The vulnerability centers on missing capability checks when removing calendar subscriptions. The calendar_delete_subscription function would be the primary entry point for subscription deletion operations. The CWE-639 (User-Controlled Key) indicates the function accepted user-provided subscription IDs without verifying ownership or proper permissions (CWE-276). Moodle's security model typically requires capability checks like 'require_capability' before sensitive operations, which appears to have been missing here based on the vulnerability description of insufficient authorization checks.