Squid before 4.15 and 5.x before 5.0.6 allows remote servers to cause a denial of service (affecting availability to all clients) via an HTTP response. The issue trigger is a header that can be expected to exist in HTTP traffic without any malicious intent by the server.
https://lists.debian.org/debian-lts-announce/2021/06/msg00014.html
https://github.com/squid-cache/squid/security/advisories/GHSA-572g-rvwr-6c7f