XSS Bypass Using Weak Input Filtering Bug

While hunting bugs in a real-world bug bounty program, I discovered a dangerous XSS bypass caused by weak input filtering.
The backend logic was only removing double quotes ("), which is a very common mistake developers make when trying to block XSS attacks.

This kind of protection may look safe at first, but in reality, it’s extremely fragile ❌.

🔍 How the XSS Bypass Worked​

The application tried to sanitize user input by stripping quotes only, assuming that would stop JavaScript execution.
However, modern browsers are very flexible when parsing HTML, which allows attackers to bypass such filters easily 😈.

Here’s the original payload that was sent: <s"vg o"nload=al"ert() />

After the backend removed all quotes, the payload became: <svg onload=alert()>

And just like that… XSS was successfully executed 🎯

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⚠️ Why This Input Filtering Is Dangerous​

Relying on poor input filtering creates a false sense of security. Attackers don’t need quotes to execute JavaScript in HTML contexts.

Common problems with weak filtering:

• ❌ Filtering specific characters instead of contexts
• ❌ Ignoring browser HTML parsing behavior
• ❌ No proper output encoding
• ❌ No Content Security Policy (CSP)

This is why many XSS vulnerabilities still appear in modern applications.

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🛡️ How to Properly Prevent XSS Attacks​

Instead of removing characters manually, always follow secure coding best practices:

✅ Use Context-Aware Output Encoding​

Escape user input based on where it’s used (HTML, JavaScript, URL, etc).

✅ Use Trusted Libraries​

Examples:

• DOMPurify
• OWASP Java Encoder
• Framework built-in escaping (React, Angular, etc)

✅ Apply Content Security Policy (CSP)​

CSP can block inline scripts even if XSS exists.

✅ Never Trust User Input​

Treat all input as untrusted, no matter where it comes from.

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💡 Key Takeaway for Developers & Pentesters​

🚫 Never rely on poor input filtering
✅ Always use proper encoding and security controls
If you’re a developer, weak filters can expose your users.
If you’re a penetration tester or bug bounty hunter, this type of logic flaw is pure gold 🪙.