Handle custom status codes in error handler

Fixes an issue where custom status codes in the error response cause an
IllegalArgumentException to be thrown when resolving an HttpStatus.

Closes gh-9741

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/http/OAuth2ErrorResponseErrorHandler.java

@@ -52,7 +52,7 @@ public class OAuth2ErrorResponseErrorHandler implements ResponseErrorHandler {
 
 	@Override
 	public void handleError(ClientHttpResponse response) throws IOException {
-		if (!HttpStatus.BAD_REQUEST.equals(response.getStatusCode())) {
+		if (HttpStatus.BAD_REQUEST.value() != response.getRawStatusCode()) {
 			this.defaultErrorHandler.handleError(response);
 		}
 		// A Bearer Token Error may be in the WWW-Authenticate response header

oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/http/OAuth2ErrorResponseErrorHandlerTests.java

@@ -16,12 +16,17 @@
 
 package org.springframework.security.oauth2.client.http;
 
+import java.io.IOException;
+
 import org.junit.Test;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.client.ClientHttpResponse;
+import org.springframework.mock.http.MockHttpInputMessage;
 import org.springframework.mock.http.client.MockClientHttpResponse;
 import org.springframework.security.oauth2.core.OAuth2AuthorizationException;
+import org.springframework.web.client.UnknownHttpStatusCodeException;
 
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
@@ -67,4 +72,48 @@ public class OAuth2ErrorResponseErrorHandlerTests {
 				.isThrownBy(() -> this.errorHandler.handleError(response)).withMessage("[server_error] ");
 	}
 
+	@Test
+	public void handleErrorWhenErrorResponseWithInvalidStatusCodeThenHandled() {
+		CustomMockClientHttpResponse response = new CustomMockClientHttpResponse(new byte[0], 596);
+		assertThatExceptionOfType(UnknownHttpStatusCodeException.class)
+				.isThrownBy(() -> this.errorHandler.handleError(response)).withMessage("596 : [no body]");
+	}
+
+	private static final class CustomMockClientHttpResponse extends MockHttpInputMessage implements ClientHttpResponse {
+
+		private final int statusCode;
+
+		private CustomMockClientHttpResponse(byte[] content, int statusCode) {
+			super(content);
+			this.statusCode = statusCode;
+		}
+
+		@Override
+		public HttpStatus getStatusCode() throws IOException {
+			return HttpStatus.valueOf(getRawStatusCode());
+		}
+
+		@Override
+		public int getRawStatusCode() {
+			return this.statusCode;
+		}
+
+		@Override
+		public String getStatusText() throws IOException {
+			HttpStatus httpStatus = HttpStatus.resolve(this.statusCode);
+			return (httpStatus != null) ? httpStatus.getReasonPhrase() : "";
+		}
+
+		@Override
+		public void close() {
+			try {
+				getBody().close();
+			}
+			catch (IOException ex) {
+				// ignore
+			}
+		}
+
+	}
+
 }