Protocol command injection happens when attacker-controlled input is interpreted as part of a network protocol command instead of plain data. In other words, the client or parser fails to keep a strict boundary between user input and protocol instructions, so a string can change how the connection behaves.
This matters because it can let an attacker trigger unexpected requests, alter headers or options, bypass authentication assumptions, or redirect traffic to a different service. In real systems, the risk appears in clients, proxies, libraries, and automation tools that build protocol messages from reusable state or untrusted fields. Defenders reduce the risk by validating and encoding inputs, separating command data from control data, resetting protocol state between requests, and testing edge cases such as redirects, proxy settings, and handle reuse. Secure protocol handling is about making sure user input never becomes protocol syntax.