mod_http_oauth2: Reject invalid attempt to register client without credentials The implicit flow works without a client_secret since the token is delivered directly, but all other currently supported grant types require client to authenticate using credentials, so it makes no sense to not issue credentials then.

This module authenticates server-to-server connections by looking for an already established connection that uses the exact same certificate, reusing
the earlier validation results and letting Prosody skip performing slower validation methods such as [POSH][mod_s2s_auth_posh] twice.