A request for content is received at a content filter of a computer system, and a determination is made, based on a referrer included in the request, regarding a root site directly or indirectly associated with the request. Thereafter, the content is permitted or not permitted according to whether or not the root site is a permitted site and a site associated with the content is categorized in a semantically equivalent content category with the root site. The determination regarding the root site may involve consulting a referrer chain cache storing lists of referrers which associate referring Web sites with root sites from which referrals originate.