Comment spam has become a popular means for spammers to attract direct visits to target websites, or to manipulate search ranks of the target websites. Through posting a small number of spam messages on each victim website (e.g., normal websites such as forums, wikis, guest-books, and blogs, which the authors term as spam harbors in this paper) but spamming on a large variety of harbors, spammers can not only directly inherit some reputations from these harbors but also avoid content-based detection systems deployed on these harbors. To find such qualified harbors, spammers always have their own preferred ways based on their available resources and the cost (e.g., easiness of automatic posting, chances of content sanitization on the website).