The SQL injection vulnerability in the Hide My WP WordPress plugin (versions <= 6.2.3) is possible because of how the IP address is retrieved and used inside a SQL query. The function "hmwp_get_user_ip" tries to retrieve the IP address from multiple headers, including IP address headers that the user can spoof, such as "X-Forwarded-For." As a result, the malicious payload supplied in one of these IP address headers will be directly inserted into the SQL query, making SQL injection possible.