Fwknop stands for the “FireWall KNock OPerator” and implements an authorization scheme called Single Packet Authorization (SPA). This method of authorization is based on a default-drop packet filter and libpcap. A server might appear to have no open ports available, but it could still grant access to certain services if authorized fwknop packets are received. Companies commonly use this service on exposed systems and need to diminish the attack surface of this service.
wknop contains several vulnerabilities. The most critical of these might allow remote, authenticated attackers to leverage flaws to execute code and produce denial-of-service conditions.