# Denial of Service Vulnerability in Action View
Impact
------
Specially crafted accept headers can cause the Action View template location code to consume 100% CPU, causing the server unable to process requests. This impacts all Rails applications that render views.
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 wrapping `render` calls with `respond_to` blocks. For example, the following example is vulnerable:
``` ruby
class UserController < ApplicationController
def index
render "index"
end
end
```
But the following code is not vulnerable:
```ruby
class UserController < ApplicationController
def index
respond_to |format|
format.html { render "index" }
end
end
end
```
Implicit rendering is impacted, so this code is vulnerable:
```ruby
class UserController < ApplicationController
def index
end
end
```
But can be changed this this:
```ruby
class UserController < ApplicationController
def index
respond_to |format|
format.html { render "index" }
end
end
end
```
Alternatively to specifying the format, the following monkey patch can be applied in an initializer:
```
$ 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)
```
Please note that only the 5.2.x, 5.1.x, 5.0.x, and 4.2.x series are supported at present. Users of earlier unsupported releases are advised to upgrade as soon as possible as we cannot guarantee the continued availability of security fixes for unsupported releases.
Also note that the patches for this vulnerability are the same as CVE-2019-5418.
Credits
-------
Thanks to John Hawthorn <
john@hawthorn.email> of GitHub