CVE-2019-12523

An issue was discovered in Squid before 4.9. When handling a URN request, a corresponding HTTP request is made. This HTTP request doesn’t go through the access checks that incoming HTTP requests go through. This causes all access checks to be bypassed and allows access to restricted HTTP servers, e.g., an attacker can connect to HTTP servers that only listen on localhost.

Mitigation

Access to manager services can be prevented by enabling the Via header: (in /etc/squid/squid.conf)

 via on  

There are no reliable workarounds to prevent access to restricted upstream servers.