Unbound before 1.9.4 accesses uninitialized memory, which allows remote attackers to trigger a crash via a crafted NOTIFY query. The source IP address of the query must match an access-control rule.
https://www.debian.org/security/2019/dsa-4544
https://usn.ubuntu.com/4149-1/
https://seclists.org/bugtraq/2019/Oct/23
https://nlnetlabs.nl/downloads/unbound/CVE-2019-16866.txt
https://github.com/NLnetLabs/unbound/blob/release-1.9.4/doc/Changelog