Knot Resolver before 5.1.1 allows traffic amplification via a crafted DNS answer from an attacker-controlled server, aka an "NXNSAttack" issue. This is triggered by random subdomains in the NSDNAME in NS records.
https://www.knot-resolver.cz/2020-05-19-knot-resolver-5.1.1.html
https://lists.debian.org/debian-lts-announce/2024/04/msg00017.html