Introduction

ModSecurity is a plug-in module for Apache that works like a firewall. It functions through rule sets, which allow you to customize and configure your server security.

ModSecurity can also monitor web traffic in real time and help you detect and respond to intrusions. It can be used with Apache, Nginx, and IIF and is compatible with Debian, Ubuntu, and CentOS.

This tutorial explains how to install and configure ModSecurity on Apache web servers.

Prerequisites

  • The LAMP stack (Linux, Apache, MySQL, PHP) installed and configured
  • Access to a user account with sudo or root privileges
  • A package manager (APT or YUM), included by default
  • A command line/terminal window (Ctrl-Alt-T, Ctrl-Alt-F1)
  • A text editor, like nano

Step 1: Update Software Repositories

Open a terminal window, and enter the following:

On Debian / Ubuntu

sudo apt update -y

Update Ubuntu/Debian repository list.

On CentOS

sudo yum update -y

Step 2: Installing ModSecurity On Apache

Install ModSecurity on Debian

1. In a terminal window, enter the following:

sudo apt install libapache2-modsecurity

If prompted, pres y and hit Enter to allow the process to complete.

2. Restart the Apache service:

sudo systemctl restart apache2

There will be no output if Apache was restarted successfully.

3. Check the software version (it should be 2.8.0 or later):

apt-cache show libapache2-modsecurity

Note: Ubuntu has a slightly different syntax for the ModSecurity package.


Install ModSecurity on Ubuntu 18.04

1. In a terminal window, enter:

sudo apt install libapache2-mod-security2

If prompted, pres y and hit Enter to allow the process to complete.

Installing modevasive on Ubuntu.

2. Restart the Apache service:

sudo systemctl restart apache2

There will be no output if Apache was restarted successfully.

3. Check the software version (should be 2.8.0 or later):

apt-cache show libapache2-mod-security2

Check mod_evasive on Ubuntu.

Install ModSecurity on CentOS 7

1. Enter the following into a terminal window:

sudo yum install mod_security

If prompted, pres y and hit Enter to allow the process to complete.

2. Restart the Apache service:

sudo systemctl restart httpd.service

3. Check the software version (should be 2.8.0 or later):

yum info mod_security

Step: 3 Configure ModSecurity

Upon installation, ModSecurity is set to log events according to default rules. You’ll need to edit the configuration file to adjust the rules to detect and block traffic.

The default configuration file is /etc/modsecurity/modsecurity.conf-recommended.

1. Copy and rename the file:

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

2. Next, change the ModSecurity detection mode. First, move into the /etc/modsecurity folder:

sudo cd /etc/modsecurity

3. Open the configuration file in a text editor (we will be using nano):

sudo nano modsecurity.conf

Near the top, you should see an entry labeled:

SecRuleEngine DetectionOnly

Change this to read as follows:

SecRuleEngine On

Configure the SecRuleEngine option to set up ModSecurity.

4. Use CTRL+X to exit, then press y then Enter to save the changes.

5. Navigate away from the /etc/modsecurity folder:

cd

6. Restart Apache:

On Debian/Ubuntu

sudo systemctl restart apache2

Restart Apache to load the updated configuration file.

On CentOS

sudo systemctl restart httpd.service

This will turn on ModSecurity using the basic default rules. In some versions of Linux, this includes the OWASP Core Rule Set. However, this might differ from the latest version maintained by the developers.


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Step 4: Download Latest OWASP ModSecurity Rules

The latest Core Rule Set (CRS) for ModSecurity is maintained on GitHub.

1. Install Git if it’s not already included on your system.

Install Git on Debian/Ubuntu:

sudo apt install git

Install Git on CentOS:

sudo yum install git

2. Download a copy of the CRS:

git clone https://github.com/SpiderLabs/owasp-modsecurity-crs.git

Clone the OWASP GitHub repository.

This places a copy of the directory as a subdirectory of your current working location.

3. Open a new directory:

cd owasp-modsecurity-crs

4. Move the crs-setup file:

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

Image of a command that moves the OWASP csr setup file.

5. Then move the rules/ directory:

sudo mv rules/ /etc/modsecurity

If you encounter an error trying to move this directory, enter:

sudo mkdir /etc/modsecurity/rules
cd rules 
sudo cp *.* /etc/modsecurity/rules

6. Next, check your security2.conf file to verify it’s set to load the ModSecurity rules:

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

Edit the OWASP scp configuration file.

Verify you have the following lines included and uncommented:

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

If they are not there, add them. Do not duplicate them, or you risk disabling your Apache service.

The OWASP configration file.

7. Restart the Apache service:

On Debian/Ubuntu

sudo systemctl restart apache2

On CentOS

sudo systemctl restart httpd.service

Step 5: Test Apache Configuration

1. Open the default Apache configuration file:

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

2. Locate the </VirtualHost> tag at the bottom and add the following lines:

SecRuleEngine On
SecRule ARGS:testparam "@contains test" "id:1234,deny,status:403,msg:'phoenixNAP test rule was triggered'"

You can change the msg to whatever you prefer.

Nano editor showing the apache configuration file.

Save and quit the file (CTRL+X > y > Enter).

3. Restart the Apache service:

On Debian/Ubuntu

sudo systemctl restart apache2

On CentOS

sudo systemctl restart httpd.service

4. Then, enter the following command:

curl localhost/index.html?testparam=test

The system responds by attempting to display the default webpage. Instead of the content, it generates error codes and messages inside the tags:

ModSecurity error messages in the terminal.

5. You can confirm that ModSecurity worked by looking for code 403 at the Apache error logs with the command:

sudo tail -f /var/log/apache2/error.log

One of the entries towards the bottom should be the ModSecurity error code:

Test ModSecurity and OWASP CRS With Bash Script

Another method you can use to test ModSecurity is to use a Bash script.

1. Enter the following command in the terminal:

curl localhost/index.html?exec=/bin/bash

The output shows the same error messages as the last time.

2. View the Apache error.log file again and you will find that the the rule kicked in:

sudo tail -f /var/log/apache2/error.log

OWASP CRS error message in the Apache log

The output displays the OWASP-related ModSecurity error message.

Step 6: Create ModSecurity Rules

Below is a test example how you can use ModSecurity to block specific keywords on a PHP form.

1. Create a PHP file inside the html directory with the command:

sudo nano /var/www//html/test.php

2. Enter the following code into the file:

<html>
<body>
<?php
if(isset($_POST['data']))
echo $_POST['data'];
else
{
?>
<form method="post" action="">
Enter text here:<textarea name="data"></textarea>
<input type="submit"/>
</form>
<?php
}
?>
</body>
</html>

Save the file and exit.

3. Next, create a new ModSecurity custom rules file:

sudo nano /etc/modsecurity/modsecurity_custom_rules.conf

Add the following lines:

SecRule REQUEST_FILENAME "test.php" "id:'400001',chain,deny,log,msg:'Spam detected'"
SecRule REQUEST_METHOD "POST" chain
SecRule REQUEST_BODY "@rx (?i:(enlarge|Nigerian|gold))"

Of course, change the keywords in the last line to anything you want.

Save the file and exit.

4. Reload the Apache service:

On Debian/Ubuntu

sudo systemctl restart apache2

On CentOS

sudo systemctl restart httpd.service

5. Launch the form in a web browser

localhost/test.php

Browser showing the php test form.

6. Type one of the keywords from the rule into the form. In this example: enlarge, Nigerian, or gold.

You should receive a 403 Forbidden error message.

A browser window showing forbidden message after testing a form.

You can also check the /var/log/apache2/error.log file to verify ModSecurity’s action.


Note: We don’t need to add this custom_rules file to the security2.conf file, because we specified a wildcard (IncludeOptional /etc/modsecurity/*.conf). If we had specified an individual .conf file, we would need to add this custom_rules file to the security2.conf file.


Conclusion

You should now have a solid understanding of how to install, set up, and configure ModSecurity on Apache. Make sure you installed the LAMP stack properly before following the steps in this guide.


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