Debugging unexpected access denials in Spring Security is a systematic process of checking configuration, roles, and logs in order—from the security rules themselves down to what authorities the user actually holds at runtime.
Key Points: • Re-check the SecurityFilterChain rules to confirm the URL pattern and required role/authority actually match what's expected, including matcher ordering. • Enable Spring Security debug logging (logging.level.org.springframework.security=DEBUG) to see exactly which filter and rule rejected the request. • Verify the authenticated user's actual GrantedAuthority values, since a role name mismatch (e.g., missing "ROLE_" prefix) is a common silent cause. • Check for conflicting method-level annotations (@PreAuthorize, @Secured) that might impose stricter conditions than the URL rule. • Confirm the token or session is valid and not expired, since an unnoticed expiry can look like a permissions problem.
Example: Debug logging reveals that a request is being matched by an earlier, more general requestMatcher than the intended specific one, because the general rule was declared first in the chain.
Interview Tip: A concise interview answer is:
"I'd start by re-checking the security configuration for matcher ordering and role names, then enable Spring Security's debug logging to see exactly which rule rejected the request. I'd also verify the user's actual granted authorities and check for any conflicting method-level annotations that might be more restrictive than the URL rule."