When using copy_headers in a forward_auth block, client-supplied headers with
the same names were not being removed before being forwarded to the backend.
This happens because PR #6608 added a MatchNot guard that skips the Set
operation when the auth service does not return a given header. That guard
prevents setting headers to empty strings, which is the correct behavior,
but it also means a client can send X-User-Id: admin in their request and
if the auth service validates the token without returning X-User-Id, Caddy
skips the Set and the client value passes through unchanged to the backend.
The fix adds an unconditional delete route for each copy_headers entry,
placed just before the existing conditional set route. The delete always runs
regardless of what the auth service returns. The conditional set still only
runs when the auth service provides that header.
The end result is:
- Client-supplied headers are always removed
- When the auth service returns the header, the backend gets that value
- When the auth service does not return the header, the backend sees nothing
Existing behavior is unchanged for any deployment where the auth service
returns all of the configured copy_headers entries.
Fixes GHSA-7r4p-vjf4-gxv4
See https://caddy.community/t/using-forward-auth-and-writing-my-own-authenticator-in-php/16410, apparently it didn't work when `copy_headers` wasn't used. This is because we were skipping adding a handler to the routes in the "good response handler", but this causes the logic in `reverseproxy.go` to ignore the response handler since it's empty. Instead, we can just always put in the `header` handler, even with an empty `Set` operation, it's just a no-op, but it fixes that condition in the proxy code.
