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Also known as: 401 Unauthorized
An HTTP client error indicating missing or invalid authentication credentials for the target resource.
DESCRIPTION
HTTP 401 Unauthorized indicates the request lacks valid authentication credentials for the target resource. Servers typically return a WWW-Authenticate header describing how to obtain credentials. Clients should prompt for login, refresh tokens, or attach proper credentials. Unlike 403, 401 signals authentication, not authorization, failure. It commonly appears with bearer tokens, API keys, or basic authentication. Security best practices include not leaking sensitive details in error messages. Repeated 401s may reflect token expiration or clock skew. Proper usage improves security posture and UX by clearly directing clients to reauthenticate.
ALTERNATIVE DEFINITIONS
MDN Web Docs
The HTTP 401 Unauthorized response status code indicates that the client request has not been completed because it lacks valid authentication credentials for the requested resource.
Clarifies authentication failure and incomplete request due to missing credentials.
IETF RFC
The 401 (Unauthorized) status code indicates that the request has not been applied because it lacks valid authentication credentials for the target resource.
Normative definition distinguishing authentication needs for the target resource.
RELATED TERMS
OAuth 2.0 OpenID Connect WWW-Authenticate Bearer Token API Key Basic Authentication TLS Access Token Token Introspection
COMPETING TERMS
KNOWLEDGE AREAS
Data Security Software Engineering Data Integration & Interoperability
TARGET AUDIENCE
Software Developer DevOps Engineer Cloud Architect AI Engineer
EXTERNAL SOURCES
- Wikipedia - HTTP 401
- MDN Web Docs - Explains auth challenge header
- IETF - Bearer token usage in HTTP
TOP QUESTIONS
When should an API return HTTP 401?
Return 401 when a request lacks valid authentication credentials. Include WWW-Authenticate to inform clients how to obtain or present credentials.
Category: Requirements
How is HTTP 401 different from HTTP 403 for authentication?
401 means authentication is missing or invalid; 403 means authentication succeeded but access is not allowed. They signal distinct client actions.
Category: Misunderstanding
What are common causes of HTTP 401 in token-based systems?
Expired tokens, invalid signatures, incorrect audience, or missing Authorization headers commonly trigger 401 responses.
Category: Example
How should clients handle repeated HTTP 401 responses?
Refresh tokens if available, reauthenticate, or prompt the user. Ensure clock synchronization to avoid premature expirations.
Category: Management
Is it safe to include error details with HTTP 401?
Limit details to avoid information leaks; use generic messages and standard headers, logging specifics server-side.
Category: Risk
Does HTTP 401 support multiple authentication schemes?
Yes, servers can include multiple WWW-Authenticate challenges to indicate supported schemes like Bearer and Basic.
Category: Overview
What’s a hot topic around HTTP 401 today?
Short-lived access tokens with rotating refresh tokens and proof-of-possession schemes are trending for better security.
Category: Topic
Can CORS errors be mistaken for HTTP 401?
Yes, browsers may block responses due to CORS, appearing as auth issues. Check network logs and headers carefully.
Category: Mistakes
How does OAuth 2.0 relate to HTTP 401?
OAuth 2.0 uses 401 with WWW-Authenticate to indicate invalid or expired bearer tokens and convey error codes.
Category: Overview