Puma is a simple, fast, multi-threaded, parallel HTTP 1.1 server for Ruby/Rack applications. When using Puma behind a proxy that does not properly validate that the incoming HTTP request matches the RFC7230 standard, Puma and the frontend proxy may disagree on where a request starts and ends. This would allow requests to be smuggled via the front-end proxy to Puma. The vulnerability has been fixed in 5.6.4 and 4.3.12. Users are advised to upgrade as soon as possible. Workaround: when deploying a proxy in front of Puma, turning on any and all functionality to make sure that the request matches the RFC7230 standard.
https://www.debian.org/security/2022/dsa-5146
https://security.gentoo.org/glsa/202208-28
https://lists.debian.org/debian-lts-announce/2022/08/msg00015.html
https://github.com/puma/puma/security/advisories/GHSA-h99w-9q5r-gjq9
https://github.com/puma/puma/commit/5bb7d202e24dec00a898dca4aa11db391d7787a5