What is a recommended approach to mitigate replay attacks on intercepted SAML assertions?

Implementing time-based expiration is a robust approach to mitigate replay attacks on intercepted SAML assertions. In the context of SAML (Security Assertion Markup Language), a replay attack occurs when an attacker captures a valid authentication assertion and reuses it to gain unauthorized access to a system. By incorporating a time-based expiration, the assertions have a limited lifespan and are only valid for a specific period. After this time has elapsed, any intercepted assertion will be rejected by the service provider, making it ineffective for unauthorized access.

This method increases security significantly because even if an attacker manages to capture and store a valid assertion, the assertion will no longer be usable after the defined expiration date and time. This forces the attacker to contend with the constraints of time, which limits their ability to exploit the stolen assertion.

While ensuring assertions are signed can provide integrity and authenticity, it doesn’t address the problem of replay attacks specifically. Reducing token size might have other performance benefits but would not inherently secure the assertion against replay. Exploring alternative authentication methods could also be valid, but it does not directly address protections for SAML assertions themselves. Thus, time-based expiration stands out as the most effective method in this context.