XSS Payload WAF Bypass: Advanced Techniques to Evade Microsoft’s 2025 Security

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Introduction:

Cross-Site Scripting (XSS) remains a critical web vulnerability, and modern Web Application Firewalls (WAFs) are evolving to detect and block traditional payloads. However, attackers continuously refine techniques to bypass these protections. This article dissects an advanced XSS payload designed to evade Microsoft’s 2025 WAF by leveraging double-encoding, indirect function calls, and DOM manipulation.

Learning Objectives:

  • Understand how double-encoded HTML entities bypass single-layer WAF decoding.
  • Learn to use array dereferencing and indirect property access to evade signature-based detection.
  • Explore DOM-based triggers (e.g., onchange) as alternatives to monitored events like onclick.

1. Double-Encoded HTML Entities for WAF Evasion

Payload Snippet:

&%2362;

Step-by-Step Explanation:

  1. Encoding Layers: `&%2362;` is a double-encoded HTML entity:

– First layer: `%23` decodes to “, resulting in &62;.
– Second layer: `&62;` decodes to >, the closing angle bracket.
2. WAF Bypass: Many WAFs decode only once, seeing `>` (harmless) instead of >.
3. Usage: Embed in payloads where angle brackets are filtered (e.g., `