protected AuthenticationHandlerExecutionResult buildPrincipalFromResponse(
        final UsernamePasswordCredential credential,
        final HttpResponse response) throws Throwable {
        try {
            try (val content = ((HttpEntityContainer) response).getEntity().getContent()) {
                val result = IOUtils.toString(content, StandardCharsets.UTF_8);
                LOGGER.debug("REST authentication response received: [{}]", result);
                val principalFromRest = MAPPER.readValue(result, Principal.class);
                val principal = principalFactory.createPrincipal(principalFromRest.getId(), principalFromRest.getAttributes());
                return createHandlerResult(credential, principal, getWarnings(response));
            }
        } catch (final Throwable e) {
            LoggingUtils.error(LOGGER, e);
            throw new FailedLoginException("Unable to detect the authentication principal for " + credential.getUsername());
        }
    }
Header name that explains the warnings.
support/cas-server-support-rest-authentication/src/main/java/org/apereo/cas/adaptors/rest/RestAuthenticationHandler.java