The Customize WordPress Emails and Alerts WordPress plugin before 1.8.7 does not have authorisation and CSRF check in its bnfw_search_users AJAX action, allowing any authenticated users to call it and query for user e-mail prefixes (finding the first letter, then the second one, then the third one etc.).
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Check my exposure →This product uses data from the NVD API but is not endorsed or certified by the NVD. Informational only; not professional security advice.