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.).
[
{
"vendor": "Unknown",
"product": "Customize WordPress Emails and Alerts",
"versions": [
{
"status": "affected",
"versionType": "custom",
"version": "0",
"lessThan": "1.8.7"
}
],
"defaultStatus": "unaffected",
"collectionURL": "https://wordpress.org/plugins"
}
]