Keypoints
- Unpacked payload identified as hash 376165CCD556CD74658AFEA9F6F428F9; sample is custom-packed and required unpacking to analyze.
- On execution the binary checks for specific mutexes and exits if present to avoid re-infection of the same host.
- Embedded configuration is decrypted via CryptoAPI and contains blocklisted extensions/folders, excluded country Language IDs, targeted extensions, a base64 RSA public key, and ransom note templates.
- Cerber traverses filesystem data according to the configuration and encrypts file contents using RC4 and RSA via CryptoAPI, skipping the first 1800 bytes, then renames files to a 10-character random string plus “.a769”.
- The ransomware manipulates Windows firewall rules and disables/impairs AV/anti-spyware services to block outbound traffic from security binaries, aiding evasion and persistence.
- C2 connections are made to port 6893 against CIDR-specified IP ranges; communication packets are prefixed by an MD5 of the Machine GUID and include parameters like PARTNER_ID, OS, IS_X64, IS_ADMIN, COUNT_FILES, STOP_REASON, and STATUS.
- After encryption it drops ransom notes named “__R_E_A_D__T_H_I_S__.html” and TXT files, drops two files containing parts of the RSA key, and then deletes its own executable using ShellExecuteA.
MITRE Techniques
- [T1486] Data Encrypted for Impact – Encrypts victim files using RSA and RC4, skipping the first 1800 bytes and appending “.a769” to filenames (‘It implements RSA and RC4 algorithms in its encryption routine… appends a “.a769” extension’).
- [T1083] File and Directory Discovery – Traverses filesystem entries and uses decrypted configuration to select target extensions and folders for encryption (‘Then it traverses the decrypted data and uses its value for further encryption processes.’).
- [T1082] System Information Discovery – Collects and transmits host details (OS, architecture, admin status, file counts) as part of C2 packets (‘The packet concludes with parameters such as PARTNER_ID, OS details, IS_X64 (indicating whether the system is 64-bit), IS_ADMIN… COUNT_FILES’).
- [T1562.001] Impair Defenses: Disable or Modify Security Tools – Identifies and configures Windows firewall rules to block outbound traffic from security product binaries and disables AV services to hinder detection (‘identifying and configuring Windows firewall rules to obstruct outbound traffic from the executable binaries of installed firewalls, antivirus, and anti-spyware products’).
- [T1071] Application Layer Protocol (Command and Control) – Communicates with C2 hosts on port 6893 using CIDR-listed IPs and a Machine GUID-prefixed hash in packets (‘Cerber ransomware establishes connections to port 6893 on IPs specified by CIDR… The communication packet initiation involves a hash prefixed with the Machine GUID (MD5_KEY).’).
- [T1070.004] Indicator Removal on Host: File Deletion – Removes its own executable after completing its actions using ShellExecuteA to hinder analysis and cleanup (’employs the ShellExecuteA() API function with specific arguments to eliminate its own file from the compromised system’).
- [T1489] Service Stop – Stops or interferes with security services as part of its evasion process (figure captions and description reference disabling AV services: ‘Disabling AV services’).
Indicators of Compromise
- [File Hash] payload/sample identifiers – 376165CCD556CD74658AFEA9F6F428F9, FE1BC60A95B2C2D77CD5D232296A7FA4
- [Network CIDR] C2 infrastructure ranges – 93.107.12.0/27, 95.1.200.0/27, 87.98.176.0/22
- [File Name] dropped ransom note – “__R_E_A_D__T_H_I_S__.html” and corresponding TXT ransom note in affected folders
- [File Extension] encrypted file marker – files renamed to a 10-character random string with appended extension “.a769”
- [Artifacts] RSA key fragments and temp files – two dropped files containing the RSA key parts and temporary key fragments (e.g., parts added to tmp files shown in figures), and other temp key artifacts
The technical analysis begins with unpacking a custom-packed Cerber sample (identified by hash 376165CCD556CD74658AFEA9F6F428F9). On execution the binary checks for pre-existing mutexes and exits if they exist; it then decrypts an embedded configuration with CryptoAPI that contains blocklisted extensions/folders, excluded country Language IDs, targeted file extensions, a base64-encoded RSA public key, and HTML/TXT ransom-note templates.
Using the decrypted configuration, Cerber enumerates files and applies an encryption routine combining RC4 and RSA via CryptoAPI: it reads each file, skips the first 1800 bytes, encrypts the remainder, and writes back the data. Encrypted files are renamed to a 10-character alphanumeric string matching [0-9a-zA-Z_-]{10} and appended with “.a769”; the malware also drops the HTML/TXT ransom notes (“__R_E_A_D__T_H_I_S__”), writes parts of the RSA key to temporary files, and changes artifacts as part of its workflow.
For evasion and control, Cerber modifies Windows firewall rules and disables or interferes with AV/anti-spyware services to block outbound traffic from security binaries, creates C2 sessions to port 6893 against CIDR-specified IP ranges (packets prefixed by an MD5 of the Machine GUID and including PARTNER_ID, OS, IS_X64, IS_ADMIN, COUNT_FILES, STOP_REASON, STATUS), and finally removes its own executable using ShellExecuteA to impede forensic analysis.