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XSS injections

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CrossSiteScripting injections, also called XSS injections are one of the most well-known and easy ways to inject some code on a website. When you are making a form taking a username, the user can also write JavaScript inside the field. Once submitted, nothing happens, but once someone will go on the page where the username is displayed, then some script may be executed.

Setting your Content-Security-Policy (CSP) header will usually prevent most of the scripts from running, as you should have only allowed trusted websites to do cross-site requests.

Stored XSS

The easiest example is to test with this username

<!-- use this as your username -->
<!-- or maybe add some tag after your username -->
<span onload="alert('virus')">Calistro</span>


  • in a chat, you may use strip_tags (PHP)
  • if you want to escape the HTML
    • use htmlentities (ex: echo htmlentities($username))
    • do not use htmlspecialchars as this is escaping everything (and for French people, this means that you will have problems with special characters not being properly rendered because they are escaped twice)

You need to serialize the data, either before reading it, or before storing it and reading it.

Reflected XSS

Sometimes, you also got JavaScript inside URLs. Let's say someone hidden a link inside another/or simply the link was too long so you didn't check, you may have clicked on this

# of course this URL won't work

To summarize, a hacker is sending you a link to a website with a vulnerability with some code inside the link to exploit the vulnerability and hack you.