Install and use ModSecurity

ModSecurity (mod_security) is an open-source IDS and prevention engine, originally developed for Apache, and now also available for Nginx (and other platforms) in a “standalone” version.

ModSecurity works as a supplemental firewall for the web server, allowing you to monitor traffic in real-time, and disabling host connections if the module suspects potential brute-force password attacks.


To install it without (re)compiling Nginx, install the dependencies:

# apt-get install libxml2 libxml2-dev libxml2-utils libaprutil1 libaprutil1-dev

And download, compile and install mod_security:

# git clone mod_security
# cd mod_security
~/mod_security# ./
~/mod_security# ./configure --enable-standalone-module
~/mod_security# make

To compile Nginx from source with the modsecurity module (check for latest version here:

# wget
# tar -xvpzf nginx-1.9.5.tar.gz
# cd nginx-1.9.5
~/nginx-1.9.5# ./configure --add-module=../mod_security/nginx/modsecurity
~/nginx-1.9.5# make
~/nginx-1.9.5# make install

The ModSecurity configuration file must be defined in the nginx.conf file, something like this:

server {
   listen       80;
   server_name  localhost;

   location / {
      ModSecurityEnabled on;
      ModSecurityConfig modsecurity.conf;


For custom rules applied to different directories, create new mod_security.conf files, for example:

location /secured {
   ModSecurityConfig modsecurity3.conf; 
   proxy_read_timeout 180s;

To turn it off for a particular directory:

location /unsecured/ {
   ModSecurityEnabled off;
   proxy_read_timeout 180s;

Restart Nginx.


Compile and embed the ModSecurity module or install from repository:

# apt-get install libapache2-modsecurity

Check it was loaded:

# apachectl -M | grep --color security

Rename the recommended-labeled configuration file:

# mv /etc/modsecurity/modsecurity.conf-recommended /etc/modsecurity/modsecurity.conf

Restart Apache.


A new logfile named /var/log/apache2/modsec_audit.log has been created. Check that the default configuration file is not set to DetectionOnly, which logs requests according to rule matches and doesn’t block anything. Edit the /etc/modsecurity/modsecurity.conf file to change it if need be (set it to On). Some possible changes:

# Prevent path traversal (..) attacks
SecFilter "../"

# Weaker XSS protection but allows common HTML tags
SecFilter "<[[:space:]]*script"

# Prevent XSS atacks (HTML/Javascript injection)
SecFilter "<(.|n)+>"

# Very crude filters to prevent SQL injection attacks

SecFilter "delete[[:space:]]+from"
SecFilter "insert[[:space:]]+into"
SecFilter "select.+from"
SecFilter "drop[[:space:]]table"

# Protecting from XSS attacks through the PHP session cookie
SecFilterSelective ARG_PHPSESSID "!^[0-9a-z]*$"
SecFilterSelective COOKIE_PHPSESSID "!^[0-9a-z]*$"

Restart Apache.

Configuration resources

Have a look at the OWASP ModSecurity Core Rule Set Project (CRS) for making more secure changes. The CRS aims to protect web applications from a wide range of attacks, with a minimum of false alerts.