A fallback mechanism is a secondary method a system uses when its primary route, service, or control fails. In software, this can mean switching to another server, transport, authentication path, or local process when the preferred one is unavailable.
In cyber security, fallback mechanisms matter because attackers can abuse them to keep malware working even after defenders block the main channel. For example, if direct internet communication is detected or filtered, malicious code may try a local relay, peer-to-peer path, or another protocol to move data or receive commands. Defenders also use fallback designs for resilience, but they should be tightly controlled, logged, and tested, because hidden fallback paths can create blind spots and expand the attack surface.