Install LibModsecurity with Apache on Debian 10


Welcome to our guide on how to install LibModsecurity with Apache on Debian 10. Libmodsecurity (Modsecurity v3), is an open source, cross platform web application firewall (WAF) developed by Trustwave’s SpiderLabs. It is a complete rewrite of ModSecurity v2 and it provides a robust event-based programming language which protects web applications against a wide range of attacks such as SQL injection, Cross-site Scripting (XSS), Local File Include, Remote File Include It also allows for HTTP traffic monitoring, logging and real-time analysis.

Install LibModsecurity with Apache on Debian 10


Run System Update

Resynchronize your system packages to their latest versions.

apt update
apt upgrade

Install LAMP Stack

Well, if you are running a web application, that needs LAMP stack, go ahead and setup LAMP stack.

However, in this demo setup, we will just install Apache, which is enough to demonstrate how ModSecurity 3 works.

apt install apache2

Install Required Build Tools and Dependencies

To install Libmodsecurity or Modsecurity v3 on Debian 10, we are going to build it from source. Hence, you need to install some required build tools and dependencies for a successful build.

apt install g++ flex bison curl apache2-dev doxygen libyajl-dev ssdeep liblua5.2-dev libgeoip-dev libtool dh-autoreconf libcurl4-gnutls-dev libxml2 libpcre++-dev libxml2-dev git

Compile and Install LibModsecurity on Debian 10

Once your system is setup, you can proceed to install Modsecurity v3 on Debian 10.

Download LibModsecurity Source Code

To install the latest stable version of LibModsecurity, you need to compile it from the source.

Therefore, navigate to ModSecurity releases page and download ModSecurity source code. You can simply use wget to pull it.


Extract the ModSecurity source code.

tar xzf modsecurity-v3.0.4.tar.gz

Compile and Install LibModsecurity

Navigate to the LibModsecurity source directory, configure, compile and install it

cd modsecurity-v3.0.4

Configure LibModsecurity to adapt it to your system and check if any required dependency is missing.


You can safely ignore the fatal: not a git repository (or any of the parent directories): .git error messages.


Be sure to fix any dependency issue, if any, before you can proceed to compile and install LibModsecurity with Apache on Debian 10.

If the configure script above completes with no error, proceed to compile and install LibModSecurity on Debian 10.

make install

LibModSecurity aka ModSecurity v3 has now been installed on Debian 10

Install ModSecurity-Apache Connector

The ModSecurity-apache connector provides a communication channel between Apache and libModsecurity. Now that libmodsecurity is installed, follow through the following steps to install Modsecurity Apache connector.

Clone the git repository for the ModSecurity Apache connector.

cd ~
git clone

Navigate to ModSecurity-apache directory and run the following commands to compile and install it.

cd ModSecurity-apache
./configure --with-libmodsecurity=/usr/local/modsecurity/
make install

Configure Apache with LibModsecurity

Load the Apache ModSecurity Connector Module

Configure Apache to load Modsecurity Apache connector module by adding the line below to the main Apache configuration file.

echo "LoadModule security3_module /usr/lib/apache2/modules/" | sudo tee -a /etc/apache2/apache2.conf

Configure ModSecurity

Create ModSecurity configuration directory under /etc/apache2

sudo mkdir /etc/apache2/modsecurity.d

Copy the sample ModSecurity configuration file from the source code directory to the ModSec configuration directory created above renaming it as follows.

sudo cp ~/modsecurity-v3.0.4/modsecurity.conf-recommended /etc/apache2/modsecurity.d/modsecurity.conf

Also copy the unicode.mapping file from ModSecurity source directory to Apache Modsecurity configuration directory.

sudo cp ~/modsecurity-v3.0.4/unicode.mapping /etc/apache2/modsecurity.d/

Turn on ModSecurity by changing the value of SecRuleEngine to On.

sed -i 's/SecRuleEngine DetectionOnly/SecRuleEngine On/' /etc/apache2/modsecurity.d/modsecurity.conf

Next, you need to configure ModSecurity rules. Therefore, create a file where you can define the rules to include, say, /etc/apache2/modsecurity.d/modsec_rules.conf.

cat > /etc/apache2/modsecurity.d/modsec_rules.conf << 'EOL'
Include "/etc/apache2/modsecurity.d/modsecurity.conf"
Include "/etc/apache2/modsecurity.d/owasp-crs/crs-setup.conf"
Include "/etc/apache2/modsecurity.d/owasp-crs/rules/*.conf"

Since we have included the OWASP Rules as part of ModSecurity 3 rules, proceed to install them.

Install OWASP ModSecurity Core Rule Set (CRS) on Debian

The OWASP ModSecurity Core Rule Set (CRS) is a set of generic attack detection rules for use with ModSecurity. It aims at protecting the web applications from a wide range of attacks, including the OWASP Top Ten, minimum of false alerts.

Clone the CRS from GitHub repository to /etc/apache2/modsecurity.d/ as shown below;

git clone /etc/apache2/modsecurity.d/owasp-crs

Next, rename crs-setup.conf.example to crs-setup.conf.

sudo cp /etc/apache2/modsecurity.d/owasp-crs/crs-setup.conf{.example,}

Activate ModSecurity

After all that, activate the modsecurity on the default site configuration file or on any virtual host configuration file. In this guide, we are using Apache’s default site configuration file.

cp /etc/apache2/sites-available/000-default.conf{,.old}

See our sample default site virtual host configuration with no comments;

cat > /etc/apache2/sites-available/000-default.conf << 'EOL'
<VirtualHost *:80>
	modsecurity on
	modsecurity_rules_file /etc/apache2/modsecurity.d/modsec_rules.conf 
	ServerAdmin [email protected]
	DocumentRoot /var/www/html
	ErrorLog ${APACHE_LOG_DIR}/error.log
	CustomLog ${APACHE_LOG_DIR}/access.log combined

The lines;

	modsecurity on
	modsecurity_rules_file /etc/apache2/modsecurity.d/modsec_rules.conf 

Turns on Modsecurity and specifies the location of the Modsecurity rules.

Check Apache for configuration errors and restart it.

apachectl -t

If there is no error, you should get the output, Syntax OK.

systemctl restart apache2

Testing Modsecurity

Next, test the effectiveness of Modsecurity, for example, command injection. Run the command below;

curl localhost?doc=/bin/ls
<title>403 Forbidden</title>
<p>You don't have permission to access this resource.</p>
<address>Apache/2.4.41 (Debian) Server at localhost Port 80</address>

Check Modsecurity logs;

tail /var/log/modsec_audit.log
ModSecurity: Warning. Matched "Operator `PmFromFile' with parameter `' against variable `ARGS:doc' (Value: `/bin/ls' ) [file "/etc/apache2/modsecurity.d/owasp-crs/rules/REQUEST-932-APPLICATION-ATTACK-RCE.conf"] [line "496"] [id "932160"] [rev ""] [msg "Remote Command Execution: Unix Shell Code Found"] [data "Matched Data: bin/ls found within ARGS:doc: /bin/ls"] [severity "2"] [ver "OWASP_CRS/3.2.0"] [maturity "0"] [accuracy "0"] [tag "application-multi"] [tag "language-shell"] [tag "platform-unix"] [tag "attack-rce"] [tag "paranoia-level/1"] [tag "OWASP_CRS"] [tag "OWASP_CRS/WEB_ATTACK/COMMAND_INJECTION"] [tag "WASCTC/WASC-31"] [tag "OWASP_TOP_10/A1"] [tag "PCI/6.5.2"] [hostname ""] [uri "/"] [unique_id "161428003146.366446"] [ref "o1,6v10,7t:urlDecodeUni,t:cmdLine,t:normalizePath,t:lowercase"]
ModSecurity: Access denied with code 403 (phase 2). Matched "Operator `Ge' with parameter `5' against variable `TX:ANOMALY_SCORE' (Value: `5' ) [file "/etc/apache2/modsecurity.d/owasp-crs/rules/REQUEST-949-BLOCKING-EVALUATION.conf"] [line "80"] [id "949110"] [rev ""] [msg "Inbound Anomaly Score Exceeded (Total Score: 5)"] [data ""] [severity "2"] [ver "OWASP_CRS/3.2.0"] [maturity "0"] [accuracy "0"] [tag "application-multi"] [tag "language-multi"] [tag "platform-multi"] [tag "attack-generic"] [hostname ""] [uri "/"] [unique_id "161428003146.366446"] [ref ""]

Access the site from browser and test command injection attack,

Install LibModsecurity with Apache on Debian 10

Tailing the Apache error logs;

tail -f /var/log/apache2/error.log
[Thu Feb 25 19:29:06.385428 2021] [:error] [pid 38210] [client] ModSecurity: Warning. Matched "Operator `PmFromFile' with parameter `' against variable `ARGS:exec' (Value: `/bin/bash' ) [file "/etc/apache2/modsecurity.d/owasp-crs/rules/REQUEST-932-APPLICATION-ATTACK-RCE.conf"] [line "496"] [id "932160"] [rev ""] [msg "Remote Command Execution: Unix Shell Code Found"] [data "Matched Data: bin/bash found within ARGS:exec: /bin/bash"] [severity "2"] [ver "OWASP_CRS/3.2.0"] [maturity "0"] [accuracy "0"] [tag "application-multi"] [tag "language-shell"] [tag "platform-unix"] [tag "attack-rce"] [tag "paranoia-level/1"] [tag "OWASP_CRS"] [tag "OWASP_CRS/WEB_ATTACK/COMMAND_INJECTION"] [tag "WASCTC/WASC-31"] [tag "OWASP_TOP_10/A1"] [tag "PCI/6.5.2"] [hostname ""] [uri "/"] [unique_id "161428134623.117562"] [ref "o1,8v11,9t:urlDecodeUni,t:cmdLine,t:normalizePath,t:lowercase"]

As a result, this confirms that the Modsecurity is functioning as expected.

That is just about it on our guide on how to install LibModsecurity with Apache on Debian 10.




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