Medusa Ransomware Detection Guide
Behavioral Indicators
Medusa ransomware exhibits several distinct runtime behaviors that can be detected through process monitoring and API call analysis. The malware typically initiates with a primary executable that spawns child processes to perform encryption tasks. Look for processes with names like medusa.exe, svchost.exe (impersonating legitimate Windows services), or randomly generated 8-character alphanumeric names (e.g., a1b2c3d4.exe). These processes often create multiple threads to parallelize file encryption across different directories.
Key API calls to monitor include CryptEncrypt and CryptDeriveKey from the Windows CryptoAPI, which Medusa uses for its encryption routines. The malware also calls FindFirstFileW and FindNextFileW in rapid succession to enumerate filesystem contents, typically skipping Windows system directories and specific file extensions critical for system operation. Credential access patterns include querying the Security Account Manager (SAM) via registry reads at HKEY_LOCAL_MACHINE\SAM\SAM\Domains\Account\Users to identify local user accounts for targeting. The malware also attempts to disable Volume Shadow Copy Service (VSS) by executing vssadmin delete shadows /all /quiet through CreateProcess or ShellExecute, and may attempt to stop backup-related services like SQLWriter and MSSQLSERVER.
Process injection techniques are common, where Medusa injects its payload into legitimate processes such as explorer.exe or rundll32.exe to evade detection. Monitor for VirtualAllocEx and WriteProcessMemory calls targeting these processes. Additionally, the malware sets the FILE_ATTRIBUTE_HIDDEN attribute on encrypted files and drops ransom notes, which involves calls to SetFileAttributesW.
Network Indicators
Medusa communicates with command-and-control (C2) servers over HTTPS, using TLS 1.2 encryption to obscure traffic. The malware beacons to domains that often follow a pattern of random subdomains (e.g., [random].medusa-c2[.]com or [hex-string].onion for Tor-based C2s). DNS queries for these domains may appear at regular intervals, typically every 5 to 10 minutes during initial infection, and increase in frequency during data exfiltration or encryption phases.
Beaconing HTTP/S requests use POST methods with encrypted payloads, and the User-Agent string is often generic (e.g., Mozilla/5.0 or Windows-Update-Agent) to blend in with normal traffic. The C2 response usually contains encryption keys or commands for further actions. Data exfiltration, if performed, occurs before encryption and uses similar HTTPS channels, often with larger upload sizes (megabytes to gigabytes) to external IP addresses hosted on bulletproof hosting services.
Network signatures include SSL certificate anomalies, such as self-signed certificates or certificates issued to recently registered domains. Additionally, look for connections to known malicious IP ranges associated with ransomware operations, particularly those hosted in Eastern Europe or using cloud services for resilience. In later variants, Medusa may use Tor proxies (127.0.0.1:9050) for C2 communication, indicated by SOCKS5 protocol traffic on non-standard ports.
File System Artifacts
Medusa drops several files during execution. The primary executable is often placed in %AppData%, %LocalAppData%, or %Temp% directories, with names like medusa.exe, update.exe, or random strings (e.g., f7g8h9j0.exe). Encrypted files are appended with the .medusa extension by default, though variants may use custom extensions like .encrypted or .[id]-[contact].medusa, where id is a victim identifier and contact is an email address for ransom communication.
Ransom notes are typically dropped as README.txt or HOW_TO_DECRYPT.html in every encrypted directory, and on the desktop. These files contain ransom demands, contact information (often Telegram or email), and instructions for payment. File sizes for the primary executable range from 200 KB to 2 MB, depending on the variant and packing. Timestamps for dropped files are often set to match the system’s current time during infection, but the malware may alter timestamps of encrypted files to obscure activity.
Additional artifacts include temporary files in %Temp% with .tmp extensions, used for storing encryption keys or file lists. Log files may be created at C:\ProgramData\medusa.log or similar paths, containing encryption progress or error messages. In some cases, Medusa deletes itself after execution, leaving only encrypted files and ransom notes.
Registry and Persistence Mechanisms
Medusa establishes persistence through several registry keys and scheduled tasks. Common registry entries include adding an executable path to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run or HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run, with keys named after the malware file (e.g., MedusaUpdate or generic names like SecurityHealth). It may also create services via HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services, with service names mimicking legitimate ones (e.g., WindowsDefender or WinUpdate).
Scheduled tasks are frequently used for persistence, with tasks named \Microsoft\Windows\SystemMaintenance\Medusa or \Microsoft\Windows\Backup\BackupRunner. These tasks are configured to run at system startup or at regular intervals (e.g., every hour) and execute the malware from a hidden location. Additionally, Medusa may modify file associations via HKEY_CLASSES_ROOT to trigger execution when specific file types are opened.
Other persistence methods include creating shortcut files (.lnk) in the Startup folder (%AppData%\Microsoft\Windows\Start Menu\Programs\Startup) or using WMI event subscriptions to react to system events. Registry keys related to disabling security software, such as HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender\DisableAntiSpyware set to 1, may also be present.
YARA Rule Guidance
When developing YARA rules for Medusa, focus on unique strings and byte patterns that distinguish it from other ransomware. Key strings to target include ransom note content such as “MEDUSA RANSOMWARE”, “Your files are encrypted”, and contact strings like @medusa_support or [email protected]. Also, look for embedded configuration data, such as encryption extension patterns (e.g., .medusa or [id]-[contact]), and C2 URLs within the binary, which may appear as https:// or .onion strings.
Byte patterns should include sequences from Medusa’s encryption routines, such as calls to CryptEncrypt or RtlCompressBuffer (used in some variants for file compression before encryption). Structural characteristics include the use of the AES-256 algorithm, indicated by constants like AES256_KEY_SIZE or CBC mode initialization vectors. Additionally, target packer signatures if the sample is obfuscated; Medusa often uses custom packers or common ones like UPX, with modified headers.
Consider including patterns for API function names hashed or obfuscated, as Medusa may use runtime resolution via GetProcAddress. Look for imports from ADVAPI32.dll related to registry manipulation and WININET.dll for network communication. Avoid overly generic patterns to reduce false positives, and prioritize combinations of strings and opcode sequences unique to Medusa’s codebase.
For updated samples and indicators of compromise, refer to the following resources:
This guide provides actionable steps for detecting Medusa ransomware through behavioral analysis, network monitoring, file system inspection, registry checks, and YARA rule development. Regularly update detection mechanisms based on new IOCs and sample analysis.