The s2Member plugin for WordPress is vulnerable to privilege escalation via account takeover in all versions up to, and including, 260127. This is due to the plugin not properly validating a user's identity prior to updating their password. This makes it possible for unauthenticated attackers to change arbitrary user's passwords, including administrators, and leverage that to gain access to their account.
https://plugins.trac.wordpress.org/changeset/3461625/s2member#file5