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://cert.ssi.gouv.fr/site/CERTFR-2014-AVI-512/index.html
http://doc.powerdns.com/md/security/powerdns-advisory-2014-02/
http://www.debian.org/security/2014/dsa-3096
http://www.kb.cert.org/vuls/id/264212