Time-sensitive networking (TSN) is a promising technology for real-time communication in industrial and automotive networks. One of its key features is frame preemption, which allows high-priority traffic to interrupt the transmission of low-priority traffic, thereby reducing the delay of high-priority critical traffic. However, the deterministic nature of TSN frame preemption also makes it vulnerable to denial of service (DoS) attacks, which can severely impact flow quality of service (QoS) by increasing delays and packet loss. In this paper, we introduce the concept of preemptive DoS attacks and evaluate their impact on TSN QoS performance. We describe a strategy that attackers can use to estimate the preemption scheme configured in the switch and then demonstrate how an active attacker can use this information to degrade TSN QoS. Our simulation results indicate that even a single attacker can significantly deteriorate the QoS of TSN traffic. It is important to address this vulnerability in TSN and develop countermeasures to prevent preemptive DoS attacks from occurring.