Malicious code injection into header.php file

Hi All,

It appears that my header.php files were once again infected with malicious code pointing to some dodgy website in Russia. I also found a “blog73.php” file in my twentytwelve directory (I use a child theme) that didn’t come with the original set of files.

I removed all the “new” plugins that I installed around the time of the first attack but it has happened again since.

Does anyone know how I can prevent this from happening in the future?

Many thanks,
Cristina