An issue was discovered in Ruby through 2.5.8, 2.6.x through 2.6.6, and 2.7.x through 2.7.1. WEBrick, a simple HTTP server bundled with Ruby, had not checked the transfer-encoding header value rigorously. An attacker may potentially exploit this issue to bypass a reverse proxy (which also has a poor header check), which may lead to an HTTP Request Smuggling attack.
https://github.com/ruby/webrick/commit/8946bb38b4d87549f0d99ed73c62c41933f97cc7
https://hackerone.com/reports/965267
https://lists.debian.org/debian-lts-announce/2023/04/msg00033.html