MaraDNS before 1.3.07.12 and 1.4.x before 1.4.08 computes hash values for DNS data without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted queries with the Recursion Desired (RD) bit set.
https://bugzilla.redhat.com/show_bug.cgi?id=771428
http://samiam.org/blog/20111229.html