Commit 33893c92 authored by Nick Malcolm's avatar Nick Malcolm Committed by Kati Paizee

Secure coding guidelines for dynamic method definition / metaprogramming

parent 1d154fa0
......@@ -592,3 +592,90 @@ In order to prevent this from happening, it is recommended to use the method `us
forbidden!(api_access_denied_message(user))
end
```
## Guidelines when defining missing methods with metaprogramming
Metaprogramming is a way to define methods **at runtime**, instead of at the time of writing and deploying the code. It is a powerful tool, but can be dangerous if we allow untrusted actors (like users) to define their own arbitrary methods. For example, imagine we accidentally let an attacker overwrite an access control method to always return true! It can lead to many classes of vulnerabilities such as access control bypass, information disclosure, arbitrary file reads, and remote code execution.
Key methods to watch out for are `method_missing`, `define_method`, `delegate`, and similar methods.
### Insecure metaprogramming example
This example is adapted from an example submitted by [@jobert](https://hackerone.com/jobert?type=user) through our HackerOne bug bounty program.
Thank you for your contribution!
Before Ruby 2.5.1, you could implement delegators using the `delegate` or `method_missing` methods. For example:
```ruby
class User
def initialize(attributes)
@options = OpenStruct.new(attributes)
end
def is_admin?
name.eql?("Sid") # Note - never do this!
end
def method_missing(method, *args)
@options.send(method, *args)
end
end
```
When a method was called on a `User` instance that didn't exist, it passed it along to the `@options` instance variable.
```ruby
User.new({name: "Jeeves"}).is_admin?
# => false
User.new(name: "Sid").is_admin?
# => true
User.new(name: "Jeeves", "is_admin?" => true).is_admin?
# => false
```
Because the `is_admin?` method is already defined on the class, its behavior is not overridden when passing `is_admin?` to the initializer.
This class can be refactored to use the `Forwardable` method and `def_delegators`:
```ruby
class User
extend Forwardable
def initialize(attributes)
@options = OpenStruct.new(attributes)
self.class.instance_eval do
def_delegators :@options, *attributes.keys
end
end
def is_admin?
name.eql?("Sid") # Note - never do this!
end
end
```
It might seem like this example has the same behavior as the first code example. However, there's one crucial difference: **because the delegators are meta-programmed after the class is loaded, it can overwrite existing methods**:
```ruby
User.new({name: "Jeeves"}).is_admin?
# => false
User.new(name: "Sid").is_admin?
# => true
User.new(name: "Jeeves", "is_admin?" => true).is_admin?
# => true
# ^------------------ The method is overwritten! Sneaky Jeeves!
```
In the example above, the `is_admin?` method is overwritten when passing it to the initializer.
### Best practices
- Never pass user-provided details into method-defining metaprogramming methods.
- If you must, be **very** confident that you've sanitized the values correctly.
Consider creating an allowlist of values, and validating the user input against that.
- When extending classes that use metaprogramming, make sure you don't inadvertently override any method definition safety checks.
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