In SAML and other federated login systems, the Service Provider is the application or website that wants to authenticate a user by trusting an external identity system. It does not usually check the user’s password itself. Instead, it receives a signed assertion from an Identity Provider and decides whether to create a local session.
This role matters because the SP is the trust boundary. If the SP validates the assertion incorrectly, accepts a bad signature, or treats a failed verification as success, an attacker may log in without valid proof of identity. In real attacks, flaws in SP-side code can turn a single logic mistake into authentication bypass. Defenders should treat SP validation as security-critical: verify signatures strictly, compare expected audience and recipient values, reject all verification errors, and keep the SP software patched. Strong MFA and access controls help, but they do not replace correct SP-side verification.