AST SQL Bypass in Modern WAFs

🔍 Understanding AST-Based WAF Filters

Modern Web Application Firewalls (WAFs) go beyond regex checks. They parse incoming payloads into an Abstract Syntax Tree (AST) to match the structure of known SQL injections.

       WAF Input: SELECT * FROM users WHERE id = 1 UNION SELECT null, null
                                    │
                                    ▼
                          ┌──────────────────┐
                          │   AST Parser     │
                          └────────┬─────────┘
                                   │
                    ┌──────────────┴──────────────┐
                    ▼                             ▼
       [SELECT Statement]                 [UNION SELECT Node]
     (Parsed structure matched)        (Identified as SQL Injection!)

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💡 The Evasion Vectors

In this article, I demonstrate how stateful AST tokenizers fail when payloads use nested subqueries and character set escapes. By rewriting SQL queries to manipulate token nodes, we force the WAF parser to misclassify malicious payloads as harmless tokens, while they resolve perfectly in the database engine.