How many different ways XSS attacks can occur?

Answer:

XSS attacks can be categorized into following three categories:

1) Stored: The attack is saved into a persistent storage medium such as a database or file.

2) Reflected: Non-persistent attacks needing a delivery method to initiate the attack such an email or malicious form.

3) DOM based XSS: This attack manipulates the DOM to execute the attack payload. It is less well known.