Chinese Edition: Ruby on Rails 路径穿越与任意文件读取漏洞分析 - 【CVE-2019-5418】

Security Advisory

https://groups.google.com/forum/#!topic/rubyonrails-security/pFRKI96Sm8Q

Analysis

The render method can use a view that’s entirely outside of your application. So in actionview-5.2.1/lib/action_view/renderer/template_renderer.rb:22, it will call find_file to determine which template to be rendered。

module ActionView   
  class TemplateRenderer < AbstractRenderer #:nodoc:
    # Determine the template to be rendered using the given options.
      def determine_template(options)
        keys = options.has_key?(:locals) ? options[:locals].keys : []
        if options.key?(:body)
          ...
        elsif options.key?(:file)
          with_fallbacks { find_file(options[:file], nil, false, keys, @details) }
        ...
      end
end

In the find_file method:

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def find_file(name, prefixes = [], partial = false, keys = [], options = {})
    @view_paths.find_file(*args_for_lookup(name, prefixes, partial, keys, options))
end

step into args_for_lookup method which to generate the options. When it returns, our payload will be saved in details[formats] :

then it will execute @view_paths.find_file which located in actionview-5.2.1/lib/action_view/path_set.rb：

class PathSet #:nodoc:
    def find_file(path, prefixes = [], *args)
      _find_all(path, prefixes, args, true).first || raise(MissingTemplate.new(self, path, prefixes, *args))
    end
    private
        def _find_all(path, prefixes, args, outside_app)
            prefixes = [prefixes] if String === prefixes
            prefixes.each do |prefix|
            paths.each do |resolver|
                if outside_app
                  templates = resolver.find_all_anywhere(path, prefix, *args)
                else
                  templates = resolver.find_all(path, prefix, *args)
                end
                return templates unless templates.empty?
            end
            end
            []
        end

Because the view is outside of your application，so outside_app equalsTrue and then will call find_all_anywhere

def find_all_anywhere(name, prefix, partial = false, details = {}, key = nil, locals = [])
    cached(key, [name, prefix, partial], details, locals) do
    find_templates(name, prefix, partial, details, true)
    end
end

Skip cached part， the find_templates will according the options to find the template to render:

 # An abstract class that implements a Resolver with path semantics.
class PathResolver < Resolver #:nodoc:
    EXTENSIONS = { locale: ".", formats: ".", variants: "+", handlers: "." }
    DEFAULT_PATTERN = ":prefix/:action{.:locale,}{.:formats,}{+:variants,}{.:handlers,}"
    ...
    private
        def find_templates(name, prefix, partial, details, outside_app_allowed = false)
            path = Path.build(name, prefix, partial)
            # 注意 details 与 details[:formats] 的传入
            query(path, details, details[:formats], outside_app_allowed)
        end
        def query(path, details, formats, outside_app_allowed)
            query = build_query(path, details)
            template_paths = find_template_paths(query)
            ...
            end
        end

After build_query , the variables ：

SO here we use ../ to make directory traversal，and use double { to make sure syntax right. After File.expand_path , the result is:

1	/etc/passwd{{},}{+{},}{.{raw,erb,html,builder,ruby,coffee,jbuilder},}

so the /etc/passwd will be treated the template to be rended ，which lead to a arbitrary file read attack.

Reproduce

install vulnerable Rails (e.g 5.2.1)

# echo "gem 'rails', '5.2.1'" >> Gemfile
# echo "gem 'sqlite3', '~> 1.3.6', '< 1.4'" >> Gemfile
# echo "source 'https://rubygems.org'" >> Gemfile
# bundle exec rails new . --force --skip-bundle

Generate controller:

1	# rails generate controller chybeta

Inapp/controllers/chybeta_controller.rb ：

class ChybetaController < ApplicationController
  def index
    render file: "#{Rails.root}/some/file"
  end
end

add resources in config/routes.rb:

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Rails.application.routes.draw do
  resources :chybeta
end

Patch

https://github.com/rails/rails/commit/f4c70c2222180b8d9d924f00af0c7fd632e26715