Deserialization Cheat Sheet

Introduction

This article is focused on providing clear, simple, 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 attacks.

Guidance on Deserializing Objects Safely

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

Java

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

Implementation: 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. Implementation: 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. Implementation: Use a Java agent to override the internals of ObjectInputStream to prevent exploitation of known dangerous types as seen in rO0 and NotSoSerial

Prevent Data Leakage and Trusted Field Clobbering

If there are members of the object graph that should never be controlled by end users during deserialization or exposed to users during serialization, they should be marked with the transient field.

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() 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");
}

Language-Agnostic Methods for Deserializing Safely

Using Alternative Data Formats

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

Many applications rely on a data-transfer object pattern that involves creating separate objects 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.

References

• Deserialization of untrusted data

[1] [2]

Authors and Primary Editors

Arshan Dabirsiaghi - arshan [at] contrastsecurity dot org

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