SAML certificate verification
FortiGate 7.2.12, 7.2.13, 7.4.9, and 7.6.4 verify the signature for SAML response messages. Please turn on Sign SAML response and assertion or similar options in corresponding IDP settings. Lack of signature for signing response messages or assertions may cause authentication to fail.
For more information on how the SAML response signing is configured, see Identify Providers.