Theme support / old version

Hi

I have inherited a site that appears to be using an old version of your Enfold theme (the site is here https://www.petsabroaduk.co.uk/). The problem I am facing is that there must be some sort of vulnerability somewhere in this version of the theme as it keeps getting injected with a small malware link near the footer. I have disabled all plugins and the issue remains, only changing the theme makes it go away – hence why I am sure it must be the theme that is the issue. This issue has resulted in Google pulling the clients AdWords campaign until it is fixed – we can rollback to earlier backups but the site is reinfected again within days.

The malware injects “https://www.9iwp.org/jquery.js” into the footer, but not directly as we cannot find any trace of this when checking the individual files or database.

My question is – are you aware of a vulnerability like this in older versions and if so – if we purchased the updated version of this theme and replace the core files is that likely to fix it?

Thank you in advance.