  Single Sign-On (SSO) protocols streamline user authentication with a unified
login for multiple online services, improving usability and security. One of
the most common SSO protocol frameworks - the Security Assertion Markup
Language V2.0 (SAML) Web SSO Profile - has been in use for more than two
decades, primarily in government, education and enterprise environments.
Despite its mission-critical nature, only certain deployments and
configurations of the Web SSO Profile have been formally analyzed. This paper
attempts to bridge this gap by performing a comprehensive formal security
analysis of the SAML V2.0 SP-initiated SSO with POST/Artifact Bindings use
case. Rather than focusing on a specific deployment and configuration, we
closely follow the specification with the goal of capturing many different
deployments allowed by the standard. Modeling and analysis is performed using
Tamarin prover - state-of-the-art tool for automated verification of security
protocols in the symbolic model of cryptography. Technically, we build a
meta-model of the use case that we instantiate to eight different protocol
variants. Using the Tamarin prover, we formally verify a number of critical
security properties for those protocol variants, while identifying certain
drawbacks and potential vulnerabilities.
