What is 'HTTP 401 vs 403' and why does the distinction matter for API security?

• A.401 Unauthorized means authentication is required or has failed - the client should authenticate. 403 Forbidden means the authenticated client is not authorized to access the resource - reauthenticating won't help. For security: returning 403 for unauthenticated requests reveals the resource exists; returning 401 is more accurate. Some security-sensitive implementations return 404 (Not Found) for unauthorized access to hide resource existence (security through obscurity)
• B.401 Unauthorized means the token is valid yet expired (the client should refresh it); 403 Forbidden means the server is blocking the client IP (changing network or VPN exit will help). For security: returning 401 for missing resources reveals the endpoint exists; returning 404 is more accurate. Some security-sensitive implementations return 410 (Gone) for unauthorized access to hide resource existence (a form of obscurity by design) on public APIs exposed to anonymous internet traffic