URLDNS链分析

​ 之前学URLDNS的时候学的迷迷糊糊的，昨天学到shiro发现可以用URLDNS探测shiro反序列化是否存在，所以今天心血来潮又跟了一遍URLDNS,虽然之前跟了一遍但是这次收获颇多。

反序列化入口在HashMap类的readObject,先看readObject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[] tab = (Node[])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);
        }
    }
}

在最后会调用hash

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

跟进一下这个hash方法

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

发现调用了key.hashCode,查看ysoserial的URLDSN链发现这个key指的是URL类。

所以查看URL类的hashcode方法

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

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

当hashcode != -1时会返回hashcode变量，跟进一下hashCode,发现

private int hashCode = -1;

跟进一下handler

transient URLStreamHandler handler;

发现调用了URLStreamHandler，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();
        }

跟进getHostAddress

    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;
            }
        }

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。

这样Gadget就出来了

1. HashMap->readObject()
2. HashMap->hash()
3. URL->hashCode()
4. URLStreamHandler->hashCode()
5. URLStreamHandler->getHostAddress()
6. InetAddress->getByName()

然后我们再查看ysoserial的URLDNS链

package ysoserial.payloads;

import java.io.IOException;
import java.net.InetAddress;
import java.net.URLConnection;
import java.net.URLStreamHandler;
import java.util.HashMap;
import java.net.URL;

import ysoserial.payloads.annotation.Authors;
import ysoserial.payloads.annotation.Dependencies;
import ysoserial.payloads.annotation.PayloadTest;
import ysoserial.payloads.util.PayloadRunner;
import ysoserial.payloads.util.Reflections;



@SuppressWarnings({ "rawtypes", "unchecked" })
@PayloadTest(skip = "true")
@Dependencies()
@Authors({ Authors.GEBL })
public class URLDNS implements ObjectPayload