unbound is vulnerable to denial of service. There is no available reproducer or proof of concept for this issue, nor it was ever proven the infinite loop can be triggered in practice.
access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/8.4_release_notes/
access.redhat.com/errata/RHSA-2021:1853
access.redhat.com/security/updates/classification/#moderate
bugzilla.redhat.com/show_bug.cgi?id=1954799
lists.debian.org/debian-lts-announce/2021/05/msg00007.html
ostif.org/our-audit-of-unbound-dns-by-x41-d-sec-full-results/
security.netapp.com/advisory/ntap-20210507-0007/