To detect malicious executables, often spread as email attachments, two types of algorithms are usually applied under instance-based statistical learning paradigms: (1) Signature-based template matching, which finds unique tell-tale characteristics of a malicious executable and thus is capable of matching those with known signatures; (2) Two-class supervised learning, which determines a set of features that allow benign and malicious patterns to occupy a disjoint regions in a feature vector space and thus probabilistically identifies malicious executables with the similar features. Nevertheless, given the huge potential variety of malicious executables, we cannot be confident that existing training sets adequately represent the class as a whole. In this study, we
investigated the use of byte sequence frequencies to profile only benign data. The malicious executables are identified as outliers or anomalies that significantly deviate from the normal profile. A multivariate Gaussian likelihood model, fit with a Principal
Component Analysis (PCA), was compared with a one-class Support Vector Machine (SVM) model for characterizing the benign executables. We found that the Gaussian model substantially outperformed the one-class SVM in its ability to distinguish
malicious from benign files. Complementing to the capabilities in reliably detecting those malicious files with known or similar features using two aforementioned methods, the one-class unsupervised approach may provide another layer of safeguard in identifying those novel computer viruses.