Big Issue on multiple WP websites

For anyone whose site has been hacked, please see these:

https://codex.www.ads-software.com/FAQ_My_site_was_hacked
https://www.ads-software.com/support/topic/268083#post-1065779
https://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
https://ottopress.com/2009/hacked-wordpress-backdoors/

Additional Resources:
https://sitecheck.sucuri.net/scanner/
https://www.unmaskparasites.com/
https://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html

Note that if you do not completely clean up and secure the site, the hack will very likely keep happening. Also talk to your hosting company as a hack can result from a server that has been hacked.

Also note that using an outdated version of WP is a major security RISK – updating WP does not cause hacks.

See also:

https://codex.www.ads-software.com/Hardening_WordPress