To diagnose code 401 in the HTTP domain, start with the following check: Reconnect. Then compare the result to this probable cause: Session expired. Save the context and time of appearance to compare behavior before and after correction. Priority level indicated: Average…
Operational context
HTTP 401 means the request lacks valid authentication credentials for the target resource. Diagnose the authentication challenge, token/session state, clock and proxy behavior before changing application authorization rules.
Step-by-step checks
Capture the response status and WWW-Authenticate header to identify the authentication scheme and issuer expected by the server.
Confirm the request actually carries the intended Authorization header or session cookie and that an intermediate proxy is not stripping it.
Validate token expiry, audience, issuer and client/server time when OAuth/JWT is used, or credential/SPN negotiation for integrated authentication.
Reproduce with a known-valid account and a clean session to separate authentication failure from application-specific authorization that should normally return 403.
Useful verification commands
Use commands only on systems you administer and capture the read-only output before making a configuration change.
curl -I https://example.com/protected
curl -vk https://example.com/protected
How to validate the result
The same resource must return its normal success response after valid credentials are supplied, while unauthenticated requests continue to receive the intended 401 challenge.
Evidence to keep
Keep request URL/method, 401 response headers, authentication scheme, sanitized token claims or account identity, client/server time and reverse-proxy/authentication logs.
Frequently asked question
What is the practical difference between HTTP 401 and 403?
401 indicates authentication is missing or invalid. 403 indicates the server understood the identity/request but refuses access under authorization or policy.