501
HTTPSERVER_ERRORCritical5xx Server ErrorHIGH confidence
Not Implemented
Production Risk
Moderate. While it correctly communicates a server capability issue, it may point to a client trying to use a feature that the server is expected to have.
What this means
The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.
Why it happens
- 1A client sends a request with an obscure or custom HTTP method (e.g., 'TRACE') that the server does not handle.
- 2The server is a simple file server and receives a request for a feature it does not have, like 'LOCK'.
- 3The feature is planned but has not been built yet.
How to reproduce
A client sends a request using the 'PATCH' method to an old server that was built before the PATCH method was standardized.
trigger — this will error
trigger — this will error
PATCH /api/resource/123 HTTP/1.1 Host: old-server.example.com
expected output
HTTP/1.1 501 Not Implemented
Fix
Implement the Feature
WHEN You are the server developer and the feature is desired.
Implement the Feature
Add a handler for the requested method or functionality.
Why this works
Server-Side Development
What not to do
✕
Sources
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