unpoly-rails Denial of Service vulnerability

There is a possible Denial of Service (DoS) vulnerability in the unpoly-rails gem that implements the Unpoly server protocol for Rails applications.

Impact

This issues affects Rails applications that operate as an upstream of a load balancer's that uses passive health checks.

The unpoly-rails gem echoes the request URL as an X-Up-Location response header. By making a request with exceedingly long URLs (paths or query string), an attacker can cause unpoly-rails to write a exceedingly large response header.

If the response header is too large to be parsed by a load balancer downstream of the Rails application, it may cause the load balancer to remove the upstream from a load balancing group. This causes that application instance to become unavailable until a configured timeout is reached or until an active healthcheck succeeds.

Patches

The fixed release 2.7.2.2+ is available via RubyGems and GitHub.

Workarounds

If you cannot upgrade to a fixed release, several workarounds are available:

• Configure your load balancer to use active health checks, e.g. by periodically requesting a route with a known response that indicates healthiness.

• Configure your load balancer so the maximum size of response headers is at least twice the maximum size of a URL.

• Instead of changing your server configuration you may also configure your Rails application to delete redundant X-Up-Location headers set by unpoly-rails:

  class ApplicationController < ActionController::Base
  
    after_action :remove_redundant_up_location_header
  
    private
  
    def remove_redundant_up_location_header
      if request.original_url == response.headers['X-Up-Location']
        response.headers.delete('X-Up-Location')
      end
    end
  
  end