This tweet reveals a critical vulnerability involving a logical flaw in a Web Application Firewall (WAF) system that blindly trusts the 'XMLHttpRequest' header. This blind trust can completely bypass authentication and filtering layers, leading to a Blind SQL Injection (SQLi) attack. The vulnerability arises when the WAF allows requests with this header to pass without proper verification, allowing attackers to exploit the backend databases through unauthorized queries. The tweet's example points to a POST request, suggesting the attack vector requires a crafted POST request with manipulated headers to trigger the bypass. The exact WAF vendor is not mentioned, but the issue demonstrates a critical design flaw in WAFs that trust client-side headers without validation, making them vulnerable to blind SQLi. Security professionals should be wary of relying solely on header values like 'XMLHttpRequest' for critical security decisions and instead implement comprehensive verification mechanisms.