XSS from 0! All you need to know!

My name is Luis Madero (_Y000! _) I am a computer systems engineer, bug hunter and above all a lover of web security, in this writing I would like to touch on the topic of xss injections from a theoretical point of view until I see some examples of injections along the history.

To start we have to answer a question:

What is an xss injection?

According to The Open Web Application Security Project® (OWASP) an xss injection is:

Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.

An attacker can use XSS to send a malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by the browser and used…