WAF - Web application firewall
One of the first things we should do when starting to poke on a website is see what WAF it has.
Identify the WAF
wafw00f http://example.com
http://securityidiots.com/Web-Pentest/WAF-Bypass/waf-bypass-guide-part-1.html
https://medium.com/secjuice/waf-evasion-techniques-718026d693d8
https://medium.com/secjuice/web-application-firewall-waf-evasion-techniques-2-125995f3e7b0