The Hidden Dangers of Deserialization in Ruby: A Developer’s Guide to Security
In the world of Ruby development, we often need to store or transmit complex objects. Serialization is the go-to process for this, converting an object’s state into a byte stream that can be easily saved to a file, stored in a database, or sent across a network. When we need the object back, we deserialize the byte stream. While this is a powerful feature, it hides a critical security risk that has been responsible for some of the most severe vulnerabilities in the Ruby ecosystem: insecure deserialization.
When an application deserializes data from an untrusted source, it can be tricked into executing arbitrary code, leading to a full system compromise. Understanding this threat is essential for any developer working with Ruby or Ruby on Rails.
What is Serialization in Ruby? The Role of Marshal
Ruby’s standard library provides a module called Marshal for this exact purpose. The Marshal.dump method takes a Ruby object and converts it into a string of bytes. Later, the Marshal.load method can take that byte stream and reconstruct the original object in memory, complete with its instance variables and state.
This is incredibly useful for tasks like:
- Caching complex query results.
- Storing session data in cookies.
- Sending objects between processes in a distributed system.
The problem, however, doesn’t lie in the process itself, but in the source of the data being deserialized.
The Core Vulnerability: Deserializing Untrusted Data
The danger of Marshal.load and similar methods is that they do more than just restore data; they reconstruct objects. A cleverly crafted byte stream can create unexpected object types in your application’s memory. If an attacker can control the serialized data your application loads, they can instantiate objects of almost any class available to your application.
This leads to the primary risk: Remote Code Execution (RCE). Attackers achieve this by creating a malicious object payload that, upon being deserialized, triggers a chain of method calls using existing code within your application or its dependencies. This is known as a “gadget chain.”
A gadget chain does not inject new code. Instead, it cleverly stitches together pieces of legitimate, existing code (the “gadgets”) in your application’s codebase to perform malicious actions. For example, a gadget chain could be designed to ultimately call a method like system('rm -rf /'), using only the classes and methods already present in your application’s environment.
A Brief History of Major Exploits
The threat of Ruby deserialization is not theoretical. It came into the spotlight in 2013 with a series of critical vulnerabilities in Ruby on Rails. Attackers found they could pass malicious serialized data through application parameters or session cookies. When Rails deserialized this data to reconstruct objects, it would inadvertently trigger RCE gadget chains.
This discovery forced the community to re-evaluate how data was handled. The key takeaway was that any data coming from a user—including cookies, form inputs, and API parameters—must be considered untrusted and should never be passed directly into a deserialization method like Marshal.load.
Beyond Marshal: The Threat from YAML
The vulnerability isn’t limited to the Marshal module. YAML, a popular human-readable data format, has also been a vector for deserialization attacks. The YAML.load method in older versions of Ruby’s Psych gem could be exploited in a similar way, as it was capable of instantiating arbitrary objects.
To combat this, safer alternatives were introduced. Modern Ruby development strongly encourages the use of Psych.safe_load (or its equivalent, YAML.safe_load), which restricts parsing to simple data types like strings, numbers, arrays, and hashes, preventing the creation of arbitrary objects.
How to Protect Your Ruby Applications: Actionable Security Tips
Protecting your application from deserialization attacks requires a defensive mindset and adherence to security best practices. Here are essential steps every developer should take:
Never Deserialize Untrusted Data. This is the golden rule. Audit your codebase for any instances of
Marshal.load,YAML.load, orOj.load(from the popularOjgem) that operate on user-controlled input. This includes HTTP headers, cookies, session data, and API request bodies.Use Safer Data Formats for Communication. Whenever possible, use JSON as your data interchange format. Standard JSON parsers do not execute code or instantiate complex objects; they only parse data into simple, safe data structures. It is inherently more secure for handling data from external sources.
Implement Data Integrity Checks. If you must serialize data that will later be processed (e.g., for internal caching), ensure it cannot be tampered with. You can do this by signing the serialized data with a secret key using a mechanism like HMAC (Hash-based Message Authentication Code). Before deserializing, verify the signature to ensure the data is authentic and unchanged.
Favor Safe Loading Methods. If you are working with YAML, make
YAML.safe_loadyour default choice. Only useYAML.loadwhen you have absolute certainty that the data comes from a trusted, secure source that you control.Keep Your Dependencies Updated. Security vulnerabilities are frequently discovered in third-party gems. Use tools like
bundler-auditto scan yourGemfile.lockfor known vulnerabilities and update your dependencies regularly to ensure you have the latest security patches.
Final Thoughts
Insecure deserialization remains a potent threat to web applications. While powerful, methods like Marshal.load are a loaded gun when aimed at user-supplied data. By treating all external input with suspicion, favoring safer data formats like JSON, and actively auditing your codebase for unsafe practices, you can effectively mitigate this risk and build more secure, resilient Ruby applications.
Source: https://blog.trailofbits.com/2025/08/20/marshal-madness-a-brief-history-of-ruby-deserialization-exploits/
