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Evolutionary study of phishing

We study the evolution of phishing email messages in a corpus of over 380,000 phishing messages collected from August 2006 to December 2007. Our first result is a classification of phishing messages into two groups: flash attacks and non-flash attacks. Phishing message producers try to extend the usefulness of a phishing message by reusing the same message. In some cases this is done by sending a large volume of phishing messages over a short period of time (flash-attack) versus the same phishing message spread over a relatively longer period (nonflash attacks). Our second result is a corresponding classification of phishing features into two groups: transitory features and pervasive features. Features which are present in a few attacks and have a relatively short life span (transitory) are generally strong indicators of phishing, whereas features which are present in most of the attacks and have a long life span (pervasive) are generally weak selectors of phishing. One explanation of this is that phishing message producers limit the utility of transitory features in time (by avoiding them in future generations of phishing) and limit the utility of pervasive features by choosing features that also appear in legitimate messages. While useful in improving the understanding of phishing messages, our results also show the need for further study.