What is Naxsi?
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 website 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 the Naxsi’s administrator duty to add specific rules that will whitelist legitimate behaviors. The administrator can either add whitelists manually by analyzing nginx’s error log or (recommended) start the project with an intensive auto-learning phase that will automatically generate whitelisting rules regarding a website’s behavior.
In short, it behaves like a DROP-by-default firewall, the only task is to add required ACCEPT rules for the target website to work properly.
Why is it different?
Contrary to most Web Application Firewalls, 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 WAFs, it filters only GET and POST requests, is Free software (as in freedom) and free (as in free beer) to use.
What does it run on?
Naxsi is compatible with any nginx version, although it currently doesn’t play well with the new HTTPv2 protocol added in recent nginx versions. See issue #227 for more details.
It depends on libpcre for its regexp support and is reported to work great on NetBSD, FreeBSD, OpenBSD, Debian, Ubuntu, and CentOS.
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