What is Cross-Site Scripting (XSS)?

Definition: Cross-site scripting, or XSS for short, is one of the most common security flaws in web apps. In an XSS attack, attackers target websites. They slip harmful scripts into content that other users view. These scripts can be written in many languages, but most often in JavaScript.

Types of cross-site scripting

There are several types of cross-site scripting. Each one works in a different way:

Reflected XSS

In reflected XSS, user data goes straight into the HTTP response. It is not cached or stored. Attackers often spread this type of attack through harmful links.

Stored XSS

Stored XSS is also called persistent XSS. Here, the harmful data is stored on the server. The scripts reach many users each time someone opens the affected page.

DOM-based XSS

In DOM-based XSS, the web app's own client-side code lets harmful scripts run in the browser. The server does not have to be involved at all.

Risks of cross-site scripting

XSS poses serious risks. Attackers can get hold of user data such as cookies, session IDs and other sensitive information. They can also:

steal login data
add harmful code
send users to phishing websites

Protective measures against cross-site scripting

To protect your app against XSS attacks, take these steps:

Validate and sanitize input: Check and clean all user input before it goes into the app.
Use HttpOnly and Secure flags: Set the HttpOnly flag on cookies so harmful scripts cannot read them. Set the Secure flag so cookies are only sent over HTTPS.
Implement a Content Security Policy (CSP): A CSP controls which resources a page may load or run.

Have your systems tested for XSS vulnerabilities

Test your security on a regular basis. This is the only way to find and fix XSS flaws in time.

Best practices for protecting against cross-site scripting

Good protection against XSS starts during development. It combines several layers:

Context-aware output encoding: Encode output to suit where it is used (HTML, attribute, JavaScript, URL) before the browser shows it.
Use secure frameworks: Use template engines and frameworks that escape output on their own. Avoid unsafe functions such as innerHTML or eval.
Content Security Policy: Add a strict CSP as an extra layer. It blocks inline scripts and unknown sources.
Testing and code reviews: Run security tests (SAST/DAST) and penetration tests on a regular basis. Train your developers in secure coding.

Common challenges

Many output contexts: The same data must be encoded in different ways, based on where it is used.
Legacy apps and third-party scripts: Old code and embedded outside scripts make it harder to bring in a strict CSP.
Client-side logic: DOM-based XSS happens in the browser. Server-side filters do not catch it.

Measuring success and KPIs

number of XSS flaws found and fixed
time to fix critical flaws
share of apps with an active Content Security Policy

Your next step

Have your web apps checked for XSS flaws. Our experts help you rate the risks and put strong defenses in place.

Get started today:

📞 Free consultation: Book a talk with no strings attached
📋 Security assessment: Have your current security posture rated
🎯 Penetration test: Have your web apps tested for XSS and other flaws
🚀 Implementation: Get expert help to fix and harden your systems