cross-site scripting attack (XSS) and how do you prevent it?
Answer
XSS refers to client-side code injection where the attacker injects malicious scripts into a legitimate website or web application. This is often achieved when the application does not validate user input and freely injects dynamic HTML content.
For example, a comment system will be at risk if it does not validate or escape user input. If the comment contains unescaped HTML, the comment can inject a <script>
tag into the website that other users will execute against their knowledge.
The malicious script has access to cookies which are often used to store session tokens. If an attacker can obtain a userโs session cookie, they can impersonate the user.
The script can arbitrarily manipulate the DOM of the page the script is executing in, allowing the attacker to insert pieces of content that appear to be a real part of the website.
The script can use AJAX to send HTTP requests with arbitrary content to arbitrary destinations.
Good to hear
On the client, using
textContent
instead ofinnerHTML
prevents the browser from running the string through the HTML parser which would execute scripts in it.On the server, escaping HTML tags will prevent the browser from parsing the user input as actual HTML and therefore won't execute the script.
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