Squid by default comes with rules to block access to the Cache Manager, which serves detailed server information meant for the maintainer. This rule is implemented via url_regex. The handler for url_regex rules URL decodes an incoming request. This allows an attacker to encode their URL to bypass the url_regex check, and gain access to the blocked resource.
Upstream Issue: https://gitlab.com/jeriko.one/security/-/blob/master/squid/CVEs/CVE-2019-12524.txt
Patch: http://www.squid-cache.org/Versions/v4/changesets/SQUID-2019_4.patch
External References: http://www.squid-cache.org/Advisories/SQUID-2019_4.txt
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s): https://access.redhat.com/security/cve/cve-2019-12524
This issue has been addressed in the following products: Red Hat Enterprise Linux 8 Via RHSA-2020:4743 https://access.redhat.com/errata/RHSA-2020:4743