wpForo Forum plugin: authenticated file deletion can lead to code execution

The wpForo Forum plugin for WordPress is vulnerable to arbitrary file deletion due to insufficient file path validation in the theme_delete function in all versions up to, and including, 3.2.2 This makes it possible for authenticated attackers, with subscriber-level access and above, to delete arbitrary files on the server, which can easily lead to remote code execution when the right file is deleted (such as wp-config.php). This exploit chain is reachable only on sites where an administrator has created a custom wpForo secondary usergroup with a blank WordPress role and the 'mth' (Manage Themes) capability; on such sites, an attacker can self-register and claim that group via the unguarded wpforo_secondary_groupids[] POST field, bypassing the role-based group filter and gaining theme-management access as a Subscriber.