Characterization of HTTP flood DDoS attacks

A method for characterizing application layer denial-of-service (DDoS) attacks comprises generating a plurality of dynamic applicative signatures by analyzing at the application layer application layer requests received during an on-going DDoS attack, a dynamic applicative signature characterizing each received request based on frequent application layer attributes appearing in the received requests, wherein the requests are represented as a set of paraphrases, each paraphrase representing a specific aspect of a request's structure, the frequent application layer attributes being determined based on frequency of paraphrases in the set; characterizing each of the received requests based on one of the dynamic applicative signatures, the characterization providing an indication for each request whether a request is generated by an attack tool executing the on-going DDoS attack; and causing a mitigation action on the received request generated by the attack tool based on the generated dynamic applicative signature.