What are the differences between 403 Forbidden and 401 Unauthorized HTTP responses?

The 401 Unauthorized and 403 Forbidden HTTP status codes both indicate client access issues, but they address distinct scenarios. Here’s a breakdown of their differences:

1. 401 Unauthorized

• Meaning: The request lacks valid authentication credentials (e.g., username/password, API token, JWT).
• When to Use:
• The client failed to authenticate (e.g., no credentials provided).
• The provided credentials are invalid or expired.
• Server Response:
• Includes a WWW-Authenticate header specifying how to authenticate (e.g., Basic, Bearer).
• Example: Accessing an admin dashboard without logging in.
• Client Action:
• Re-send the request with valid credentials (e.g., via login prompt).

2. 403 Forbidden

• Meaning: The client is authenticated but not authorized to access the resource.
• When to Use:
• The user lacks permissions (e.g., a regular user trying to access an admin-only endpoint).
• The server denies access regardless of credentials (e.g., IP blacklisting, file/directory permissions).
• Server Response:
• No authentication headers (credentials won’t help).
• Example: A logged-in user attempting to view another user’s private data.
• Client Action:
• Do not retry with the same credentials (access is permanently denied).

Key Differences

Feature	401 Unauthorized	403 Forbidden
Authentication	Credentials missing/invalid	Credentials valid but insufficient permissions
Authorization	Not checked (authentication failed first)	Explicitly denied
Response Headers	Includes WWW-Authenticate	No authentication headers
Client Behavior	Retry with valid credentials	Do not retry (access blocked)
Common Use Cases	Login required, expired token	Insufficient roles, IP blocked, file denied

Examples

401 Unauthorized

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="API"

Cause: Missing/invalid JWT token for an API endpoint.

403 Forbidden

HTTP/1.1 403 Forbidden
Content-Type: application/json

{ "error": "User lacks admin privileges" }

Cause: A non-admin user tries to delete a resource.

Flowchart: Choosing Between 401 and 403

Is the client authenticated?
├─ **No** → Return **401** (Provide credentials).
└─ **Yes** → Does the client have permission?
   ├─ **Yes** → Allow access (200 OK).
   └─ **No** → Return **403** (Forbidden).

Summary

• 401: “Who are you?” (Authentication issue).
• 403: “I know who you are, but you can’t do this.” (Authorization issue).