Vulnerability Instance

Path Traversal in Action View
# File Content Disclosure in Action View

Impact 
------ 
There is a possible file content disclosure vulnerability in Action View.  Specially crafted accept headers in combination with calls to `render file:`  can cause arbitrary files on the target server to be rendered, disclosing the  file contents. 

The impact is limited to calls to `render` which render file contents without  a specified accept format.  Impacted code in a controller looks something like this: 

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

Rendering templates as opposed to files is not impacted by this vulnerability. 

All users running an affected release should either upgrade or use one of the workarounds immediately. 

Releases 
-------- 
The 6.0.0.beta3, 5.2.2.1, 5.1.6.2, 5.0.7.2, and 4.2.11.1 releases are available at the normal locations. 

Workarounds 
----------- 
This vulnerability can be mitigated by specifying a format for file rendering, like this: 

``` ruby
class UserController < ApplicationController 
  def index 
    render file: "#{Rails.root}/some/file", formats: [:html] 
  end 
end 
``` 

In summary, impacted calls to `render` look like this: 

``` 
render file: "#{Rails.root}/some/file" 
``` 

The vulnerability can be mitigated by changing to this: 

``` 
render file: "#{Rails.root}/some/file", formats: [:html] 
``` 

Other calls to `render` are not impacted. 

Alternatively, the following monkey patch can be applied in an initializer: 

``` ruby
$ cat config/initializers/formats_filter.rb 
# frozen_string_literal: true 

ActionDispatch::Request.prepend(Module.new do 
  def formats 
    super().select do |format| 
      format.symbol || format.ref == "*/*" 
    end 
  end 
end) 
``` 

Credits 
------- 
Thanks to John Hawthorn <john@hawthorn.email> of GitHub