C3 is a C++ framework from ReversecLabs that lets authorized red teams rapidly prototype esoteric command and control Channels and bridge them to frameworks like Cobalt Strike.
| Tool | ReversecLabs/C3 — open-source framework for rapid prototyping of custom C2 channels |
| Category | command and control framework (C++, ~1.7k stars) |
| Primary Use | Building and chaining esoteric Channels (e.g. LDAP, MSSQL, Slack, Dropbox) to route traffic back to Cobalt Strike via ExternalC2 |
| Safe Use | Intended exclusively for authorized red team engagements and controlled lab environments; operators are expected to run Gateways on engagement-owned infrastructure |
| Telemetry Note | Public detection resources exist: a Sigma rule for rundll32-launched C3 DLLs, a YARA rule for C3 reflective DLL usage, and the RelayRumbler relay config parser for defenders |
C3, short for Custom Command and Control, is a framework built by ReversecLabs whose explicit purpose is letting red teams rapidly develop and deploy esoteric command and control channels without having to write a full C2 stack from scratch. Rather than competing with existing post-exploitation tooling, it is designed to extend it: at release it supports Cobalt Strike through the ExternalC2 specification, treating the commercial product as the brains of the operation while C3 supplies the transport plumbing. The idea is that an operator should only have to concern themselves with the channel they want to implement, delegating reliability, routing, and stability to the framework. That division of labor is what makes it interesting from an engineering perspective as much as a tradecraft one.
The README positions this architecture as a safety feature as much as an efficiency feature. By relying on the robustness of C3 and the underlying CS tooling, teams can operate in critical client environments with a professional level of stability, while still leaving room for safe experimentation and rapid deployment of customized Tactics, Techniques and Procedures. In other words, the framework's value proposition is that bespoke, weird C2 transports become cheap to build without inheriting the fragility of a hand-rolled implant. That framing matters for anyone evaluating whether to adopt it on an authorized engagement: the goal is emulating an adaptive real-world attacker, not improvising unreliable code on a client's network.
Internally, C3 is organized around a clear set of abstractions documented in an unusually thorough glossary. The fundamental unit is the Relay, a stand-alone piece of a C3 network that communicates over Interfaces. There are two kinds of Relays: Gate Relays (or Gateways) and Node Relays. The Gateway controls one C3 network, acts as the bridge back to attacker infrastructure, and is responsible for talking to a third-party C2 server such as Cobalt Strike's Teamserver. The README is explicit that Gateways should always be hosted within attacker-controlled infrastructure — a sensible operational constraint that also serves as a boundary marker for lawful use.
Node Relays, by contrast, are the executables that would run on hosts within the engagement scope, and they communicate through Devices either peer-to-peer or back toward the Gateway. The term Device generalizes two interface types: Channels and Peripherals. A Channel transports data between two Relays and works strictly in pairs, with no one-to-many transmission; that limitation is worked around by Negotiation Channels, a special variant that can automatically negotiate regular channel connections with multiple Relays but carries nothing except the negotiation protocol itself. This is a classic mesh-routing design compressed into a small vocabulary, and the glossary suggests the implementers thought hard about keeping the concepts teachable.
Routing is handled through a few identifier types that reveal how the framework tracks topology. Every Relay carries a static Build ID that survives reboots, while Agent IDs are dynamic and unique per running Node Relay instance — even relays instantiated from the same executable get different Agent IDs. A Route ID pairs an Agent ID with a Device ID to describe one path to a given relay, and every Relay maintains a routing table of its descendants so that when a packet arrives from the Gateway, the correct Channel can be selected for the next hop. The first channel configured on a Node Relay automatically becomes its Gateway Return Channel, or GRC, which may itself route through another relay.
The transport layer has its own careful constraints. The README defines a C3 Minimal MTU of 64 bytes — the smallest chunk every channel must be able to send. If a chunk shorter than 64 bytes arrives without being a complete packet, the receiving relay ignores it and the sender re-transmits the last portion. Update Delay Jitter, the delay between successive interface updates, can be randomized across a configured range, which is a straightforward way to break up periodic beaconing patterns that detection engineering tends to key on. These small details are exactly the kind of thing defenders should read closely, because they shape what C3 traffic looks like on the wire.
Where C3 really distinguishes itself is the breadth of its channel catalog, much of it community-contributed. External channels listed in the README include Mattermost, Asana, GitHub, Dropbox, Cisco WebEx Teams, JIRA, Discord, GoogleDrive, Slack, EWS Tasks, and two non-functional OneDrive 365 variants. Internal channels — those that abuse legitimate intra-network services — include MSSQL, UNC Share File, LDAP, and Printer Jobs. The split is meaningful: external channels ride SaaS platforms that typically traverse egress filtering, while internal channels like LDAP and printer jobs enable lateral movement across segmented networks without touching the internet at all. Contributors such as @mariuszbit, @tvgdb2, @sunn_y_k, @adm1nPanda, and @checkymander are credited inline.
Integration with other tooling is handled by two more abstractions: Peripherals, which wrap a third-party implant (the README cites Cobalt Strike's SMB beacon as the example), and Connectors, which integrate with a third-party C2 framework — the ExternalC2 interface exposed via the externalc2_start command on the Teamserver. Collectively these are called Binders, and they are the reason C3 can slot into an existing toolkit rather than replace it. For an authorized operator already invested in CS, this is the feature that lowers adoption cost to near zero.
Unusually for an offensive tool, the README carries an extensive Detection section, which is where much of this project's community value lives. It links WithSecure's hunting guide for C3, several Attack Detection Fundamentals labs covering C2 and exfiltration over Dropbox plus discovery and lateral movement over UNC Share File, a writeup on detecting C2 printer pivoting, and conference material from Black Hat USA 2021 on breaking network segregation with esoteric C2 channels. Concrete detection artifacts include a Sigma rule for C3 DLL launch via rundll32, a YARA rule for C3 reflective DLL usage, and RelayRumbler, a parser that extracts configuration from captured relay binaries. Blue teams should treat this section as required reading; it maps the offensive surface to countermeasures directly.
Building the framework requires a Windows-centric toolchain. The README specifies adding .NET 7.0, the .NET Core 3.1 runtime (flagged as out of support), and MSVC v142 VS 2019 C++ build tools through the Visual Studio installer, with optional Clang components for LLVM-based builds. After that, running CreateBuild.bat from the Visual Studio Developer Command Prompt produces output in the Builds directory. CI badges for both MSVC and Clang builds are present, indicating the project maintains dual-compiler coverage, and a CONTRIBUTING.md documents how to develop custom channels for anyone extending the framework.
The repository shows roughly 1,787 stars with NOASSERTION licensing, meaning anyone considering operational or derivative use should review the actual license terms before redistribution. The C++ codebase, the clean glossary-driven architecture, and the strong detection ecosystem make C3 a genuinely instructive project even for readers who never run it: it is a compact masterclass in how modern overlay C2 is structured, how SaaS and internal enterprise services can be abused as transport, and — through the linked detection resources — how defenders can go about spotting exactly that behavior during authorized assessments and defensive research.
ReversecLabs/C3.Educational analysis for authorized security professionals. Use only in controlled, authorized environments.
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