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New .NET-Based Ransomware Uses Open Source Code

Two newly discovered .NET-based ransomware families are using open source repositories to encrypt users’ files, Zscaler security researchers say.

Dubbed Vortex and BUGWARE, the two ransomware families have been seen in live attacks carried out via spam emails containing malicious URLs. Both of the new malware families are compiled in Microsoft Intermediate Language (MSIL) and have been packed with the 'Confuser' packer.

The Vortex ransomware is written in Polish and makes use of the AES-256 cipher to encrypt image, video, audio, document, and other potentially important data files on the victim’s machine, Zscaler notes in an analysis report shared with SecurityWeek.

The same as other ransomware variants out there, the malware drops a ransom note once it has completed the encryption process, informing the victim on how they can restore their data and how to send the ransom money.

The malware allows users to decrypt two of their files for free and demands a $100 ransom, which supposedly increases to $200 in four days. Victims are encouraged to contact the attackers using the [email protected] or [email protected] email addresses.

After installation, the malware attempts to achieve persistence through creating a registry entry, as well as a registry key called “AESxWin.” The malware was also observed deleting shadow copies to prevent users from restoring their data without paying.

While analyzing the malware’s command and control (C&C) communication, the security researchers observed it sending system information and requesting a password API used for the encryption and decryption key.

Vortex is entirely based on AESxWin, a freeware encryption and decryption utility hosted on GitHub and created by Egyptian developer Eslam Hamouda. Thus, files can be decrypted using AESxWin, as long as the password used for encryption is known, Zscaler suggests.

BUGWARE, on the other hand, is based on the open source Hidden Tear code, which has been abused to create various ransomware families before.

The new threat also uses an invalid certificate pretending to be for GAS INFORMATICA LTDA and asks victims to pay the equivalent of a thousand Brazilian reals in Monero.

The malware creates a list of paths to encrypt and stores it in a file called Criptografia.pathstoencrypt. It also searches for all fixed, network, and removable drives and adds those paths to the list.

BUGWARE was observed generating the encryption key and using the AES 256-bit algorithm to encrypt users’ files, as well as renaming the encrypted files. The AES key is encrypted too, using a RSA public key, and the base64 encoded key is saved in the registry.

To achieve persistence, the malware creates a run key that ensures it is executed each time the user logs into the computer. If removable drives are detected, the threat drops a copy of itself on them, with the name “fatura-vencida.pdf.scr.”

The ransomware changes the victim’s desktop background using image files downloaded from “i[.]imgur.com/NpKQ3KZ.jpg."

Related: Variants Spawn From Hidden Tear Ransomware

Related: HDDCryptor Leverages Open Source Tools to Encrypt MBR

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