fix: Deserialization checks valid class types for HttpTransportFactory

appengine/java/com/google/auth/appengine/AppEngineCredentials.java

@@ -43,6 +43,8 @@
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.IOException;
 import java.io.ObjectInputStream;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
 import java.util.Collection;
 import java.util.Date;
 import java.util.Objects;
@@ -132,7 +134,29 @@ public boolean equals(Object obj) {
 
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     input.defaultReadObject();
-    appIdentityService = newInstance(appIdentityServiceClassName);
+    try {
+      // Load the class without initializing it (second argument: false) to prevent
+      // static initializers from running, which could potentially be used for
+      // malicious purposes. Use the class loader of AppIdentityService (third
+      // argument) to ensure the class is loaded from the same context as the library,
+      // preventing class loading manipulation.
+      Class<?> clazz =
+          Class.forName(
+              appIdentityServiceClassName, false, AppIdentityService.class.getClassLoader());
+      if (!AppIdentityService.class.isAssignableFrom(clazz)) {
+        throw new IOException(
+            String.format(
+                "The class, %s, is not assignable from %s.",
+                appIdentityServiceClassName, AppIdentityService.class.getName()));
+      }
+      Constructor<?> constructor = clazz.getConstructor();
+      appIdentityService = (AppIdentityService) constructor.newInstance();
+    } catch (InstantiationException
+        | IllegalAccessException
+        | NoSuchMethodException
+        | InvocationTargetException e) {
+      throw new IOException(e);
+    }
   }
 
   public static Builder newBuilder() {

appengine/javatests/com/google/auth/appengine/AppEngineDeserializationSecurityTest.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2026, Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *    * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *    * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *
+ *    * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.google.auth.appengine;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.lang.reflect.Field;
+import java.util.Collections;
+import org.junit.jupiter.api.Test;
+
+class AppEngineDeserializationSecurityTest {
+
+  /** A class that does not implement HttpTransportFactory. */
+  static class ArbitraryClass {}
+
+  @Test
+  void testArbitraryClassInstantiationPrevented() throws Exception {
+    // 1. Create valid credentials
+    AppEngineCredentials credentials =
+        AppEngineCredentials.newBuilder().setScopes(Collections.singleton("scope")).build();
+
+    // 2. Use reflection to set appIdentityServiceClassName to ArbitraryClass
+    // as the setter must be of AppIdentityService
+    Field classNameField =
+        AppEngineCredentials.class.getDeclaredField("appIdentityServiceClassName");
+    classNameField.setAccessible(true);
+    classNameField.set(credentials, ArbitraryClass.class.getName());
+
+    // 3. Serialize
+    ByteArrayOutputStream bos = new ByteArrayOutputStream();
+    ObjectOutputStream oos = new ObjectOutputStream(bos);
+    oos.writeObject(credentials);
+    oos.close();
+
+    // 4. Deserialize
+    ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
+    ObjectInputStream ois = new ObjectInputStream(bis);
+
+    // 5. Assert that IOException is thrown (validation failure)
+    assertThrows(IOException.class, ois::readObject);
+  }
+
+  @Test
+  void testValidServiceDeserialization() throws Exception {
+    // 1. Create valid credentials with MockAppIdentityService
+    MockAppIdentityService mockService = new MockAppIdentityService();
+    AppEngineCredentials credentials =
+        AppEngineCredentials.newBuilder()
+            .setScopes(Collections.singleton("scope"))
+            .setAppIdentityService(mockService)
+            .build();
+
+    // 2. Serialize
+    ByteArrayOutputStream bos = new ByteArrayOutputStream();
+    ObjectOutputStream oos = new ObjectOutputStream(bos);
+    oos.writeObject(credentials);
+    oos.close();
+
+    // 3. Deserialize
+    ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
+    ObjectInputStream ois = new ObjectInputStream(bis);
+
+    AppEngineCredentials deserialized = (AppEngineCredentials) ois.readObject();
+
+    // 4. Verify deserialization success and field type
+    assertNotNull(deserialized);
+    Field serviceField = AppEngineCredentials.class.getDeclaredField("appIdentityService");
+    serviceField.setAccessible(true);
+    Object service = serviceField.get(deserialized);
+    assertEquals(MockAppIdentityService.class, service.getClass());
+  }
+
+  @Test
+  void testNonExistentClassDeserialization() throws Exception {
+    // 1. Create valid credentials
+    AppEngineCredentials credentials =
+        AppEngineCredentials.newBuilder().setScopes(Collections.singleton("scope")).build();
+
+    // 2. Use reflection to set appIdentityServiceClassName to non-existent class
+    Field classNameField =
+        AppEngineCredentials.class.getDeclaredField("appIdentityServiceClassName");
+    classNameField.setAccessible(true);
+    classNameField.set(credentials, "com.google.nonexistent.Class");
+
+    // 3. Serialize
+    ByteArrayOutputStream bos = new ByteArrayOutputStream();
+    ObjectOutputStream oos = new ObjectOutputStream(bos);
+    oos.writeObject(credentials);
+    oos.close();
+
+    // 4. Deserialize
+    ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
+    ObjectInputStream ois = new ObjectInputStream(bis);
+
+    // 5. Assert ClassNotFoundException
+    assertThrows(ClassNotFoundException.class, ois::readObject);
+  }
+}

oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -673,6 +673,9 @@ public boolean equals(Object obj) {
 
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 

oauth2_http/java/com/google/auth/oauth2/ExternalAccountAuthorizedUserCredentials.java

@@ -355,6 +355,9 @@ static ExternalAccountAuthorizedUserCredentials fromJson(
 
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -605,6 +605,9 @@ public CredentialSource getCredentialSource() {
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     // Properly deserialize the transient transportFactory.
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -378,6 +378,9 @@ public Builder toBuilder() {
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     // properly deserialize the transient transportFactory.
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 

oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java

@@ -911,6 +911,9 @@ public ImpersonatedCredentials build() {
 
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 }

oauth2_http/java/com/google/auth/oauth2/OAuth2Credentials.java

@@ -35,6 +35,7 @@
 import com.google.auth.Credentials;
 import com.google.auth.RequestMetadataCallback;
 import com.google.auth.http.AuthHttpConstants;
+import com.google.auth.http.HttpTransportFactory;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.MoreObjects;
 import com.google.common.base.Preconditions;
@@ -51,6 +52,8 @@
 import java.io.IOException;
 import java.io.ObjectInputStream;
 import java.io.Serializable;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
 import java.net.URI;
 import java.time.Duration;
 import java.util.ArrayList;
@@ -475,11 +478,60 @@ private void readObject(ObjectInputStream input) throws IOException, ClassNotFou
     refreshTask = null;
   }
 
-  @SuppressWarnings("unchecked")
-  protected static <T> T newInstance(String className) throws IOException, ClassNotFoundException {
+  /**
+   * Best-effort safe mechanism to attempt to instantiate an {@link HttpTransportFactory} from a
+   * class name.
+   *
+   * <p>This method attempts to avoid Arbitrary Code Execution (ACE) vulnerabilities by:
+   *
+   * <ol>
+   *   <li>Checking if the class name matches the default or ServiceLoader-provided factory, and
+   *       returning that instance if so.
+   *   <li>If not, loading the class using reflection without running static initializers.
+   *   <li>Verifying that the loaded class is assignable to {@link HttpTransportFactory}.
+   *   <li>Only after verification, instantiating the class using its default constructor.
+   * </ol>
+   *
+   * @param className The fully qualified name of the class to instantiate.
+   * @return An instance of {@link HttpTransportFactory}.
+   * @throws IOException If the class cannot be loaded, is the wrong type, or cannot be
+   *     instantiated.
+   * @throws ClassNotFoundException If the class cannot be found.
+   */
+  protected static HttpTransportFactory newInstance(String className)
+      throws IOException, ClassNotFoundException {
+    // Check if the requested class matches the default or ServiceLoader-provided
+    // factory. This avoids unsafe reflection for the most common use cases.
+    // This check runs first to replicate the logic in Credential constructor.
+    HttpTransportFactory currentFactory =
+        getFromServiceLoader(HttpTransportFactory.class, OAuth2Utils.HTTP_TRANSPORT_FACTORY);
+    // It is possible that there is a custom implementation of HttpTransportFactory
+    if (className.equals(currentFactory.getClass().getName())) {
+      return currentFactory;
+    }
+
+    // Fallback to reflection if the requested class differs from the ServiceLoader
+    // default. This handles cases where a custom factory was used during
+    // serialization but is not the currently active ServiceLoader provider.
     try {
-      return (T) Class.forName(className).newInstance();
-    } catch (InstantiationException | IllegalAccessException e) {
+      // Load the class without initializing it (second argument: false) to prevent
+      // static initializers from running, which could potentially be used for
+      // malicious purposes. Use the class loader of HttpTransportFactory (third argument)
+      // to ensure the class is loaded from the same context as the library, preventing
+      // class loading manipulation.
+      Class<?> clazz = Class.forName(className, false, HttpTransportFactory.class.getClassLoader());
+      if (!HttpTransportFactory.class.isAssignableFrom(clazz)) {
+        throw new IOException(
+            String.format(
+                "The class, %s, is not assignable from %s.",
+                className, HttpTransportFactory.class.getName()));
+      }
+      Constructor<?> constructor = clazz.getDeclaredConstructor();
+      return (HttpTransportFactory) constructor.newInstance();
+    } catch (InstantiationException
+        | IllegalAccessException
+        | NoSuchMethodException
+        | InvocationTargetException e) {
       throw new IOException(e);
     }
   }

oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java

@@ -1126,6 +1126,9 @@ private Map<String, List<String>> getRequestMetadataWithSelfSignedJwt(URI uri)
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     // properly deserialize the transient transportFactory
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }
 

oauth2_http/java/com/google/auth/oauth2/UserCredentials.java

@@ -389,6 +389,9 @@ public boolean equals(Object obj) {
 
   private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
     input.defaultReadObject();
+    // Use Oauth2Credential's newInstance() to try to safely instantiate the transport factory,
+    // with best-effort prevention against RCE attacks by validating the class name and loading
+    // behavior.
     transportFactory = newInstance(transportFactoryClassName);
   }