During the development of the DNS Knot Resolver, CZ.NIC Labs have managed to reveal a security flaw that makes it possible to bypass DNSSEC security on F5 load balancers and cause denial of service. These products are being used, for example, in some internet banking applications, including those of Czech banks and public authorities. From the perspective of a user attempting to access an internet banking service, a successful attack exploiting this error would manifest in the browser suddenly reporting an “address not found” error and the service becoming unavailable.