Inside Pay2Key: Technical Analysis of a Linux Ransomware Variant
Morphisec Threat Labs analyzed a Linux variant of the Iranian-attributed Pay2Key ransomware. The malware requires root privileges to execute, utilizes a JSON configuration file, disables system defenses like SELinux and AppArmor, and employs ChaCha20 for full or partial file encryption while lacking built-in network C2 or exfiltration capabilities.
Authors: Ilia Kulmin
Key Takeaways
- Pay2Key has developed a Linux ransomware variant that requires root privileges and is driven by a JSON configuration file.
- Pre-encryption actions include disabling SELinux and AppArmor, stopping services, killing processes, and establishing persistence via a reboot-time cron entry.
- The malware enumerates /proc/mounts, intentionally skipping read-only mounts, ELF/MZ binaries, and zero-length files to avoid crashing the host.
- Encryption is performed using the ChaCha20 algorithm, supporting both full and partial file encryption modes.
- No network C2 loop or data exfiltration routines were observed; the malware relies on a hardcoded string 'DontDecompileMePlease' for metadata key derivation.
Affected Systems
- Linux
Attack Chain
The malware first verifies it has root privileges before parsing a JSON configuration file. It then executes pre-encryption actions, which include disabling SELinux and AppArmor, stopping services, killing processes, and creating a cron entry for persistence across reboots. Next, it enumerates the filesystem via /proc/mounts, skipping read-only mounts and specific file types like ELF/MZ binaries to maintain system stability. Finally, it generates per-file keys, writes metadata (using a hardcoded string for key derivation), and encrypts files using ChaCha20 in either full or partial mode.
Detection Availability
- YARA Rules: No
- Sigma Rules: No
- Snort/Suricata Rules: No
- KQL Queries: No
- Splunk SPL Queries: No
- EQL Queries: No
- Other Detection Logic: No
The article does not provide specific detection rules, but outlines behavioral indicators such as the disabling of SELinux/AppArmor and the use of a specific hardcoded string for key derivation.
Detection Engineering Assessment
EDR Visibility: High — EDR solutions on Linux can monitor process execution, service stopping, modification of SELinux/AppArmor states, and cron job creation. Network Visibility: None — The article explicitly states there is no evidence of a network C2 loop or data exfiltration routines in this variant. Detection Difficulty: Moderate — While the encryption phase is fast, the pre-encryption behaviors (disabling security tools, stopping services en masse, adding cron jobs) are highly anomalous and detectable if proper Linux telemetry is collected.
Required Log Sources
- Linux Auditd
- Syslog
- Cron logs
- File Integrity Monitoring (FIM)
Hunting Hypotheses
| Hypothesis | Telemetry | ATT&CK Stage | FP Risk |
|---|---|---|---|
| Look for processes disabling SELinux or AppArmor followed shortly by mass service termination or process killing. | Process execution logs (Auditd/EDR) | Defense Evasion | Low |
| Monitor for unexpected cron job creation by the root user, especially if associated with unknown binaries or scripts. | Cron logs, File creation events | Persistence | Medium |
| Detect rapid, sequential file modifications across multiple mounts, specifically excluding read-only mounts and ELF binaries. | File system events (FIM/EDR) | Impact | Low |
Control Gaps
- Lack of purpose-built Linux anti-ransomware controls
- Over-reliance on network detection (which fails here due to the absence of C2 communication)
Key Behavioral Indicators
- Commands or API calls disabling SELinux/AppArmor
- Mass process/service termination by a single parent process
- Creation of reboot-time cron entries by anomalous processes
- Reading /proc/mounts followed by high-volume file I/O
- Presence of the 'DontDecompileMePlease' string in memory or written files
False Positive Assessment
- Low
Recommendations
Immediate Mitigation
- Ensure critical Linux servers have immutable backups stored offline.
- Review and restrict root access and privilege escalation paths on Linux systems.
Infrastructure Hardening
- Implement purpose-built Linux anti-ransomware solutions that do not rely solely on signatures.
- Enforce strict SELinux or AppArmor policies and alert on any attempts to disable them.
- Deploy Automated Moving Target Defense (AMTD) to disrupt execution consistency.
User Protection
- Enforce MFA for all administrative access (SSH, console) to Linux infrastructure.
Security Awareness
- Train administrators on the rising threat of Linux-targeted ransomware and the importance of defense-in-depth for server infrastructure.
MITRE ATT&CK Mapping
- T1562.001 - Impair Defenses: Disable or Modify Tools
- T1489 - Service Stop
- T1053.003 - Scheduled Task/Job: Cron
- T1082 - System Information Discovery
- T1486 - Data Encrypted for Impact
Additional IOCs
- File Paths:
/proc/mounts- Enumerated by the ransomware to classify mounts and determine filesystem traversal scope.
