In an embodiment, a statistical approach for augmenting signature detection in a Web application firewall includes receiving a new request including a parameter in a uniform resource identifier (URI), tokenizing the new request, and determining a compound probability that tokens in a value that is associated with the parameter of the URI and that is included in the new request are associated with an attack. The compound probability is determined based at least in part on component probabilities of tokens of historical values associated with the parameter of the URI.