GOFFEE Dissects Custom COW Agent Forked from Poseidon Mythic Implant
Angara Security researchers have published a detailed technical analysis of a custom Go agent for the Mythic C2 framework used by the Russian-oriented APT group GOFFEE (Paper Werewolf). The implant, internally named COW, was discovered on compromised Linux servers during an incident response investigation and represents a standalone development branch forked from the public Poseidon agent rather than its Freyja derivative.
Initial Discovery and Obfuscation
During one investigation, the team identified servers where attackers had established persistence using Go-based Mythic agents. The binaries were protected with Garble obfuscation and packed with a modified version of UPX. Standard UPX unpacking failed because service headers had been partially removed or corrupted, requiring manual restoration of the packer structures. Even after unpacking, Garble had stripped most original function, package, and type names, complicating static analysis.
Origin from Poseidon
Initial indicators pointed to Poseidon, previously observed in GOFFEE operations. Comparison with public Poseidon and Freyja codebases revealed that COW is not a direct copy of either. A surviving file path in an early unobfuscated sample, /Mythic/agent_code/pkg/utils/p2p/poseidon_tcp.go, confirmed the Poseidon codebase as the direct ancestor. Additional architectural similarities in profile handling, Mythic tasking logic, and P2P module structure reinforced this conclusion.
Key Differences and Enhancements
Researchers documented multiple deviations from public Poseidon and Freyja versions:
- Configuration continues to use ldflags instead of the later Base64/JSON approach adopted by the public projects.
- Support for additional addresses inside HTTP profiles not present in Poseidon or Freyja.
- Custom P2P implementation (p2p_tcp) with compatibility for Poseidon and webshell-based communication.
- Expanded Windows capabilities including registry manipulation, .NET execution, screenshot capture, and persistence mechanisms.
- Platform-specific command sets for both Linux and Windows environments.
These changes were observed across multiple related samples, indicating stable, intentional modifications rather than one-off build variations.
Additional Samples and Attribution
Retrospective searches on VirusTotal using surviving strings and build parameters located several additional Linux and Windows builds. Some samples lacked Garble obfuscation, preserving package and function names that aided further comparison. Attribution to GOFFEE was established through matching C2 infrastructure previously linked to the group in public incident reports, not merely code similarity.
Group Context
GOFFEE has conducted targeted espionage operations since at least 2022, primarily via spear-phishing with RAR archives, malicious Office documents, and HTA files. The group targets government organizations, energy, telecom, media, construction, industry, and defense sectors. Its toolkit includes PowerTaskel, PowerModul, MiRat, Sauropsida, BindSycler, DQuic, and a modified Owowa component, with heavy use of Garble, Ebowla, and custom encryption for evasion.
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