Squid 2.5, when processing the configuration file, parses empty Access Control Lists (ACLs), including proxy_auth ACLs without defined auth schemes, in a way that effectively removes arguments, which could allow remote attackers to bypass intended ACLs if the administrator ignores the parser warnings.
http://www.squid-cache.org/bugs/show_bug.cgi?id=1166
http://www.squid-cache.org/Versions/v2/2.5/bugs/squid-2.5.STABLE7-empty_acls.patch
http://www.squid-cache.org/Versions/v2/2.5/bugs/#squid-2.5.STABLE7-empty_acls
http://www.kb.cert.org/vuls/id/260421
http://www.debian.org/security/2005/dsa-667
http://marc.info/?l=bugtraq&m=110901183320453&w=2
http://fedoranews.org/updates/FEDORA--.shtml
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000923