phpIPAM version 1.5.1 contains a vulnerability where an attacker can bypass the IP block mechanism to brute force passwords for users by using the 'X-Forwarded-For' header. The issue lies in the 'get_user_ip()' function in 'class.Common.php' at lines 1044 and 1045, where the presence of the 'X-Forwarded-For' header is checked and used instead of 'REMOTE_ADDR'. This vulnerability allows attackers to perform brute force attacks on user accounts, including the admin account. The issue is fixed in version 1.7.0.