Hello, is there a reason why you found the hack during update ?
The thing was accidental.Having arrived the notice of automatic update I went to check out the sites and I found three hacked, on three different servers, with the same template. But trying to use as the default template also the header.php was continually overwritten.
Thanks to kindly provided suggestions by Andrew Nevins I set patiently to work. Downloaded locally a copy of one of the sites, and scanned with Kaspersky and other antivirus tool, I could not find malicious code or viruses.
Restoring a backup copy of your website does not solve the problem.
From the reading of the site log files were not unusual access via ftp.
I then tried to disable all the administrator except my changing my password, which was already strong with a extrastrong created on the fly from the admin panel of wordpress site.
For more than 24 hours in any of three sites vine longer overwritten the header.php template and I’m happy, really happy!
I make this relationship that will perhaps help some other user with the same problem.
TANKS wordpress volunteers for the wonderful work!!!