squid 2.3 and earlier allows local users to overwrite arbitrary files via a symlink attack in some configurations.
https://exchange.xforce.ibmcloud.com/vulnerabilities/5921
http://www.securityfocus.com/bid/2184
http://www.linux-mandrake.com/en/security/2001/MDKSA-2001-003.php3
http://www.debian.org/security/2001/dsa-019
http://marc.info/?l=bugtraq&m=97916374410647&w=2
http://archives.neohapsis.com/archives/bugtraq/2001-01/0212.html