According to the CHFI v11 Web Application Forensics and Network & Web Attacks module , attackers commonly use encoding and obfuscation techniques to bypass input validation mechanisms, web application firewalls (WAFs), and intrusion detection systems. One such advanced technique is double URL encoding , which involves encoding already URL-encoded characters a second time.
In URL encoding, the forward slash / is represented as %2F. When this value is encoded again, % becomes %
25, resulting in %252F. In Option A , multiple occurrences of %252f clearly indicate that characters such as / and comment markers (/* */) have been double encoded . When processed by the web server or application, the input may be decoded twice, ultimately reconstructing a valid SQL injection payload like UNION SELECT, thereby bypassing security filters.
Options B and C rely on case manipulation and keyword splitting , which are evasion techniques but not double encoding . Option D uses hex-encoded control characters , which is a different obfuscation method and does not represent double URL encoding.
CHFI v11 explicitly highlights double encoding as a common technique used in SQL injection attacks to evade detection and filtering mechanisms. Therefore, the URL that clearly demonstrates double-encoded SQL injection payloads is Option A , making it the correct and CHFI-aligned answer.