BIND 9 DNSSEC validation code could cause bogus NXDOMAIN responses

There was an error in the DNSSEC NSEC/NSEC3 validation code that could cause bogus NXDOMAIN responses (that is, NXDOMAIN responses for records proven by NSEC or NSEC3 to exist) to be cached as if they had validated correctly, so that future queries to the resolver would return the bogus NXDOMAIN with the AD flag set.

Impact:
This problem affects all DNSSEC-validating resolvers. It would be difficult to exploit due to other existing protections against cache poisoning (including transaction ID and source port randomization), but it could impair the ability of DNSSEC to protect against a denial-of-service attack on a secure zone.