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.
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.
This vulnerability can be mitigated by wrapping render
calls with respond_to
blocks. For example, the following example is vulnerable:
class UserController < ApplicationController
def index
render "index"
end
end
But the following code is not vulnerable:
class UserController < ApplicationController
def index
respond_to |format|
format.html { render "index" }
end
end
end
Implicit rendering is impacted, so this code is vulnerable:
class UserController < ApplicationController
def index
end
end
But can be changed this this:
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.
Thanks to John Hawthorn john@hawthorn.email of GitHub