Due to incorrect data validation Squid is vulnerable to a denial of service attack when processing specially crafted DNS packets.
Squid still using the obsolete dnsserver are not vulnerable.
The ignore_unknown_nameservers option affects the severity of this vulnerability. When set to "on" (the default) risk is low. When set to "off" the vulnerability risk is increased. All unpatched Squid-3.0 versions up to and including 3.0.STABLE21 are vulnerable. All unpatched Squid-3.1 versions up to and including 18.104.22.168 are vulnerable. All unpatched Squid-2.x versions are vulnerable.
Using all of the following steps are required to protect a vulnerable Squid from this and other forms of DNS attack.
- Ensuring the ignore_unknown_nameservers is turned on.
- Ensuring that DNS packets cannot be sent to Squid from untrusted nameservers or other machines.