ModSecurity – Open Source Web Application Firewall

Last updated: November 15, 2015 | 6,732 views

ModSecurity is an open source web application firewall (WAF) module that is cross platform capable. Known as the “Swiss Army Knife” of WAFs, it enables web application defenders to gain visibility into HTTP(S) traffic and provides a power rules language and API to implement advanced protections.

ModSecurity - Open Source Web Application Firewall

ModSecurity is a toolkit for real-time web application monitoring, logging, and access control. It’s 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.

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.

Features

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.
Virtual patching
Virtual patching is a concept of vulnerability mitigation in a separate layer, where you get to fix problems in applications without having to touch the applications themselves. Virtual patching is applicable to applications that use any communication protocol, but it is particularly useful with HTTP, because the traffic can generally be well understood by an intermediary device. ModSecurity excels at virtual patching because of its reliable blocking capabilities and the flexible rule language that can be adapted to any need. It is, by far, the activity that requires the least investment, is the easiest activity to perform, and the one that most organizations can benefit from straight away.
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?

Deployment Options

ModSecurity supports two deployment options: embedded and reverse proxy deployment. There is no one correct way to use them; choose an option based on what best suits your circumstances. There are advantages and disadvantages to both options:

Embedded
Because ModSecurity is an Apache module, you can add it to any compatible version of Apache. At the moment that means a reasonably recent Apache version from the 2.0.x branch, although a newer 2.2.x version is recommended. The embedded option is a great choice for those who already have their architecture laid out and don’t want to change it. Embedded deployment is also the only option if you need to protect hundreds of web servers. In such situations, it is impractical to build a separate proxybased security layer. Embedded ModSecurity not only does not introduce new points of failure, but it scales seamlessly as the underlying web infrastructure scales. The main challenge with embedded deployment is that server resources are shared between the web server and ModSecurity.
Reverse proxy
Reverse proxies are effectively HTTP routers, designed to stand between web servers and their clients. When you install a dedicated Apache reverse proxy and add ModSecurity to it, you get a ‘proper‘ network web application firewall, which you can use to protect any number of web servers on the same network. Many security practitioners prefer having a separate security layer. With it you get complete isolation from the systems you are protecting. On the performance front, a standalone ModSecurity will have resources dedicated to it, which means that you will be able to do more (i.e. have more complex rules). The main disadvantage of this approach is the new point of failure, which will need to be addressed with a high-availability setup of two or more reverse proxies.

You can install ModSecurity on most operating systems from the repository as it’s been around a fair while.

For Ubuntu/Debian