Bypassing Flawed WAF Regex with JavaScript BigInt: A Cybersecurity Deep Dive

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Introduction

Web Application Firewalls (WAFs) often rely on regex patterns to block malicious payloads, but JavaScript’s quirks—like BigInt notation—can bypass flawed filters. Gareth Heyes’ recent post demonstrates how `0o1337n` (octal BigInt) can evade detection, highlighting the need for robust WAF rule design.

Learning Objectives

  • Understand how JavaScript BigInt notation can bypass WAF regex.
  • Learn defensive coding practices to prevent such bypasses.
  • Explore real-world exploitation and mitigation techniques.

1. How JavaScript BigInt Evades WAF Regex

Payload Example:

throw-0o1337n,x=onerror=alert,1

Step-by-Step Explanation:

1. `0o1337n` – An octal BigInt literal, often missed by regex-based WAFs.
2. `x=onerror=alert` – Assigns `alert` to onerror, triggering XSS when an error occurs.

3. `throw` – Forces an error, executing `onerror`.

Mitigation:

  • Normalize input before regex checks.
  • Use strict content security policies (CSP).
    1. Testing WAF Bypasses with Alternative Number Notations

Hex & Binary BigInt Examples:

throw-0xdeadn; // Hex BigInt 
throw-0b1010n; // Binary BigInt

Why This Works:

  • Many WAFs fail to parse non-decimal number formats.
  • Combine with obfuscation (e.g., eval('ale' + 'rt')) for further evasion.

Defense:

  • Implement syntax-aware parsing (e.g., AST-based analysis).
  • Block dynamic `onerror` assignment.

3. Exploiting Implicit Error Handling

Malicious Script:

x=onerror=eval;throw"alert(1)"

Breakdown:

1. Assigns `eval` to `onerror`.

2. `throw` passes the string `”alert(1)”` to `eval`.

Mitigation:

  • Disallow arbitrary `onerror` overwrites.
  • Use `’unsafe-eval’` CSP directives cautiously.

4. Real-World WAF Weaknesses

Example Regex Flaw:

/alert(.?)/gi

Bypass Methods:

  • Unicode Escape: `\u0061lert(1)`
  • Template Literals: alert`1`
  • Concatenation: `a=’ale’;b=’rt’;window[a+b](1)`

Solution:

  • Use multi-layered validation (e.g., lexical analysis + behavioral checks).

5. Hardening WAF Rules Against JS Quirks

Improved Regex:

/(?:alert|eval|onerror)\s(|[0-9boxn]+n/gi

Key Improvements:

  • Catches BigInt (n suffix).
  • Blocks common JS attack vectors.

Additional Measures:

  • Rate-limiting unusual syntax (e.g., excessive `throw` statements).
  • Sandboxing untrusted JS execution.

What Undercode Say:

  • Key Takeaway 1: JavaScript’s flexibility is a double-edged sword—enabling both innovation and exploitation.
  • Key Takeaway 2: Regex-only WAF rules are insufficient; context-aware filtering is critical.

Analysis:

Heyes’ finding underscores the cat-and-mouse game between attackers and defenders. As JavaScript evolves, WAFs must adopt deeper semantic analysis to keep pace. Future-proofing requires:
– Machine learning for anomaly detection.
– Browser-level security enhancements (e.g., Trusted Types).

Prediction:

As attackers refine JS obfuscation, WAFs will shift toward runtime behavior monitoring. Meanwhile, zero-day exploits leveraging numeric notations (BigInt, hex, etc.) will rise, pushing enterprises to adopt stricter CSP and sandboxing.

Final Thought:

Security teams must treat WAFs as one layer—not the only layer—of defense. Proactive code reviews and adversarial testing remain essential.

Ready to test your WAF? Try these payloads in a controlled environment and share your findings! 🚀

IT/Security Reporter URL:

Reported By: Gareth Heyes – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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