Mutual TLS, or mTLS, is a certificate-based authentication method in which both sides of a connection prove their identity before any application data is exchanged. In normal TLS, the server is authenticated to the client. With mTLS, the client also presents a certificate, and the server verifies it against trusted issuers and policy controls. That creates a stronger trust boundary for APIs, payment systems, service-to-service traffic, and administrative interfaces.
In cyber security, mTLS matters because stolen passwords alone are not enough if access also depends on valid private keys and certificates. Attackers often target certificate stores, key material, and enrollment systems so they can impersonate approved systems or move through trusted pathways. Defenders use mTLS to reduce unauthorized access, limit lateral movement, and make machine-to-machine communications easier to verify and monitor. It is especially valuable in payment environments, where a connection that looks legitimate may be used to submit fraudulent activity unless the trust layer is protected.