A downgrade is a fallback from a modern, stronger protocol or network mode to an older, weaker one. In cyber security, that matters because the weaker option may offer less encryption, weaker authentication, or more visible metadata, making interception and manipulation easier.
Attackers sometimes try to trigger or exploit downgrade behavior by blocking newer features, spoofing failures, or forcing a device to accept legacy options. Defenders work to prevent this by disabling obsolete protocols, requiring secure negotiation, and rejecting unsafe fallback paths. On mobile networks, for example, automatic blocking of 2G reduces exposure to legacy links that are easier to monitor or abuse.