A method, including identifying, in network data traffic, multiple scans, each of the scans including an access, in the traffic, of multiple ports on a given destination node by a given source node during a time period. A group of high-traffic ports are identified in the traffic that include one or more ports that receive respective volumes of the traffic that exceed a threshold, and respective signatures are generated for the identified port scans that indicate the ports other than the high-traffic ports that were accessed in each of the port scans. A respective frequency of occurrence of each of the signatures over the set of the port scans is computed, and a whitelist of the signatures for which the respective frequency of occurrence is greater than a threshold is assembled. Upon detecting a port scan for which the respective signature is not whitelisted, a preventive action is initiated.