The best way to mitigate a replay attack against a system using identity federation and Security Assertion Markup Language (SAML) implementation is to use timed sessions and Secure Socket Layer (SSL). A replay attack is a type of network attack that involves capturing and retransmitting a valid message or data to gain unauthorized access or perform malicious actions. Identity federation is a process that enables the sharing of identity information across different security domains, such as different organizations or applications. SAML is a standard protocol that enables identity federation by using XML-based assertions to exchange authentication and authorization information. To prevent a replay attack, the system can use timed sessions and SSL. Timed sessions are sessions that have a limited duration and expire after a certain period of time or inactivity. SSL is a protocol that provides encryption and authentication for data transmission over the internet. By using timed sessions and SSL, the system can ensure that the SAML assertions are valid, fresh, and secure, and that they cannot be reused or tampered with by an attacker. Two-factor authentication, digital certificates and hardware tokens, and passwords with alpha-numeric and special characters are not the best ways to mitigate a replay attack against a system using identity federation and SAML implementation, as they do not address the specific vulnerabilities of the SAML protocol or the network transmission. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 4, Communication and Network Security, page 462. Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 4, Communication and Network Security, page 478.