Solution:
There’s a number of things that could be causing it. Check all of the following to help remove security holes:
- Are you running the latest version of WordPress? If not, there could be a known vulnerability that is being exploited.
- Are all your plugins up to date? For the same reason as above – a poorly written WordPress plugin can open up security holes.
- Do the WordPress files and folders have the correct CHMOD permissions on the server? If not, you’re asking for trouble.