CVE-2026-3568: MStore API <= 4.18.3 - Authenticated (Subscriber+) Insecure Direct Object Reference to Arbitrary User Meta Update

The MStore API plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 4.18.3. This is due to the updateuserprofile() function in controllers/flutter-user.php processing the 'metadata' JSON parameter without any allowlist, blocklist, or validation of meta keys. The function reads raw JSON from php://input (line 1012), decodes it (line 1013), authenticates the user via cookie validation (line 1015), and then directly iterates over the user-supplied metadata array passing arbitrary keys and values to updateusermeta() (line 1080) with no sanitization or restrictions. This makes it possible for authenticated attackers, with Subscriber-level access and above, to modify arbitrary user meta fields on their own accounts, including sensitive fields like wpuserlevel (to escalate to administrator-level legacy checks), plugin-specific authorization flags (e.g., wpufuseractive, aiowpsaccountstatus), and billing/profile fields with unsanitized values (potentially enabling Stored XSS in admin contexts). Note that wpcapabilities cannot be directly exploited this way because it requires a serialized array value, but wpuserlevel (a simple integer) and numerous plugin-specific meta keys are exploitable.