Multiplexed SSH channels are multiple logical data streams that share one encrypted SSH connection. A single SSH session can carry a shell, file transfer, port forwarding, and other traffic at the same time, while the protocol keeps those streams separated at the application layer.
This matters because sharing one connection also means sharing state. Features such as compression or flow control can create side channels if data from one channel influences how another is processed. An attacker who can send chosen input on one channel may be able to infer information from secret-bearing traffic on another channel by watching packet sizes, timing, or behavior changes. Defenders reduce this risk by disabling risky compression modes, limiting mixed-trust use of SSH sessions, and reviewing forwarding and multiplexing settings where secrecy is important.