, along with details such as administrative privileges, screen resolution, processor architecture, computer name, username, and the MAC address of the primary network adapter.

PyCL uses a unique AES-256 encryption key for each file, saves the list of files and their decryption keys to a random named file in the CL folder, and encrypts the file using the RSA-2048 public encryption key. 

While most ransomware families replace user’s files with their encrypted counterparts, this piece of malware leaves the original files on the hard disk, meaning that users don’t have to pay the ransom to get them back (this, however, might change in future versions of the malware). Finally, PyCL displays a lock screen that contains a 4-day timer, a Bitcoin address, and the ransom amount.

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