AT_NA2000 from Nanda Automation Technology vendor has a denial-of-service vulnerability. For the processing of TCP RST packets, PLC AT_NA2000 has a wide acceptable range of sequence numbers. It does not require the sequence number to exactly match the next expected sequence value, just to be within the current receive window, which violates RFC5961. This flaw allows attackers to send multiple random TCP RST packets to hit the acceptable range of sequence numbers, thereby interrupting normal connections and causing a denial-of-service attack.