Naxsi
Open-source, high performance, low rules maintenance WAF for NGINX
...Being very simple, those patterns may match legitimate queries, it is 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, Naxsi behaves like a DROP-by-default firewall, the only task is to add required ACCEPT rules for the target website to work properly.