1. A security client comprising a processor and a memory, the security client configured to use a one-time execution code in an environment in which a server and the security client establish a connection through a network, wherein the security client automatically receives, from the server, the one-time execution code without a user interaction, wherein the received one-time execution code is included in a packet between the server and the security client, the packet being transmitted from the server to the security client, wherein the one-time execution code is repeatedly generated as irregular by the server while the connection with the security client is maintained, for determining whether a client program is appropriate, wherein the security client is configured to load the one-time execution code on a code container, the code container being pre-allocated in a part of the memory which is already occupied by the client program, and obtain an execution result of the loaded one-time execution code while executing the client program, wherein the received one-time execution code is not generated as a file but directly loaded on the pre-allocated part of the memory which is already occupied by the client program, wherein the security client transmits the execution result to the server, and deletes the one-time execution code from the code container after the transmitting of the execution result, wherein the transmitted execution result is included in a packet between the server and the security client, the packet being transmitted from the security client to the server.