PowerDNS Recursor before 3.6.2 does not limit delegation chaining, which allows remote attackers to cause a denial of service ("performance degradations") via a large or infinite number of referrals, as demonstrated by resolving domains hosted by ezdns.it.
http://www.securitytracker.com/id/1031310
http://www.securityfocus.com/bid/71545
http://www.kb.cert.org/vuls/id/264212
http://www.debian.org/security/2014/dsa-3096
http://doc.powerdns.com/md/security/powerdns-advisory-2014-02/