Quantum Key Distribution (QKD) provides a theoretically unbreakable method for exchanging cryptographic keys, grounded in the principles of quantum mechanics. However, practical deployments rely on both quantum and classical channels to facilitate key exchange, thereby introducing new vectors for potential security vulnerabilities. While much of the existing research has concentrated on threats to the quantum channel, the classical channel remains a comparatively underexplored and vulnerable component. This paper investigates Denial-of-Service (DoS) attacks targeting the classical channel in QKD networks, with a focus on the Quantum Point-to-Point Protocol (Q3P). We analyze how adversaries can exploit Q3P’s First-In-First-Out (FIFO) queuing mechanism by injecting large packets, leading to delays or disruptions in the key exchange process. Furthermore, we evaluate potential mitigation strategies and discuss the broader challenges involved in securing the classical communication infrastructure of QKD systems.