We love developing applications, but sometimes we overlook critical security aspects, especially in the authentication. This oversight can lead to serious consequences. In this session, we'll explore the vulnerabilities that arise when authentication methods are weak, particularly in large-scale applications. We'll highlight the importance of Single Sign-On (SSO) authentication in modern development, and explain how SSO components interact with each other, including Identity Providers (IdP) and Service Providers (SP). We'll compare standards like SAML, OpenID, and OAuth, and demonstrate practical implementation on .NET using third-party solutions for seamless integration. Understanding SAML request/response formats and decoding tools will be the key. We'll also discuss real-world SAML configuration errors and vulnerabilities, and share best practices for securing SSO configurations, drawing on trusted sources like OWASP for guidance.