407 Proxy Authentication Required
Meaning
The client must first authenticate itself with the proxy. Similar to 401 Unauthorized, but for proxy usage.
When to Use
When a request is being made through a proxy that requires authentication and the proper credentials have not been provided. The proxy must return a Proxy-Authenticate header with a challenge.
Example
A client trying to browse the web through a corporate proxy without logging in receives 407 Proxy Authentication Required, indicating it needs to provide valid proxy credentials.
When Not to Use
Do not use for origin server authentication (use 401 for that). This is specific to proxies requiring authentication before forwarding client requests.
Source
RFC 9110