Forbid mismatched inner element in XML flow inputs (#3104)

core/src/main/java/google/registry/flows/picker/FlowPicker.java

@@ -14,6 +14,7 @@
 
 package google.registry.flows.picker;
 
+import com.google.common.base.Ascii;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableTable;
@@ -255,6 +256,17 @@ public class FlowPicker {
     if (innerCommand == null && !(eppInput.getCommandWrapper() instanceof Hello)) {
       throw new MissingCommandException();
     }
+    if (innerCommand instanceof ResourceCommandWrapper resourceCommandWrapper) {
+      ResourceCommand resourceCommand = resourceCommandWrapper.getResourceCommand();
+      if (resourceCommand != null) {
+        String wrapperName = innerCommand.getClass().getSimpleName();
+        String commandName = resourceCommand.getClass().getSimpleName();
+        if (!wrapperName.equals(commandName)) {
+          throw new MismatchedCommandException(
+              Ascii.toLowerCase(wrapperName), Ascii.toLowerCase(commandName));
+        }
+      }
+    }
     // Try the FlowProviders until we find a match. The order matters because it's possible to
     // match multiple FlowProviders and so more specific matches are tried first.
     for (FlowProvider flowProvider : FLOW_PROVIDERS) {
@@ -279,4 +291,14 @@ public class FlowPicker {
       super("Command missing");
     }
   }
+
+  /** Command wrapper and inner resource command do not match. */
+  static class MismatchedCommandException extends SyntaxErrorException {
+    public MismatchedCommandException(String wrapperName, String commandName) {
+      super(
+          String.format(
+              "EPP command wrapper <%s> does not match resource command <%s>",
+              wrapperName, commandName));
+    }
+  }
 }

core/src/test/java/google/registry/flows/picker/FlowPickerTest.java

@@ -0,0 +1,96 @@
+// Copyright 2026 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.flows.picker;
+
+import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import google.registry.flows.Flow;
+import google.registry.flows.domain.DomainCheckFlow;
+import google.registry.flows.domain.DomainCreateFlow;
+import google.registry.model.domain.DomainCommand;
+import google.registry.model.eppcommon.EppXmlTransformer;
+import google.registry.model.eppinput.EppInput;
+import org.junit.jupiter.api.Test;
+
+/** Unit tests for {@link FlowPicker}. */
+class FlowPickerTest {
+
+  @Test
+  void testGetFlowClass_matchingWrapperAndCommand_returnsFlow() throws Exception {
+    EppInput checkEppInput = EppInput.create(EppInput.Check.create(new DomainCommand.Check()));
+    Class<? extends Flow> checkFlow = FlowPicker.getFlowClass(checkEppInput);
+    assertThat(checkFlow).isEqualTo(DomainCheckFlow.class);
+
+    EppInput createEppInput = EppInput.create(EppInput.Create.create(new DomainCommand.Create()));
+    Class<? extends Flow> createFlow = FlowPicker.getFlowClass(createEppInput);
+    assertThat(createFlow).isEqualTo(DomainCreateFlow.class);
+  }
+
+  @Test
+  void testGetFlowClass_mismatchedWrapperAndCommand_throwsSyntaxErrorException() {
+    EppInput mismatchedEppInput1 =
+        EppInput.create(EppInput.Check.create(new DomainCommand.Create()));
+    assertThat(
+            assertThrows(
+                FlowPicker.MismatchedCommandException.class,
+                () -> FlowPicker.getFlowClass(mismatchedEppInput1)))
+        .hasMessageThat()
+        .isEqualTo("EPP command wrapper <check> does not match resource command <create>");
+
+    EppInput mismatchedEppInput2 =
+        EppInput.create(EppInput.Create.create(new DomainCommand.Check()));
+    assertThat(
+            assertThrows(
+                FlowPicker.MismatchedCommandException.class,
+                () -> FlowPicker.getFlowClass(mismatchedEppInput2)))
+        .hasMessageThat()
+        .contains("EPP command wrapper <create> does not match resource command <check>");
+  }
+
+  @Test
+  void testGetFlowClass_mismatchedXml_throwsMismatchedCommandException() throws Exception {
+    String mismatchedXml =
+        """
+        <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+        <epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
+          <command>
+            <check>
+              <domain:create xmlns:domain="urn:ietf:params:xml:ns:domain-1.0">
+                <domain:name>example.com</domain:name>
+                <domain:authInfo>
+                  <domain:pw>fooBAR123</domain:pw>
+                </domain:authInfo>
+              </domain:create>
+            </check>
+            <clTRID>ABC-12345</clTRID>
+          </command>
+        </epp>
+        """;
+
+    // Verify JAXB successfully unmarshals the mismatched XML without throwing any errors
+    EppInput eppInput = EppXmlTransformer.unmarshal(EppInput.class, mismatchedXml.getBytes(UTF_8));
+    assertThat(eppInput).isNotNull();
+
+    // Verify that FlowPicker intercepts the unmarshalled input and blocks it
+    assertThat(
+            assertThrows(
+                FlowPicker.MismatchedCommandException.class,
+                () -> FlowPicker.getFlowClass(eppInput)))
+        .hasMessageThat()
+        .contains("EPP command wrapper <check> does not match resource command <create>");
+  }
+}