The Apache web server is highly customizable and can be configured in multiple ways to suit your needs. There are many third-party modules that you can use to configure Apache to your preference.

ModSecurity is an open-source WAF (Web Application Firewall) that is native to the Apache web server. It was initially an Apache module only but has grown over the year to become a fully-fledged web app firewall. It is now supported by Nginx and even IIS.

ModSecurity inspects incoming requests to the webserver against a predefined set of rules. Typically, it provides a set of rules known as CRS (Core Rule Set) that protect a website from an array of web application attacks such as SQL injection, XSS, and session hijacking among other exploits.

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The ModSecurity application firewall forms an integral part of PCI DSS compliance in shielding sites from external attacks. When the module is enabled it triggers a ‘403 Forbidden Error’ which simply implies that you have insufficient permissions to access the resource on the webserver.

In this guide, we will show you how to set up and configure ModSecurity to work with Apache on Debian and Ubuntu Linux.

Important Note for Debian 12 (Bookworm) Users

If you are using Debian 12 (Bookworm), be cautious when following this setup, as it may create duplicate rules that can prevent Apache from starting. In recent Debian versions, the core ruleset (modsecurity-crs) often comes bundled with the modsecurity-crs package by default, so manual installation may not be necessary.

Make sure to check your installed packages first to avoid any duplication issues.

dpkg -l | grep modsecurity-crs

If you see a line with modsecurity-crs listed, it means the package is already installed.

Step 1: Install ModSecurity on Ubuntu

The first step is to install ModSecurity. We will begin by, first, refreshing the package lists as follows:

$ sudo apt update

Next, install the ModSecurity package alongside other dependencies and libraries.

$ sudo apt install libapache2-mod-security2

Afterward, enable the module.

$ sudo a2enmod security2

Then restart the Apache webserver to apply the changes.

$ sudo systemctl restart apache2

At this point, ModSecurity is successfully installed. Let’s now configure it.

Step 2: Configure ModSecurity in Ubuntu

By default, ModSecurity is only configured to detect and log suspicious activity. We need to go an extra step and configure it to not only detect but also block suspicious activity.

Copy, the default ModSecurity configuration file – modsecurity.conf-recommended – to a new file as provided in the command below.

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

Using your preferred text editor, open the file

$ sudo nano /etc/modsecurity/modsecurity.conf

Locate the line:

SecRuleEngine DetectionOnly

Set it to:

SecRuleEngine On

Save the changes and exit the file.

To apply the changes in Apache, restart the webserver.

$ sudo systemctl restart apache2

Step 3: Download OWASP ModSecurity Core Ruleset

The next step is to download the latest OWASP ModSecurity Core Rule Set (CRS) from the GitHub page.

Clone the OWASP git repository as shown.

$ git clone https://github.com/coreruleset/coreruleset.git

Navigate into the directory.

$ cd coreruleset/

Be sure to move the crs-setup.conf.example file to the modsecurity directory and rename it as crs-setup.conf.

$ sudo mv crs-setup.conf.example /etc/modsecurity/crs-setup.conf

In addition, move the rules directory to the modsecurity directory as well.

$ sudo mv rules/ /etc/modsecurity/

Next, edit the security2.conf file.

$ sudo nano /etc/apache2/mods-enabled/security2.conf

Ensure that it contains the following lines.

IncludeOptional /etc/modsecurity/*.conf
Include /etc/modsecurity/rules/*.conf

Then restart Apache for the changes to persist.

$ sudo systemctl restart apache2

Let us now test our ModSecurity configuration.

Step 4: Testing the ModSecurity Configuration on Ubuntu

Lastly, we need to test that ModSecurity can detect and block suspicious HTTP traffic. To achieve this, we need to edit the default virtual host file.

$ sudo nano /etc/apache2/sites-available/000-default.conf

Next, we will create a blocking rule that will block access to a certain URL when accessed by a web browser.

Append these lines at the end before the ‘Virtualhost’ closing tag.

SecRuleEngine On
SecRule ARGS:testparam "@contains test" "id:254,deny,status:403,msg:'Test Successful'"

Feel free to set the ‘id’ and ‘msg’ tags to whatever desirable values.

Then restart the Apache webserver to apply the changes made to the virtual host configuration file.

$ sudo systemctl restart apache2

On your web browser, try to visit the URL shown with ?testparam=test at the end.

http://server-ip/?testparam=test

You get a ‘403 Forbidden error’ indicating that you have been blocked from accessing the resource.

You can further confirm the client was blocked by checking the error logs as follows.

$ cat /var/log/apache2/error.log | grep “Test Successful”

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This is confirmation that we have successfully set up ModSecurity to detect and block unwanted traffic. In this guide, we have walked you through the process of setting up ModSecurity with Apache on Debian/Ubuntu systems.