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NAXSI means Nginx Anti XSS & SQL Injection.
Technically, it is a third party nginx module, available as a package for
many UNIX-like platforms. This module, by default, reads a small subset of
simple (and readable) rules
containing 99% of known patterns involved in
websites vulnerabilities. For example, <, | or drop are not supposed
to be part of a URI.
Being very simple, those patterns may match legitimate queries, it is Naxsi's administrator duty to add specific rules that will whitelist those legitimate behaviours. The administrator can either add whitelists manually by analyzing nginx's error log, or (recommended) start the project by an intensive auto-learning phase that will automatically generate whitelisting rules regarding website's behaviour.
In short, Naxsi behaves like a DROP-by-default firewall, the only job needed is to add required ACCEPT rules for the target website to work properly.
On the contrary of most Web Application Firewall, Naxsi doesn't rely on a signature base, like an antivirus, and thus cannot be circumvented by an "unknown" attack pattern. Another main difference between Naxsi and other WAF, Naxsi filters only GET and POST resquests, is a Free software (as in freedom) and free (as in free beer) to use.
- The documentation
- Some rules for mainstream softwares
- The nxapi/nxtool to generate rules