Impact

Under certain circumstances response bodies will not be closed, for example a bug in a webserver or a bug in a Rack middleware. In the event a response is not notified of a close, ActionDispatch::Executor will not know to reset thread local state for the next request. This can lead to data being leaked to subsequent requests, especially when interacting with ActiveSupport::CurrentAttributes.

Upgrading to the FIXED versions of Rails will ensure mitigation if this issue even in the context of a buggy webserver or middleware implementation.

Patches

This has been fixed in Rails 7.0.2.2, 6.1.4.6, 6.0.4.6, and 5.2.6.2.

Workarounds

Upgrading is highly recommended, but to work around this problem the following middleware can be used:

class GuardedExecutor < ActionDispatch::Executor
  def call(env)
    ensure_completed!
    super
  end

  private

    def ensure_completed!
      @executor.new.complete! if @executor.active?
    end
end

# Ensure the guard is inserted before ActionDispatch::Executor
Rails.application.configure do
  config.middleware.swap ActionDispatch::Executor, GuardedExecutor, executor
end