PHP code injected onto website
I manage a website for a non-profit (volunteer work).  They have been shut down multiple times by their hosting company because of malicious software.  The attack replaces common WordPress files with a new version containing Base64 code that "includes" another file on the system that is much more thoroughly encrypted (base64 plus a key that is pulled from a cookie).  When I look at the web server log files, I see things like: - - [29/Dec/2017:22:37:39 -0600] "POST /wp-content/plugins/meta-data-filter/languages/xjwtfhlc.php HTTP/1.0" 200 69606 "" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:48.0) Gecko/20100101 Firefox/48.0"

How are they able to do this?  Or better... how do I stop this?
Update your wordpress installation, update and uninstall any uneeded plugins, and disable any file upload pages that don't require authentication. Aditionally you might want to take a look at your user credentials and change the login information for any administrative accounts.

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