CVE-2026-12949: Wishlist Member X <= 3.34.1 - Unauthenticated Account Takeover via 'mergewith' Parameter

The Wishlist Member plugin for WordPress is vulnerable to Account Takeover via Insufficient Verification of Data Authenticity in versions up to and including 3.34.1. This is due to the wpmregister() function validating the registration cookie only against the GET reg parameter while accepting the POST mergewith and POST wpmid parameters without verifying that the mergewith user ID references a temporary or incomplete registrant that is bound to the current registration transaction. This makes it possible for unauthenticated attackers to take over any existing WordPress account — including administrator accounts — by supplying an arbitrary user's numeric ID as the mergewith value, which causes wpupdateuser() to overwrite the target account's username (additionally written via a direct $wpdb UPDATE), password, email address, first name, and last name with attacker-controlled values, while WordPress password and email change notification emails are explicitly suppressed. When wpmid references a non-existent membership level, no role key is added to the update payload, causing wpupdateuser() to preserve the target user's existing role — including administrator — making full privilege escalation a direct consequence of the takeover.