Malicious URLs, also known as malicious websites, are a common and serious cyber-security threat. Malicious URLs host unsolicited content (spam, phishing, drive-by downloads, and so on) and lure unsuspecting users into becoming victims of scams (monetary loss, theft of private information, and malware installation), resulting in billions of dollars in losses each year. It is critical to detect and respond to such threats as soon as possible. Traditionally, this detection has been accomplished primarily through the use of blacklists. However, blacklists are not exhaustive and cannot detect newly generated malicious URLs. Machine learning techniques have received increased attention in recent years in order to improve the generality of malicious URL detectors.