Install mod-security for apache2

apt-get install libxml2 libxml2-dev libxml2-utils libaprutil1 libaprutil1-dev
sudo apt-get install libapache2-mod-security2
emacs /etc/modsecurity/modsecurity.conf

and change to following:

grep -i SecRuleEngine  /etc/modsecurity/modsecurity.conf
SecRuleEngine On

Restart apache2:

service apache2 restart