What is Deserialization?

OWASP Description: Data which is untrusted cannot be trusted to be well formed. Malformed data or unexpected data could be used to abuse application logic, deny service, or execute arbitrary code, when deserialized. James Forshaw has expertly discussed this attack vector in his Black Hat 2012 presentation. More recently, researchers Alvaro Muñoz and Oleksandr Mirosh have expanded upon this earlier. I’ve went ahead and posted the videos and the PDF as a reference. I hope to expand more on this topic in the future when I have more time. I am currently extremely busy studying.

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Description from the presentation:

Friday the 13th: JSON Attacks

“2016 was the year of Java deserialization apocalypse. Although Java Deserialization attacks were known for years, the publication of the Apache Commons Collection Remote Code Execution (RCE from now on) gadget finally brought this forgotten vulnerability to the spotlight and motivated the community to start finding and fixing these issues. One of the most suggested solutions for avoiding Java deserialization issues was to move away from Java Deserialization altogether and use safer formats such as JSON. In this talk, we will analyze the most popular JSON parsers in both .NET and Java for potential RCE vectors.

We will demonstrate that RCE is also possible in these libraries and present details about the ones that are vulnerable to RCE by default. We will also discuss common configurations that make other libraries vulnerable. In addition to focusing on JSON format, we will generalize the attack techniques to other serialization formats. In particular, we will pay close attention to several serialization formats in .NET. These formats have also been known to be vulnerable since 2012 but the lack of known RCE gadgets led some software vendors to not take this issue seriously. We hope this talk will change this. With the intention of bringing the due attention to this vulnerability class in .NET, we will review the known vulnerable formats, present other formats which we found to be vulnerable, and conclude with presenting several gadgets from system libraries that may be used to achieve RCE in a stable way: no memory corruption — just simple process invocation. Finally, we will provide recommendations on how to determine if your code is vulnerable, provide remediation advice, and discuss alternative approaches.”

https://www.blackhat.com/docs/us-17/thursday/us-17-Munoz-Friday-The-13th-JSON-Attacks-wp.pdf

https://www.blackhat.com/docs/us-17/thursday/us-17-Munoz-Friday-The-13th-Json-Attacks.pdf

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PDF Link to Forshaw presentation:

https://media.blackhat.com/bh-us-12/Briefings/Forshaw/BH_US_12_Forshaw_Are_You_My_Type_WP.pdf

 

“By: James Forshaw In May, Microsoft issued a security update for .NET due to a number of serious issues I found. This release was the biggest update in the product’s history, it aimed to correct a number of specific issues due to unsafe serialization usage as well as changing some of the core functionality to mitigate anything which could not be easily fixed without significant compatibility issues. This presentation will cover the process through which I identified these vulnerabilities and provide information on how they can be used to attack .NET applications, both locally and remotely, as well as demonstrating breaking out of the partial trust sandboxes used in technologies such as ClickOnce and XAML Browser Applications.”

 

 

Description from video: “Messaging can be found everywhere. It’s used by your favourite Mobile Messenger as well as in your bank’s backend system. Message Brokers such as Pivotal’s RabbitMQ, IBM’s WebSphere MQ and others often form a key component of a modern back-end system’s architecture. Furthermore, there are various messaging standards in place like AMQP, MQTT, and STOMP. When it comes to the Java World it is rather unknown that Messaging in the Java ecosystem relies heavily on Java’s serialization. Recent advances in the exploitation of Java deserialization vulnerabilities can be applied to exploit applications using Java messaging. This talk will show the attack surface of various Java messaging API implementations and their deserialization vulnerabilities. Last but not least, the Java Messaging Exploitation Tool (JMET) will be presented to help you identify and exploit message-consuming systems like a boss.”

 

Note: so far this is a collection of videos and material I have found useful while studying serialization and deserialization. My focus has been on .NET deserialization however these videos go over java vulnerabilities as well.

 

Note: the following has been copied from github and resourced for notes.

Introduction

This article is focused on providing clear, actionable guidance for safely deserializing untrusted data in your applications.

What is Deserialization?

Serialization is the process of turning some object into a data format that can be restored later. People often serialize objects in order to save them to storage, or to send as part of communications.

Deserialization is the reverse of that process, taking data structured from some format, and rebuilding it into an object. Today, the most popular data format for serializing data is JSON. Before that, it was XML.

However, many programming languages offer a native capability for serializing objects. These native formats usually offer more features than JSON or XML, including customizability of the serialization process.

Unfortunately, the features of these native deserialization mechanisms can be repurposed for malicious effect when operating on untrusted data. Attacks against deserializers have been found to allow denial-of-service, access control, and remote code execution (RCE) attacks.

Guidance on Deserializing Objects Safely

The following language-specific guidance attempts to enumerate safe methodologies for deserializing data that can’t be trusted.

 

PHP

WhiteBox Review

Check the use of unserialize() function and review how the external parameters are accepted. Use a safe, standard data interchange format such as JSON (via json_decode() and json_encode()) if you need to pass serialized data to the user.

 

Python

BlackBox Review

If the traffic data contains the symbol dot . at the end, it’s very likely that the data was sent in serialization.

 

WhiteBox Review

The following API in Python will be vulnerable to serialization attack. Search code for the pattern below.

  1. The uses of pickle/c_pickle/_pickle with load/loads:
import pickle
data = """ cos.system(S'dir')tR. """
pickle.loads(data)
  1. Uses of PyYAML with load:
import yaml
document = "!!python/object/apply:os.system ['ipconfig']"
print(yaml.load(document))
  1. Uses of jsonpickle with encode or store methods.

 

Java

The following techniques are all good for preventing attacks against deserialization against Java’s Serializable format.

Implementation advices:

  • In your code, override the ObjectInputStream#resolveClass() method to prevent arbitrary classes from being deserialized. This safe behavior can be wrapped in a library like SerialKiller.
  • Use a safe replacement for the generic readObject() method as seen here. Note that this addresses “billion laughs” type attacks by checking input length and number of objects deserialized.

 

WhiteBox Review

Be aware of the following Java API uses for potential serilization vulnerability.

1. XMLdecoder with external user defined parameters

2. XStream with fromXML method (xstream version <= v1.46 is vulnerable to the serialization issue)

3. ObjectInputSteam with readObject

4. Uses of readObjectreadObjectNodDatareadResolve or readExternal

5. ObjectInputStream.readUnshared

6. Serializable

 

BlackBox Review

If the captured traffic data include the following patterns may suggest that the data was sent in Java serialization streams

  • AC ED 00 05 in Hex
  • rO0 in Base64
  • Content-type header of an HTTP response set to application/x-java-serialized-object

 

Prevent Data Leakage and Trusted Field Clobbering

If there are data members of an object that should never be controlled by end users during deserialization or exposed to users during serialization, they should be declared as the transient keyword (section Protecting Sensitive Information).

For a class that defined as Serializable, the sensitive information variable should be declared as private transient.

For example, the class myAccount, the variable ‘profit’ and ‘margin’ were declared as transient to avoid to be serialized:

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

    private transient double margin; // declared transient
    ....

 

Prevent Deserialization of Domain Objects

Some of your application objects may be forced to implement Serializable due to their hierarchy. To guarantee that your application objects can’t be deserialized, a readObject() method should be declared (with a final modifier) which always throws an exception:

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

 

Harden Your Own java.io.ObjectInputStream

The java.io.ObjectInputStream class is used to deserialize objects. It’s possible to harden its behavior by subclassing it. This is the best solution if:

  • You can change the code that does the deserialization
  • You know what classes you expect to deserialize

The general idea is to override ObjectInputStream.html#resolveClass() in order to restrict which classes are allowed to be deserialized.

Because this call happens before a readObject() is called, you can be sure that no deserialization activity will occur unless the type is one that you wish to allow.

A simple example of this shown here, where the the LookAheadObjectInputStream class is guaranteed not to deserialize any other type besides the Bicycle class:

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);
    }
}

More complete implementations of this approach have been proposed by various community members:

  • NibbleSec – a library that allows whitelisting and blacklisting of classes that are allowed to be deserialized
  • IBM – the seminal protection, written years before the most devastating exploitation scenarios were envisioned.

 

Harden All java.io.ObjectInputStream Usage with an Agent

As mentioned above, the java.io.ObjectInputStream class is used to deserialize objects. It’s possible to harden its behavior by subclassing it. However, if you don’t own the code or can’t wait for a patch, using an agent to weave in hardening to java.io.ObjectInputStream is the best solution.

Globally changing ObjectInputStream is only safe for blacklisting known malicious types, because it’s not possible to know for all applications what the expected classes to be deserialized are. Fortunately, there are very few classes needed in the blacklist to be safe from all the known attack vectors, today.

It’s inevitable that more “gadget” classes will be discovered that can be abused. However, there is an incredible amount of vulnerable software exposed today, in need of a fix. In some cases, “fixing” the vulnerability may involve re-architecting messaging systems and breaking backwards compatibility as developers move towards not accepting serialized objects.

To enable these agents, simply add a new JVM parameter:

-javaagent:name-of-agent.jar

Agents taking this approach have been released by various community members:

A similar, but less scalable approach would be to manually patch and bootstrap your JVM’s ObjectInputStream. Guidance on this approach is available here.

 

.Net CSharp

 

WhiteBox Review

Search the source code for the following terms:

  1. TypeNameHandling
  2. JavaScriptTypeResolver

Look for any serializers where the type is set by a user controlled variable.

 

BlackBox Review

Search for the following base64 encoded content that starts with:

AAEAAAD/////

Search for content with the following text:

  1. TypeObject
  2. $type:

 

General Precautions

Don’t allow the datastream to define the type of object that the stream will be deserialized to. You can prevent this by for example using the DataContractSerializer or XmlSerializer if at all possible.

Where JSON.Net is being used make sure the TypeNameHandling is only set to None.

TypeNameHandling = TypeNameHandling.None

If JavaScriptSerializer is to be used then do not use it with a JavaScriptTypeResolver.

If you must deserialise data streams that define their own type, then restrict the types that are allowed to be deserialized. One should be aware that this is still risky as many native .Net types potentially dangerous in themselves. e.g.

System.IO.FileInfo

FileInfo objects that reference files actually on the server can when deserialized, change the properties of those files e.g. to read-only, creating a potential denial of service attack.

Even if you have limited the types that can be deserialised remember that some types have properties that are risky. System.ComponentModel.DataAnnotations.ValidationException, for example has a property Value of type Object. if this type is the type allowed for deserialization then an attacker can set the Value property to any object type they choose.

Attackers should be prevented from steering the type that will be instantiated. If this is possible then even DataContractSerializer or XmlSerializer can be subverted e.g.

// Action below is dangerous if the attacker can change the data in the database
var typename = GetTransactionTypeFromDatabase();  

var serializer = new DataContractJsonSerializer(Type.GetType(typename));

var obj = serializer.ReadObject(ms);

Execution can occur within certain .Net types during deserialization. Creating a control such as the one shown below is ineffective.

var suspectObject = myBinaryFormatter.Deserialize(untrustedData);

//Check below is too late! Execution may have already occurred.
if (suspectObject is SomeDangerousObjectType) 
{
    //generate warnings and dispose of suspectObject
}

For BinaryFormatter and JSON.Net it is possible to create a safer form of white list control useing a custom SerializationBinder.

Try to keep up-to-date on known .Net insecure deserialization gadgets and pay special attention where such types can be created by your deserialization processes. A deserializer can only instantiate types that it knows about.

Try to keep any code that might create potential gagdets separate from any code that has internet connectivity. As an example System.Windows.Data.ObjectDataProvider used in WPF applications is a known gadget that allows arbitrary method invocation. It would be risky to have this a reference to this assembly in a REST service project that deserializes untrusted data.

 

Known .NET RCE Gadgets

  • System.Configuration.Install.AssemblyInstaller
  • System.Activities.Presentation.WorkflowDesigner
  • System.Windows.ResourceDictionary
  • System.Windows.Data.ObjectDataProvider
  • System.Windows.Forms.BindingSource
  • Microsoft.Exchange.Management.SystemManager.WinForms.ExchangeSettingsProvider
  • System.Data.DataViewManager, System.Xml.XmlDocument/XmlDataDocument
  • System.Management.Automation.PSObject

 

Language-Agnostic Methods for Deserializing Safely

 

Using Alternative Data Formats

A great reduction of risk is achieved by avoiding native (de)serialization formats. By switching to a pure data format like JSON or XML, you lessen the chance of custom deserialization logic being repurposed towards malicious ends.

Many applications rely on a data-transfer object pattern that involves creating a separate domain of objects for the explicit purpose data transfer. Of course, it’s still possible that the application will make security mistakes after a pure data object is parsed.

 

Only Deserialize Signed Data

If the application knows before deserialization which messages will need to be processed, they could sign them as part of the serialization process. The application could then to choose not to deserialize any message which didn’t have an authenticated signature.

 

Mitigation Tools/Libraries

 

Detection Tools

 

References

 

Authors and Primary Editors

Arshan Dabirsiaghi – arshan@contrastsecurity.org

Tony Hsu (Hsiang-Chih) Shane Murnion

Author: McPeters Joseph

Joseph McPeters is a Security Researcher. He specializes in network and web application penetration testing. Contact: admin@incidentsecurity.com

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