A recursive DNS resolver following an infinite chain of referrals can result in high process memory and CPU usage and eventually process termination. The effect can range from increased server response time to clients to complete interruption of the service.
Resolvers that follow multiple referrals at once can cause large bursts of network traffic. These issues are addressed by limiting the maximum number of referrals followed and the number of simultaneous queries.
Read the full advisory here – http://www.kb.cert.org/vuls/id/264212