A server operator has received a legal demand to deny access to a resource or to a set of resources that includes the requested resource.
What does HTTP 451 mean?
The 451 Unavailable For Legal Reasons code is part of the 4XX class. This is a client error code — the request was faulty, unauthorized, or cannot be fulfilled. The problem is on the request side.
When is it used?
A server returns 451 Unavailable For Legal Reasons in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.