A computer device is provided with a local security mechanism, a personal firewall, for protecting the computer device from attacks from a foreign network, in addition to or instead of a firewall in the internal network which protects the computer when connected to a home network. The personal firewall is provided with different sets of security rules for the home network and foreign networks. The personal firewall is arranged to detect its current location, i.e. determine to which network it is connected to at each particular moment. The personal firewall activates one of the given sets of security rules according to the detected current location of the computer device, i.e. the personal firewall automatically uses the security rules predefined for the network to which the computer device is connected at each particular moment. Upon detecting a change in the location, the personal firewall immediately adapts to use security rules predefined for the new location.