xss xplained

I realized I never fully comprehended how xss would occur. I knew that it dealt with having to sanitize user inputs... and in my mind it was like, attacker uploads SQL query that is executed... but on their own computer? Why?

I found this link while reading Quasar docs and it explained it well.

# Examples

Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted website for the consumption of other valid users.

The most common example can be found in bulletin-board websites which provide web based mailing list-style functionality.

# Example 1

The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.

<% String eid = request.getParameter("eid"); %>

Employee ID: <%= eid %>

The code in this example operates correctly if eid contains only standard alphanumeric text. If eid has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response.


Initially, this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer?

The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL.

When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers. This mechanism of exploiting vulnerable web applications is known as Reflected XSS.