HTTP 407 Without a Usable Proxy-Authenticate Challenge

A client cannot select an authentication mechanism from a status number alone. The proxy's response needs to describe what it accepts.

HTTP requires a proxy generating 407 to send a Proxy-Authenticate challenge. For a Basic service, a simple example is:

HTTP/1.1 407 Proxy Authentication Required
Proxy-Authenticate: Basic realm="Office proxy"

The numeric status, header name and scheme-specific syntax all matter. A branded error page with the word authentication in its body is not a replacement for the header. Custom implementations must also send a complete HTTP 407 response, including the header terminator and body framing.

Inspect the response that reached this client

Capture the status and challenge fields with secret values removed. An intermediary may have transformed the response, or a custom proxy may have constructed an invalid challenge. Compare that evidence with the proxy's own transaction log.

If the service sends 405 or another error instead, do not relabel every such response as 407 inside a custom client. You would hide genuine method-policy failures and still have no reliable scheme negotiation. Repair the service behavior or use its documented supported authentication path.

Multiple challenges also require a real HTTP parser. A comma can separate parameters or challenges; splitting the entire header blindly on commas can corrupt a quoted realm. Use the client's supported authentication implementation rather than a succession of string replacements.

Only after the challenge is usable does it make sense to investigate why the credential provider did not return an account.