Gunicorn fails to properly validate Transfer-Encoding headers, leading to HTTP Request Smuggling (HRS) vulnerabilities. By crafting requests with conflicting Transfer-Encoding headers, attackers can bypass security restrictions and access restricted endpoints. This issue is due to Gunicorn's handling of Transfer-Encoding headers, where it incorrectly processes requests with multiple, conflicting Transfer-Encoding headers, treating them as chunked regardless of the final encoding specified. This vulnerability allows for a range of attacks including cache poisoning, session manipulation, and data exposure.
https://lists.debian.org/debian-lts-announce/2024/12/msg00018.html
https://lists.debian.org/debian-lts-announce/2024/06/msg00027.html
https://huntr.com/bounties/22158e34-cfd5-41ad-97e0-a780773d96c1