A method for enforcing a security policy for selectively preventing the downloading and execution of undesired Executable Objects in an individual workstation, comprising the steps of, (1) providing a security agent suitable to be installed in an individual workstation, said security agent being provided with means for introducing at least one marker in one or more data packet transmitted by a workstation to a server through a gateway, said at least one marker indicating that a security agent is installed in the transmitting workstation; (2) providing means in or coupled to the gateway for analyzing the first one or more data packet(s) received from a transmitting workstation initiating communication to a remote server, to determine whether said first one or more data packet(s) comprise at least one marker indicating that a suitable security agent is installed in the transmitting workstation; (3) If at least one marker indicating that a suitable security agent is installed in the transmitting workstation is detected in said first one or more data packet(s), deleting said marker(s) from said first one or more data packet(s), and allowing said data packet(s) to be transmitted to their destination; and (4) If no marker indicating that a suitable security agent is installed in the transmitting workstation is detected in said first one or more data packet(s), preventing any data packet(s) received from the server to which the workstation is connected from being transmittedto the workstation.