March 5th, 2025
XSS Polyglot: Why It’s So Undetectable (and How to Protect Against It)
Written by
Gurkirt Pal
What is XSS polyglot?
XSS polyglot is a specially crafted payload designed to exploit cross-site scripting (XSS) vulnerabilities in web applications. This kind of payload can be of any structure, it can contain html, css, javascript or sometimes sql queries too. These are designed to bypass security filters.
These payloads often exploit how different parsers interpret inputs, making them highly undetectable.
XSS polyglots can be embedded in various elements, such as links, images, and form inputs.
Here are the few examples:
1.
HTML Attributes with escaped values:
2.
Payload with javascript event handler:
Why Is XSS Polyglot So Undetectable?
Exploiting parser quirks is often difficult to detect because these use non-standard behavior or unexpected parsing logic. This is usually not detected by regular input validation.
Many detection systems fail, because they rely on regular expressions or predefined patterns.
Here are the few reasons why parser quirk exploits can be so undetectable:
1. Different syntax and non-standard patterns:
Attackers often use syntax that is technically valid but uncommon, allowing it to slip through the validation filters.
For example, the following JavaScript syntax is rarely used, making it difficult to flag as malicious:
2. Encoded or unicode-based payloads:
Attacker can send a payload which can be encoded in different formats, such as unicode or hexadecimal, which can go unnoticed from input validators.
When decoded, this converts to:
Since many security filters do not decode input before processing, such payloads can bypass detection.
How to Detect and Prevent XSS Polyglot Attacks?
Preventing XSS polyglots requires strict input validation and sanitization at both the frontend and backend. Here are the best practices:
1.
• Always treat user input as untrusted.
• While filtering only allow specific safe characters rather than trying to block malicious ones which provide overall better security.
2.
3.
Use backend validation libraries like:
• Express Validator (Node.js).
• ESAPI (Java)
• Django Form Validation (Python)
4.
This simple header will prevent attackers from injecting external scripts into your application.
Conclusion
Preventing your application from XSS attacks is the must to have security control, since this can allow the attackers to see user authentication cookies, redirection to the malicious websites, and any other JavaScript related malicious actions. XSS polyglots are the advance payloads, protection against this is usualy not implemented by most of the developers. We highly recommend it to implement this on your applications.
The best way to prevent these attacks is by validating and sanitizing user input, treating all input as untrusted, and using trusted libraries like DOMPurify. Additionally, enforcing Content Security Policy (CSP) and escaping user input before rendering it can further reduce the risk. By following these security measures, developers can effectively defend against XSS polyglot attacks and keep their applications safe.
To get any help in implementing security control in your applications, feel free to contact us.
