The MITRE CVE dictionary describes this issue as:
The idnsGrokReply function in Squid before 3.1.16 does not properly free memory, which allows remote attackers to cause a denial of service (daemon abort) via a DNS reply containing a CNAME record that references another CNAME record that contains an empty A record.
Find out more about CVE-2011-4096 from the
MITRE CVE dictionary dictionary and
This issue did not affect the versions of squid as shipped with Red Hat Enterprise Linux 4 and 5 as they did not include IPv6 support. This issue was introduced with the addition of IPv6 support in Squid 3.1 (in the changes made to the idnsGrokReply function).