Fifth-generation (5G) new radio (NR) millimetre wave (mmWave) is a kind of 5G network that operates in the 24GHz to 100GHz frequency range. It offers several opportunities as well as numerous challenges. One of the most prominent challenges that a 5G network faces is intrusion. Intrusion is possible because of existing vulnerabilities in the 5G NR mmWave network architecture. We exploited one such vulnerabil ity to create synchronise (SYN) flood intrusion into the network. The SYN flood intrusion is a denial of service (DoS) intrusion. The intruder who is involved in the SYN flood intrusion depletes the network's available resources. As a result, it denies genuine UEs/nodes access to the network's services and resources. Since this attack produces many open connections with the server, it slows down media access control (MAC) schedulers' ability to assign available channels to the user equipment. In this article, we proposed a method to exploit existing vulnerabilities of the 5G NR mmWave network to carry out SYN flood attacks. Further, we investigated the effect of the attack on the performance of the MAC schedulers, such as proportionate fair and round robin MAC schedulers. With the addition of SYN flood attack UEs/nodes, we observed that the throughput for proportional fair and round robin MAC schedulers drops dramatically.In the event of an attack, throughput drops by 2.34 % to 37.7%. However, in the event of an SYN flood attack, network delay and jitter increase. The performance of the network suffers as a result of that.