A Multiple Instance Learning Strategy for Combating Good Word Attacks on Spam Filters

Statistical spam filters are known to be vulnerable to adversarial attacks. One of the more common adversarial attacks, known as the good word attack, thwarts spam filters by appending to spam messages sets of “good” words, which are words that are common in legitimate email but rare in spam. We present a counterattack strategy that attempts to differentiate spam from legitimate email in the input space by transforming each email into a bag of multiple segments, and subsequently applying multiple instance logistic regression on the bags. We treat each segment in the bag as an instance. An email is classified as spam if at least one instance in the corresponding bag is spam, and as legitimate if all the instances in it are legitimate. We show that a classifier using our multiple instance counterattack strategy is more robust to good word attacks than its single instance counterpart and other single instance learners commonly used in the spam filtering domain.