URLDNS链分析

一、概述

URLDNS 是ysoserial中利用链的一个名字，通常用于检测是否存在Java反序列化漏洞。该利用链具有如下特点：

不限制jdk版本，使用Java内置类，对第三方依赖没有要求

目标无回显，可以通过DNS请求来验证是否存在反序列化漏洞

URLDNS利用链，只能发起DNS请求，并不能进行其他利用

二、流程图

三、原理

java.util.HashMap 重写了 readObject, 在反序列化时会调用 hash 函数计算 key 的 hashCode.而 java.net.URL 的 hashCode 在计算时会调用 getHostAddress 来解析域名, 从而发出 DNS 请求.

四、过程概述

1、HashMap.readObject

在调用readObject方法时添加断点

调试到断点位置，步入此步，进入HashMap.readObject

private void readObject(java.io.ObjectInputStream s)
    throws IOException, ClassNotFoundException {
    // Read in the threshold (ignored), loadfactor, and any hidden stuff
    s.defaultReadObject();
    reinitialize();
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new InvalidObjectException("Illegal load factor: " +
                                         loadFactor);
    s.readInt();                // Read and ignore number of buckets
    int mappings = s.readInt(); // Read number of mappings (size)
    if (mappings < 0)
        throw new InvalidObjectException("Illegal mappings count: " +
                                         mappings);
    else if (mappings > 0) { // (if zero, use defaults)
        // Size the table using given load factor only if within
        // range of 0.25...4.0
        float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);
        float fc = (float)mappings / lf + 1.0f;
        int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?
                   DEFAULT_INITIAL_CAPACITY :
                   (fc >= MAXIMUM_CAPACITY) ?
                   MAXIMUM_CAPACITY :
                   tableSizeFor((int)fc));
        float ft = (float)cap * lf;
        threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?
                     (int)ft : Integer.MAX_VALUE);
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] tab = (Node<K,V>[])new Node[cap];
        table = tab;

        // Read the keys and values, and put the mappings in the HashMap
        for (int i = 0; i < mappings; i++) {
            @SuppressWarnings("unchecked")
                K key = (K) s.readObject();
            @SuppressWarnings("unchecked")
                V value = (V) s.readObject();
            putVal(hash(key), key, value, false, false);
        }
    }
}

关注putVal方法，putVal是往HashMap中放入键值对的方法，这里调用了hash方法来处理key，跟进hash方法（步入putVal后，单击hash）：

2、HashMap.hash

    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

这里又调用了key.hashcode方法，而key此时是我们传入的 java.net.URL 对象，那么跟进到这个类的hashCode()方法看下

3、URL.hashCode

public synchronized int hashCode() {
    if (hashCode != -1)
        return hashCode;

    hashCode = handler.hashCode(this);
    return hashCode;
}

当hashCode字段等于-1时会进行handler.hashCode(this)计算，跟进handler发现，定义是

transient URLStreamHandler handler; // transient 关键字，修饰Java序列化对象时，不需要序列化的属性

跟进java.net.URLStreamHandler#hashCode()

4、java.net.URLStreamHandler#hashCode()

protected int hashCode(URL u) {
    int h = 0;

    // Generate the protocol part.
    String protocol = u.getProtocol();
    if (protocol != null)
        h += protocol.hashCode();

    // Generate the host part.
    InetAddress addr = getHostAddress(u);
    if (addr != null) {
        h += addr.hashCode();
    } else {
        String host = u.getHost();
        if (host != null)
            h += host.toLowerCase().hashCode();
    }

    // Generate the file part.
    String file = u.getFile();
    if (file != null)
        h += file.hashCode();

    // Generate the port part.
    if (u.getPort() == -1)
        h += getDefaultPort();
    else
        h += u.getPort();

    // Generate the ref part.
    String ref = u.getRef();
    if (ref != null)
        h += ref.hashCode();

    return h;
}

u 是我们传入的url，在调用getHostAddress方法时，会进行dns查询。

五、URL_DNS_POC构造

1、生成序列化文件

package com.mzy.URLDNS;

import java.io.*;

public class Serializable {
    public static void serializable(Object obj) throws IOException {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("mzy.ser"));
        oos.writeObject(obj);
        oos.close();
    }

    public static void main(String[] args) throws IOException {
        Student student = new Student("mzy",21);
        serializable(student);
    }
}
-----------------------------------------------------

运行此程序会在当前目录生成一个mm.ser的序列化文件

2、反序列化

package com.mzy.URLDNS;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;

public class unSerializable {
    public  static Object unserializable(String Filename) throws IOException, ClassNotFoundException {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(Filename));
        Object obj= ois.readObject();
        return obj;
    }

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Student stu = (Student) unserializable("mm.ser");
        System.out.println(stu);
    }

}

六、总结

JDK1.8下的调用路线：

HashMap->readObject()

HashMap->hash()

URL->hashCode()

URLStreamHandler->hashCode()

URLStreamHandler->getHostAddress()

InetAddress->getByName()