it's a volunarability

Cross-site Scripting,

This also known as XSS, is a way of bypassing the SOP concept in a vulnerable web application. Whenever HTML code is generated dynamically, and the user input is not sanitized and is reflected on the page an attacker could insert his own HTML code. The web browser will still show the user’s code since it pertains to the website where it is injected.

In such case an attacker can easily insert JavaScript code which would run under the site’s context. By doing so the attacker is able to access other pages on the same domain and can read data like CSRF-Tokens or the set cookies.

If the cookies, which typically contain session identifier information, can be read by the client-side JavaScript code, the attacker can use them on his own browser and login to the web application as the victim. If that does not work the attacker can still read private information from the pages, such as read CSRF tokens and make requests on behalf of the user.

Types of XSS

Stored XSS − Stored XSS also known as persistent XSS occurs when user input is stored on the target server such as database/message forum/comment field etc. Then the victim is able to retrieve the stored data from the web application.

Reflected XSS − Reflected XSS also known as non-persistent XSS occurs when user input is immediately returned by a web application in an error message/search result or the input provided by the user as part of the request and without permanently storing the user provided data.

DOM Based XSS − DOM Based XSS is a form of XSS when the source of the data is in the DOM, the sink is also in the DOM, and the data flow never leaves the browser.