nbdkit:sid is vulnerable to man-in-the-middle. A flaw was found in nbdkit due to to improperly caching plaintext state across the STARTTLS encryption boundary. A MitM attacker could use this flaw to inject a plaintext NBD_OPT_STRUCTURED_REPLY before proxying everything else a client sends to the server, potentially leading the client to terminate the NBD session.
bugzilla.redhat.com/show_bug.cgi?id=1994695
gitlab.com/nbdkit/nbdkit/-/commit/09a13dafb7bb3a38ab52eb5501cba786365ba7fd
gitlab.com/nbdkit/nbdkit/-/commit/6c5faac6a37077cf2366388a80862bb00616d0d8
listman.redhat.com/archives/libguestfs/2021-August/msg00083.html
security-tracker.debian.org/tracker/CVE-2021-3716
www.openwall.com/lists/oss-security/2021/08/18/2