WordPress Qode Membership plugin: unauthenticated login as any user, including admins

The Qode Membership plugin for WordPress is vulnerable to Authentication Bypass leading to Account Takeover in all versions up to, and including, 2.0.6.2 via the qode_membership_check_facebook_user AJAX action. This is due to a typographic error in the Facebook OAuth token verification guard within the qode_membership_login_user_from_social_network function — the guard condition tests an undefined variable ($check_social_useqoder instead of $check_social_user), causing empty() to always return true and rendering the Graph API token verification branch dead code, leaving wp_set_auth_cookie() reachable with no credential or token validation whatsoever. This makes it possible for unauthenticated attackers to log in as any existing WordPress account — including Administrator-level accounts — by supplying a known or enumerated administrator login name (rather than an email address) in the response[email] parameter, which the fallback path in get_user_by('login', $email) resolves to the matching account. Exploitation does require harvesting a valid qode_validate_facebook_login nonce, but this nonce is rendered on any public front-end page containing the [qode_user_login] or [qode_user_register] shortcode and is therefore obtainable without any prior authentication.