A new method of covert channel detection in Initial Sequence Number (ISN) of TCP/IP is proposed in the paper. The detection is based on ISN generation model of original OS. Whenever any statistical deviations of ISN network packet from the ISN model are discovered; it is considered that this ISN packet is generated by malicious software, which tries to create a covert channel. The method was tested using experimental data generated by NUSHU covert channel creation tool, which has been developed by Joanna Rutkowska.