Software-Defined Perimeter (SDP) is an access model that hides applications until a user and device are authenticated and authorized. Instead of placing a remote user onto a broad internal network like a traditional VPN, SDP creates a narrow, per-application connection only to the service allowed by policy. This reduces the visible attack surface and supports least privilege.
In cyber security, SDP matters because many attacks succeed after an initial login by moving laterally through exposed network resources. By keeping services invisible until policy checks pass, SDP can limit reconnaissance, block unauthorized access paths, and reduce the damage from stolen credentials. Defenders use SDP with strong authentication, device posture checks, and identity-based policy to control who can reach each application. If those controls are weak, however, SDP can become only a new wrapper around old trust problems.