Cross-Site Scripting (XSS) is one of the most common web vulnerabilities because it exploits something every website relies on: the user’s browser.
When XSS is exploitable, attackers can run malicious JavaScript in a victim’s session. That can lead to account takeover, data exposure, fraud, and brand damage.
What XSS is (simple definition)
XSS happens when a website displays user-controlled input without proper output encoding, allowing scripts to execute in the browser.
Types of XSS you’ll hear about
- Reflected XSS: the payload is reflected immediately in a response (often via a URL parameter).
- Stored XSS: the payload is saved (e.g., in a comment/profile field) and hits every viewer.
- DOM-based XSS: the browser-side JavaScript creates the vulnerability without the server directly returning the payload.
Why XSS is a business problem
- Session hijacking (account takeover)
- Credential theft via fake forms or script injection
- Checkout manipulation (ecommerce)
- Brand damage (defacements, malicious redirects)
- Compliance exposure if user data is accessed or altered
High-signal places XSS appears
- Search pages (q=)
- Contact forms and feedback widgets
- Comment systems / reviews
- Profile fields (name, bio)
- Any feature that renders HTML/markdown unsafely
Practical prevention checklist (what actually works)
- Output encode by context: HTML, attribute, URL, JS context all need the correct encoding.
- Don’t allow raw HTML unless you sanitize with a vetted library.
- Use safe templating and avoid string concatenation into HTML.
- Set cookies safely: HttpOnly, Secure, SameSite where appropriate.
- Add CSP (Content Security Policy) to reduce impact if something slips through.
How to verify exposure quickly
Use a scan to identify obvious XSS paths and prioritize the highest-impact endpoints first.
Run a scan here: https://scanner.skilledscan.com
FAQ
Is CSP a replacement for fixing XSS? No. CSP helps reduce impact, but you still need correct output encoding/sanitization.
What should I fix first? Any XSS that touches authenticated pages, admin areas, or checkout/account flows.