Why anti-spam and security features in an SEO-oriented Joomla! extension? because getting spammy comments on your pages will get your otherwise superb content loose weight and authority in the eyes of a search engine. Worse, getting infected by malware is the fastest way to get your site out of search engines results altogether. To help prevent such outcome, sh404SEF implements two sets of features:
Anti-spam with Project HoneyPot
Project HoneyPot is a community driven project that first identifies spammers, and then let others use this information to reject visitors from spamming IP addresses.
sh404SEF implements the Project HoneyPot protocol and can block users visitors to your site if they have been identified as spammers. In addition to preventing poor content being injected on your site, this will also saves a little bit of server CPU and memory.
Being a Joomla! extension, sh404SEF can implement a few basic security checks that are otherwise more difficult to put in place at the web server level for instance. This includes checks on some common requests parameters, but it also lets you blacklist (or white-list) visitors by their IP address or user agent string.
sh404SEF also provides you with anti-flooding protection. Though a last-resort protection, usually better done at web server level, this feature will save you CPU and memory if your site is under a flooding attack.
Please be well aware that security features provided by an extension cannot replace proper security policies. Probably the two most important things to do are: update Joomla! and any extension you use on your site as soon as a new version is available and use a well-known hosting service with good security record