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](https://unpoly.com/up.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](https://docs.nginx.com/nginx/admin-guide/load-balancer/http-health-check/#passive-health-checks). The [unpoly-rails](https://github.com/unpoly/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](https://makandracards.com/operations/537537-nginx-proxy-buffer-tuning) is at least twice the [maximum size of a URL](https://tryhexadecimal.com/guides/http/414-request-uri-too-long). - Instead of changing your server configuration you may also configure your Rails application to delete redundant `X-Up-Location` headers set by unpoly-rails: ```ruby 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 ```

Quoted source text, attributed separately from HOL analysis.