Spring Security is a comprehensive authentication and authorization library for Spring-based applications, supporting both Servlet (MVC) and Reactive (WebFlux) stacks. In production, it intercepts HTTP requests via a configurable filter chain (SecurityFilterChain for servlet, WebFilterChain for reactive) to enforce authentication, session management, CSRF protection, security headers, CORS, and access control. It provides a pluggable architecture with core abstractions (AuthenticationManager, AuthorizationManager, SecurityContextHolderStrategy) and protocol-specific modules for OAuth2/OIDC, SAML2, LDAP, CAS, WebAuthn, Kerberos, and JWT. The library is configured via annotations (@EnableWebSecurity, @EnableMethodSecurity), a Kotlin DSL, or XML namespace, and integrates deeply with Spring's configuration and dependency injection. The architecture is modular: the 'core' module defines foundational interfaces (Authentication, GrantedAuthority, SecurityContext), 'web' implements servlet and reactive security filters, 'config' provides annotation-driven and XML configuration support, and protocol modules (oauth2, saml2, ldap, webauthn, cas) handle specific authentication/authorization flows. Key entry points include the HttpSecurity/ServerHttpSecurity builders, SecurityFilterChain, and method-security annotations (@PreAuthorize, @Secured). External dependencies include Spring Framework, Jackson (for serialization), Project Reactor (for reactive support), OpenSAML, and LDAP libraries. The codebase demonstrates strong engineering practices: extensive use of strategy, composite, and builder patterns; clean separation of servlet and reactive APIs; and a consistent Jackson mixin pattern for serialization of security objects.