Security constraints are access rules that tell a web container which users, roles, HTTP methods, or URL patterns may reach a protected endpoint. In Java servers such as Tomcat, they are part of the authorization layer: before an application code path runs, the container checks whether the request matches the configured protection rules.
They matter because a small mismatch in path parsing, proxy rewriting, or endpoint mapping can make a protected resource reachable when it should not be. In practice, attackers look for these gaps to bypass login checks or reach WebSocket endpoints, admin paths, or other sensitive handlers. Defenders reduce risk by testing the exact proxy route used in production, keeping request parsing consistent across layers, and confirming that constraints apply to every equivalent URL form. If the front end and the application disagree about what a request means, security constraints can fail quietly.