GOFFEE Dissects Custom COW Agent Forked from Poseidon Mythic Implant

Angara Security researchers uncovered a custom Go-based Mythic C2 agent named COW used by the Russian-oriented APT group GOFFEE, also known as Paper Werewolf. The agent represents an independent development branch derived from the public Poseidon project rather than its Freyja fork. Samples were heavily protected with Garble obfuscation and a modified UPX packer that required manual header reconstruction for unpacking. Analysis of surviving strings, build paths, and architectural features confirmed Poseidon origins while revealing numerous custom modifications including Windows support, additional C2 profiles, and new command implementations. The group employs the agent alongside other tools such as PowerTaskel, MiRat, and BindSycler for persistent access to Linux and Windows systems across government, energy, telecom, and defense targets. Researchers traced multiple variants through VirusTotal and incident reports, documenting evolutionary changes in configuration handling and peer-to-peer communication mechanisms.

HabrAugust 21, 2026Translated from Russian

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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.

GOFFEE Poseidon Freyja Mythic Garble COW Angara Security

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