Urgent

Hi, I have received a notice from our WordPress Scan that says:

Unknown file in WordPress core: wp-includes/js/thickbox/css.php

Filename: wp-includes/js/thickbox/css.php

File type: Core

Issue first detected: 9 hours 39 mins ago.

Severity: Warning

Status New

This file is in a WordPress core location but is not distributed with this version of WordPress. This is usually due to it being left over from a previous WordPress update, but it may also have been added by another plugin or a malicious file added by an attacker.

Can you advise me on what I should do?

Many thanks