GHSA-m875-3xf6-mf78: 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

References

• Common HTTP Response Header Limits
• Nginx Proxy buffer tuning
• 414 Request-URI too long
• Unpoly server protocol

References

• GHSA-m875-3xf6-mf78
• https://nvd.nist.gov/vuln/detail/CVE-2023-28846
• unpoly/unpoly-rails@cd9ad00
• https://docs.nginx.com/nginx/admin-guide/load-balancer/http-health-check/#passive-health-checks
• https://github.com/unpoly/unpoly-rails/
• https://makandracards.com/operations/537537-nginx-proxy-buffer-tuning
• https://tryhexadecimal.com/guides/http/414-request-uri-too-long
• https://unpoly.com/up.protocol