Rewterz Threat Alert – Cyrat Ransomware Targets Windows Users

Severity

Medium

Analysis Summary

Recently, Cyrat ransomware was found targeting windows users. This ransomware was compiled with Python 3.7 and converted to a Windows PE file using PyInstaller. The malware disguises as DLL fixer 2.5 (see image below). Upon execution it will display a randomly created number of corrupted DLLs it pretends to have found on the system. After the system has been encrypted, a success message for fixing the DLLs is shown. 

Cyrat ransomware uses Fernet to encrypt files. This is a symmetric encryption method meant for small data files that fit into RAM. A public RSA key is used to encrypt the Fernet key. This public key is downloaded from Mediafire instead of shipping it with the ransomware. This adds another dependency. The encrypted Fernet key is saved in Desktop\EMAIL_US.txt. A user with an infected system is required to send this file to the criminals. Cyrat appends .CYRAT to encrypted files. It has a list of folders that it checks for target files. Those folders are ‘Desktop’, ‘Downloads’, ‘Pictures’, ‘Music’, ‘Videos’, and ‘Documents’. The target platform is undoubtedly Windows because the PyInstaller executable only works there. The trojanized DLL fixer will only lure Windows users. Registry commands and persistence mechanisms are also Windows based.

Impact

Files Encryption

Indicators of Compromise

MD5

• d427390e9fad598ec3288c9275c84628

SHA-256

• 4b76ad80e9ce4c503bde0e476a88447426fc38315d440d22926627295e1b0ec6

SHA1

• 7b88e1eaa07151fc0d7639574fc7f40fa5be8aa3

URL

• hxxp[:]//download1582[.]mediafire[.]com/c91ywpc4l7ag/xj26578psz6n9xo/public_key[.]pem

Remediation

• Block the threat indicators at their respective controls.
• Do not download untrusted file attachments from emails.
• Do not click on URLs given in untrusted emails or untrusted websites.