How dangerous is XSS security?

XSS (Cross-Site Scripting) is a security bug that allows hackers to embed malicious code (javascript) on another website.

This is one of the most common security flaws on Web sites. Systems from large to small such as Facebook, Twitter, … have been subjected to this error. Due to its popularity and danger, XSS is always honored to be in the top 10 most dangerous security flaws on OWASP (Open Web Application…