An issue was discovered in Squid 3.x and 4.x through 4.8. Due to incorrect input validation, there is a heap-based buffer overflow that can result in Denial of Service to all clients using the proxy. Severity is high due to this vulnerability occurring before normal security checks; any remote client that can reach the proxy port can trivially perform the attack via a crafted URI scheme.
https://www.debian.org/security/2020/dsa-4682
https://usn.ubuntu.com/4446-1/
https://usn.ubuntu.com/4213-1/
https://lists.debian.org/debian-lts-announce/2020/07/msg00009.html
https://github.com/squid-cache/squid/pull/275