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Collaborative filtering techniques have become popular in the past decade as an effective way to help people deal with information overload. Recent research has identified significant vulnerabilities in collaborative filtering techniques. Shilling attacks, in which attackers introduce biased ratings to influence recommendation systems, have been shown to be effective against memory-based collaborative filtering algorithms. We examine the effectiveness of two popular shilling attacks (the random attack and the average attack) on a model-based algorithm that uses Singular Value Decomposition (SVD) to learn a low-dimensional linear model. Our results show that the SVD-based algorithm is much more resistant to shilling attacks than memory-based algorithms. Furthermore, we develop an attack detection method directly built on the SVD-based algorithm and show that this method detects random shilling attacks with high detection rates and very low false alarm rates.

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Abstract. For any odd squarefree integer r, we get complete description of the Gr = Gal(Q(µr)/Q) group cohomology of the universal ordinary distribution Ur in this paper. Moreover, if M is a fixed integer which divides ℓ−1 for all prime factors ℓ of r, we study the cohomology group H * (Gr, Ur/M Ur). In particular, we explain the mysterious construction of the elements κ r ′ for r ′ |r in Rubin [10], which come exactly from a certain Z/M Z-basis of the cohomology group H 0 (Gr, Ur/M Ur) through an evaluation map.