CEH teaches that unrestricted file upload vulnerabilities are among the most dangerous in web applications because they allow attackers to bypass extension checks and upload malicious executable files. When the server fails to validate MIME types, file extensions, or execution permissions, an attacker can upload a web shell disguised as a harmless file, such as "image.php.jpg," which may pass superficial validation and still be executed by the server's interpreter. Once executed, the shell provides the attacker with command execution capabilities, allowing full control over the system. CEH emphasizes that web shells can enable privilege escalation, database compromise, lateral movement, or full server takeover. Unlike SQL injection or XSS, file upload exploitation directly affects server-side execution, making it significantly more severe. Unrestricted upload flaws are commonly tested in CEH labs with tools like Burp Suite to alter content-type headers or bypass client-side filters. This is a high-impact vulnerability requiring strict validation and sandboxing controls.