A computer security system for use in a network environment comprising at least a plurality of user computers arranged to communicate over a network, the system comprising a warning message exchange system operable to allow the communication from the user computers of warning messages relating to suspect data identified as a possible security threat; a message counting system operable to maintain a count for every particular piece or set of suspect data based on the number of warning messages communicated relating thereto; and network security means operable to act against any particular piece or set of suspect data for which the count maintained therefor exceeds at least one threshold value.