This indicates an attack attempt to exploit a Denial of Service Vulnerability in NLnet Labs Unbound.
A remote, unauthenticated attacker can exploit this vulnerability by sending a crafted NOTIFY request to the target server. Successful exploitation results will cause the server to abnormally terminate.
NLnet Labs Unbound 1.7.1 through 1.9.3
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.