unbound -- Non-Responsive Delegation Attack

A vulnerability named 'Non-Responsive Delegation Attack'
(NRDelegation Attack) has been discovered in various DNS
resolving software. The NRDelegation Attack works by having
a malicious delegation with a considerable number of non
responsive nameservers. The attack starts by querying a
resolver for a record that relies on those unresponsive
nameservers. The attack can cause a resolver to spend a lot
of time/resources resolving records under a malicious
delegation point where a considerable number of unresponsive
NS records reside. It can trigger high CPU usage in some
resolver implementations that continually look in the cache
for resolved NS records in that delegation.

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