What is ModSecurity?

ModSecurity is a free and open source web application that started out as an Apache module and grew to a fully-fledged web application firewall. It works by inspecting requests sent to the web server in real time against a predefined rule set, preventing typical web application attacks like XSS and SQL Injection.

Prerequisites & Requirements

In order to install and configure ModSecurity, you need to have a Linux server with the following services running:

• Apache 2

For instructions, see our guide on How to Install Apache Web Server on Ubuntu. Installation instructions for several other Linux distributions are also accessible from this guide.

Installing ModSecurity

1. ModSecurity can be installed by running the following command in your terminal:

   sudo apt install libapache2-mod-security2 -y
   

2. Alternatively, you can also build ModSecurity manually by cloning the official ModSecurity Github repository.

3. After installing ModSecurity, enable the Apache 2 headers module by running the following command:

   sudo a2enmod headers
   

4. After installing ModSecurity and enabling the header module, you need to restart the apache2 service, this can be done by running the following command:

   sudo systemctl restart apache2
   

   You should now have ModSecurity installed. The next steps involves enabling and configuring ModSecurity and the OWASP-CRS.

Configuring ModSecurity

ModSecurity is a firewall and therefore requires rules to function. This section shows you how to implement the OWASP Core Rule Set. First, you must prepare the ModSecurity configuration file.

1. Remove the .recommended extension from the ModSecurity configuration file name with the following command:

   sudo cp /etc/modsecurity/modsecurity.conf-recommended /etc/modsecurity/modsecurity.conf
   

2. With a text editor such as vim, open /etc/modsecurity/modsecurity.conf and change the value for SecRuleEngine from DetectionOnly to On:

   File: /etc/modsecurity/modsecurity.conf

– Rule engine initialization ———————————————-

Enable ModSecurity, attaching it to every transaction. Use detection

only to start with, because that minimises the chances of post-installation

disruption.

SecRuleEngine On … ```

1. Restart Apache to apply the changes:

   sudo systemctl restart apache2
   

ModSecurity should now be configured to run. The next step in the process is to set up a rule set to actively prevent your web server from attacks.

Setting Up the OWASP ModSecurity Core Rule Set

The OWASP ModSecurity Core Rule Set (CRS) is a set of generic attack detection rules for use with ModSecurity or compatible web application firewalls. The CRS aims to protect web applications from a wide range of attacks, including the OWASP Top Ten, with a minimum of false alerts. The CRS provides protection against many common attack categories, including SQL Injection, Cross Site Scripting, and Local File Inclusion.

To set up the OWASP-CRS, follow the procedures outlined below.

1. First, delete the current rule set that comes prepackaged with ModSecurity by running the following command:

   sudo rm -rf /usr/share/modsecurity-crs
   

2. Ensure that git is installed:

   sudo apt install git
   

3. Clone the OWASP-CRS GitHub repository into the /usr/share/modsecurity-crs directory:

   sudo git clone https://github.com/coreruleset/coreruleset /usr/share/modsecurity-crs
   

4. Rename the crs-setup.conf.example to crs-setup.conf:

   sudo mv /usr/share/modsecurity-crs/crs-setup.conf.example /usr/share/modsecurity-crs/crs-setup.conf
   

5. Rename the default request exclusion rule file:

   sudo mv /usr/share/modsecurity-crs/rules/REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf.example /usr/share/modsecurity-crs/rules/REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf
   

   You should now have the OWASP-CRS setup and ready to be used in your Apache configuration.

Enabling ModSecurity in Apache 2

To begin using ModSecurity, enable it in the Apache configuration file by following the steps outlined below:

1. Using a text editor such as vim, edit the /etc/apache2/mods-available/security2.conf file to include the OWASP-CRS files you have downloaded:

   File: /etc/apache2/mods-available/security2.conf

1. In /etc/apache2/sites-enabled/example.com.conf file VirtualHost block, include the SecRuleEngine directive set to On.

   File: /etc/apache2/sites-enabled/example.com.conf

<VirtualHost *:80> ServerAdmin webmaster@localhost DocumentRoot /var/www/html

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

    SecRuleEngine On

If you are running a website that uses SSL, add `SecRuleEngine` directive to that website's configuration file as well. See our guide on [SSL Certificates with Apache on Debian & Ubuntu](/docs/guides/ssl-apache2-debian-ubuntu/#configure-apache-to-use-the-ssl-certificate) for more information.

1. Restart the apache2 service to apply the configuration:

   sudo systemctl restart apache2
   

   ModSecurity should now be configured and running to protect your web server from attacks. You can now perform a quick test to verify that ModSecurity is running.

Testing ModSecurity

Test ModSecurity by performing a simple local file inclusion attack by running the following command:

```
curl http://<SERVER-IP/DOMAIN>/index.html?exec=/bin/bash
```

If ModSecurity has been configured correctly and is actively blocking attacks, the following error is returned:

```output
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access this resource.</p>
<hr>
<address>Apache/2.4.41 (Ubuntu) Server at 172.105.53.220 Port 80</address>
</body></html>
```