Advanced Allergy Attacks: Does a Corpus Really Help?

As research in automatic signature generators (ASGs) receives more attention, various attacks against these systems are being identified. One of these attacks is the "allergy attack" which induces the target ASG into generating harmful signatures to filter out normal traffic at the perimeter defense, resulting in a DoS against the protected network. It is tempting to attribute the success of allergy attacks to a failure in not checking the generated signatures against a corpus of known "normal" traffic, as suggested by some researchers. In this paper, we argue that the problem is more fundamental in nature; the alleged "solution" is not effective against allergy attacks as long as the normal traffic exhibits certain characteristics that are commonly found in reality. We have come up with two advanced allergy attacks that cannot be stopped by a corpus-based defense. We also propose a page-rank-based metric for quantifying the damage caused by an allergy attack. Both the analysis based on the proposed metric and our experiments with Polygraph and Hamsa show that the advanced attacks presented will block out 10% to 100% of HTTP requests to the three websites studied: CNN.com, Amazon. com and Google.com.