ModSecurity is a toolkit for real-time web application monitoring, logging, and access control. I like to think about it as an enabler: there are no hard rules telling you what to do; instead, it is up to you to choose your own path through the available features. That's why the title of this section asks what ModSecurity can do, not what it does.
The freedom to choose what to do is an essential part of ModSecurity's identity and goes very well with its open source nature. With full access to the source code, your freedom to choose extends to the ability to customize and extend the tool itself to make it fit your needs. It's not a matter of ideology, but of practicality. I simply don't want my tools to restrict what I can do.
Back on the topic of what ModSecurity can do, the following is a list of the most important usage scenarios:
- Real-time application security monitoring and access control At its core, ModSecurity gives you access to the HTTP traffic stream, in real-time, along with the ability to inspect it. This is enough for real-time security monitoring. There's an added dimension of what's possible through ModSecurity's persistent storage mechanism, which enables you to track system elements over time and perform event correlation. You are able to reliably block, if you so wish, because ModSecurity uses full request and response buffering.
- Full HTTP traffic logging Web servers traditionally do very little when it comes to logging for security purposes. They log very little by default, and even with a lot of tweaking you are not able to get everything that you need. I have yet to encounter a web server that is able to log full transaction data. ModSecurity gives you that ability to log anything you need, including raw transaction data, which is essential for forensics. In addition, you get to choose which transactions are logged, which parts of a transaction are logged, and which parts are sanitized.
- Continuous passive security assessment Security assessment is largely seen as an active scheduled event, in which an independent team is sourced to try to perform a simulated attack. Continuous passive security assessment is a variation of real-time monitoring, where, instead of focusing on the behavior of the external parties, you focus on the behavior of the system itself. It's an early warning system of sorts that can detect traces of many abnormalities and security weaknesses before they are exploited.
- Web application hardening One of my favorite uses for ModSecurity is attack surface reduction, in which you selectively narrow down the HTTP features you are willing to accept (e.g., request methods, request headers, content types, etc.). ModSecurity can assist you in enforcing many similar restrictions, either directly, or through collaboration with other Apache modules. They all fall under web application hardening. For example, it is possible to fix many session management issues, as well as cross-site request forgery vulnerabilities.
- Something small, yet very important to you Real life often throws unusual demands to us, and that is when the flexibility of ModSecurity comes in handy where you need it the most. It may be a security need, but it may also be something completely different. For example, some people use ModSecurity as an XML web service router, combining its ability to parse XML and apply XPath expressions with its ability to proxy requests. Who knew?
- For more information please visit https://modsecurity.org/
