Enforce strict fields parsing for SigningExtensionHandler

 Uses custom copy of global ObjectMapper with FAIL_ON_UNKNOWN_PROPERTIES enabled

acceptance-tests/src/test/java/tech/pegasys/web3signer/tests/signing/ProofOfValidationSigningExtAcceptanceTest.java

@@ -136,4 +136,17 @@ void invalidSignExtensionTypeCausesBadRequestStatusCode() throws Exception {
 
     signer.signExtensionPayload(PUBLIC_KEY.toString(), payload, JSON).then().statusCode(400);
   }
+
+  @Test
+  void extraJsonFieldsCausesBadRequestStatusCode() throws Exception {
+    final ProofOfValidationBody proofOfValidationBody =
+        new ProofOfValidationBody(
+            SigningExtensionType.PROOF_OF_VALIDATION,
+            "AT",
+            String.valueOf(System.currentTimeMillis()));
+    var payload = JSON_MAPPER.writeValueAsString(proofOfValidationBody);
+    payload = payload.replace("}", ",\"extraField\": \"extraValue\"}");
+
+    signer.signExtensionPayload(PUBLIC_KEY.toString(), payload, JSON).then().statusCode(400);
+  }
 }

core/src/main/java/tech/pegasys/web3signer/core/service/http/handlers/signing/SigningExtensionHandler.java

@@ -22,6 +22,7 @@
 import java.util.List;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.DeserializationFeature;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import io.vertx.core.Handler;
 import io.vertx.core.json.JsonObject;
@@ -33,7 +34,11 @@
 public class SigningExtensionHandler implements Handler<RoutingContext> {
   public static final int NOT_FOUND = 404;
   public static final int BAD_REQUEST = 400;
-  private static final ObjectMapper JSON_MAPPER = SigningObjectMapperFactory.createObjectMapper();
+  // custom copy of ObjectMapper that fails on unknown properties.
+  private static final ObjectMapper JSON_MAPPER =
+      SigningObjectMapperFactory.createObjectMapper()
+          .copy()
+          .enable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
 
   private final SignerForIdentifier<?> signerForIdentifier;