Client authentication errors cannot be serialised differently without reimplementing onAuthenticationFailure

This is a follow-up to #18285, where the same hook came up. There the answer was:

This is already possible. See Configuring Client Authentication and the clientAuthentication.errorResponseHandler(), which is an AuthenticationFailureHandler.

That is correct, and it is what we use. But it is the only hook, and it is all or nothing. If the thing you want to change is the serialisation of the error body, you have to take over the whole response writing.

Our case: legacy clients that expect an additional field in the error body. Nothing about status code, headers or when a failure happens, only how the OAuth2Error is written.

At the token endpoint this is a few lines, because the default failure handler is public and has a setter:

OAuth2ErrorHttpMessageConverter errorConverter = new OAuth2ErrorHttpMessageConverter();
errorConverter.setErrorParametersConverter(ourSerialisation());

OAuth2ErrorAuthenticationFailureHandler handler = new OAuth2ErrorAuthenticationFailureHandler();
handler.setErrorResponseConverter(errorConverter);
// tokenEndpoint.errorResponseHandler(handler)

ErrorSerializationTest asserts that the custom parameters really end up in the response body.

At OAuth2ClientAuthenticationFilter the same is not reachable (ClientAuthenticationFilterHooksTest):

• the filter is final, so no subclass
• errorHttpResponseConverter is a private final field, built in the filter itself, and there is no setter for it. The setters are setAuthenticationConverter, setAuthenticationSuccessHandler and setAuthenticationFailureHandler, nothing else
• the filter writes the error response itself in onAuthenticationFailure

So the only way to change one field is to pass an AuthenticationFailureHandler that does again what the default does. In practice that means copying the body of onAuthenticationFailure and keeping that copy correct across upgrades, for a change that has nothing to do with failure handling.

For two filters in the same package that both write an OAuth2Error this looks like an asymmetry that was not intended, especially since the class that solves it, OAuth2ErrorAuthenticationFailureHandler, is right there.

Suggestion, either one is fine for us:

1. add a setter for the error converter on OAuth2ClientAuthenticationFilter
2. or give the filter the same default OAuth2ErrorAuthenticationFailureHandler that the token endpoint uses, so the existing errorResponseHandler() hook reaches the serialisation

The second is smaller and makes both endpoints behave the same way. I am happy to open a PR for the one you prefer.

Tests: https://github.com/macstab/spring-authorization-server-issues - green on 7.1.1, on 7.0.7 and on standalone 1.5.3.

Start with OAuth2ClientAuthenticationFilter and the ClientAuthenticationFilterHooksTest, then compare its failure-response path with OAuth2ErrorAuthenticationFailureHandler and the ErrorSerializationTest. Done means a client authentication error can use a custom OAuth2Error serialization without reimplementing the complete failure handler, with tests covering the additional response field.