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Published: 4 December 2023

Squid is a caching proxy for the Web supporting HTTP, HTTPS, FTP, and more. Affected versions of squid are subject to a a Use-After-Free bug which can lead to a Denial of Service attack via collapsed forwarding. All versions of Squid from 3.5 up to and including 5.9 configured with "collapsed_forwarding on" are vulnerable. Configurations with "collapsed_forwarding off" or without a "collapsed_forwarding" directive are not vulnerable. This bug is fixed by Squid version 6.0.1. Users are advised to upgrade. Users unable to upgrade should remove all collapsed_forwarding lines from their squid.conf.


need to identify commit in 6.0.1...perhaps this one?

While this is fixed by a yet unidentified commit in 6.0.1, I
believe it is also fixed in a different way by the refactoring
in the commit to fix CVE-2023-5824. The issue no longer
reproduces with the fix for CVE-2023-5824 applied.
Ubuntu 20.04 LTS was patched in USN-6728-1, but the fix
introduced crashes and was backed out in USN-6728-2.



Cvss 3 Severity Score


Score breakdown


Package Release Status
Launchpad, Ubuntu, Debian
bionic Ignored
(end of standard support)
Released (4.10-1ubuntu1.12)
Released (5.7-0ubuntu0.22.04.4)
lunar Ignored
(end of life, was deferred [2024-01-26])
mantic Not vulnerable
noble Not vulnerable

trusty Ignored
(end of standard support)
Released (6.1-1)
xenial Ignored
(end of standard support)
Launchpad, Ubuntu, Debian
bionic Needed

focal Does not exist

jammy Does not exist

lunar Does not exist

mantic Does not exist

noble Does not exist

trusty Ignored
(end of standard support)
upstream Needs triage

xenial Needed

Severity score breakdown

Parameter Value
Base score 7.5
Attack vector Network
Attack complexity Low
Privileges required None
User interaction None
Scope Unchanged
Confidentiality None
Integrity impact None
Availability impact High
Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H