ExternalC2 is a specification/framework introduced by Cobalt Strike, which allows hackers to extend the default HTTP(S)/DNS/SMB C2 communication channels offered. Essentially this works by allowing the user to develop a number of components: Third-Party Controller - Responsible for creating a connection to the Cobalt Strike TeamServer, and communicating with a Third-Party Client on the target host using a custom C2 channel. Third-Party Client - Responsible for communicating with the Third-Party Controller using a custom C2 channel, and relaying commands to the SMB Beacon. SMB Beacon - The standard beacon which will be executed on the victim host. Using the diagram from CS’s documentation, we can see just how this all fits together.