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:
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:
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()`.
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).
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):
* 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: [https://www.gosecure.net/blog/2017/03/22/detecting-deserialization-bugs-with-dns-exfiltration/](https://www.gosecure.net/blog/2017/03/22/detecting-deserialization-bugs-with-dns-exfiltration/)
**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.
**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.
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:
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.
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.
