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The Domain Name System (DNS) is a critical infrastructure in the Internet; thus, monitoring its traffic, and protecting DNS from malicious activities are important for security in cyberspace. However, it is often difficult to determine whether a DNS query is caused by malicious or normal activity, because information available in DNS traffic is limited.We focus on the activities of mass-mailing worms and propose a method to detect hosts infected by mass-mailing worms by mining DNS traffic data. Our method begins with a small amount of a priori knowledge about a signature query. By assuming that queries sent by most hosts that have sent the signature query of worms have been sent by worm behavior, we detect infected hosts using Bayesian estimation.We apply our method to DNS traffic data captured at one of the largest commercial Internet Service Providers in Japan, and the experimental result indicates that an 89% reduction of mail exchange queries can be achieved with the method.

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