SAML (Security Assertion Markup Language) is a widely used protocol for exchanging authentication and authorization data between parties, but it can be prone to security vulnerabilities due to its XML-based nature and complex specifications. Common SAML security vulnerabilities include assertion manipulation, weak encryption, replay attacks, open redirects, man-in-the-middle attacks, XML External Entity (XXE) attacks, XML round-trip issues, signature exclusion, and more. To defend against these vulnerabilities, developers should follow best practices such as validating SAML responses, disabling DTD processing, using HTTPS, encrypting sensitive data, and employing well-known open-source libraries for parsing XML. Alternatively, consider using OpenID Connect (OIDC) or a vendor like WorkOS to handle SSO implementation and management.