What is used to detect if a SAML assertion was modified after being issued?

Digital signatures are employed to ensure the integrity and authenticity of a SAML assertion. When a SAML assertion is issued, it is digitally signed by the Identity Provider (IdP) using their private key. This signature allows the Service Provider (SP) to verify that the assertion has not been altered in transit. If any modification occurs after the assertion is issued, the signature will no longer be valid, thus indicating tampering.

This mechanism is critical for maintaining security in Single Sign-On (SSO) scenarios, as it assures the SP that it can trust the information conveyed in the assertion. The absence of a valid digital signature would mean that the assertion should not be accepted, safeguarding against potential security risks like replay attacks or modifications by unauthorized entities.

Other options do not play a role in detecting modifications of the assertion after it has been issued. While XML is the format used for the assertion, it does not inherently provide any security features. Attributes refer to the data contained within the assertion, and tokens refer more broadly to the mechanism of authentication rather than the specific integrity check required for a SAML assertion.