Inside Millenium RAT 4.x: Configuration Architecture
The native C++ rewrite — why it matters defensively, how configuration is protected, and the MaaS model behind its scale.
What is Millenium RAT 4.x? Millenium RAT 4.x is the substantially rewritten generation of the malware analyzed by Group-IB in June 2026. It is a native C++ Windows application that uses libcurl to communicate with the Telegram Bot API, stores configuration inside an embedded PE RCDATA resource protected by Base64 and a custom XOR scheme, and is marketed under a low-cost Malware-as-a-Service subscription model.
Why the rewrite matters defensively
- Removes dependency on the .NET Framework
- Changes static-analysis characteristics
- Changes malware signatures
- Alters reverse-engineering workflows
- Reduces value of detections targeting earlier .NET implementations
Important: native C++ does not automatically make malware "undetectable." It changes the analysis workflow. Modern behavior-based detection remains effective when it correlates process lineage, persistence, and network communication rather than relying on a single hash.
Configuration architecture
According to Group-IB, Millenium RAT 4.x stores configuration data inside an embedded Windows PE RCDATA resource. Reported processing:
Researchers observed additional random Base64 content placed alongside configuration data. This can alter resulting file hashes between builds even when underlying functionality remains similar.
Why hash-only detection fails
- Matches a known file fingerprint
- Config padding changes hashes between builds
- Underlying functionality can remain identical
- File reputation + behavioral activity
- Process lineage + persistence behavior
- Network communication + endpoint telemetry
Communication library: libcurl
Group-IB reported that version 4.x uses libcurl to send and receive requests involving the Telegram API. libcurl is a commonly used legitimate software library for transferring data over network protocols. Malware can abuse legitimate libraries just as legitimate applications can use them — the library itself is not malicious.
Malware-as-a-Service pricing
Group-IB reported Millenium RAT 4.x marketed using a subscription / commercial access model. Reported pricing at the time:
Pricing reported by Group-IB in June 2026. No purchase links are provided. Inexpensive MaaS lowers the skill and financial barriers to cybercrime.
- ▸4.x is a native C++ rewrite that removes the .NET dependency.
- ▸Config: embedded PE RCDATA → Base64 → custom XOR → runtime config.
- ▸Config padding causes hash variation — hash-only detection is insufficient.
- ▸Uses libcurl (a legitimate library) for Telegram API communication.
- ▸MaaS pricing (~US$50 first month) lowers barriers to cybercrime.
