In February 2021, I started looking for vulnerabilities in forward-proxies, and found various issues in Squid. Some more information about what's here can be found on my blog: https://joshua.hu/squid-security-audit-35-0days-45-exploits
Explanations and reproducers for each of the vulnerabilities are documented in each of the markdown files. IDs are assigned where possible, however since the majority of these remain unfixed, there are no identifiers.
The Squid Team have been helpful and supportive during the process of reporting these issues. However, they are effectively understaffed, and simply do not have the resources to fix the discovered issues. Hammering them with demands to fix the issues won't get far.
With any system or project, it is important to reguarly review solutions used in your stack to determine whether they are still appropriate. If you are running Squid in an environment which may suffer from any of these issues, then it is up to you to reassess whether Squid is the right solution for your system.