In previous posts we described Single Sign-On (SSO) and the messages within the authentication flow in detail. Additionally, we showed implementation pitfalls on the Service Provider (SP) side resulting in critical vulnerabilities. In 2012 we started a study about the security of SAML based Identity Provider (IdP). The motivation to make this study was very simple – if the Identity Provider is vulnerable, all Service Providers are affected. In other words – even if the Service Provider is implemented correctly, an attacker can successfully get illegitimate access to restricted resources, e.g. victim's account.