There are a few users reporting hacks/spams over the last weeks.
And if I'm not mistaken I think there was a - (one) - "suspicious" post in this (stackideas) forum too !

I want to know if it is a configuration problem or a real threat.

Therefore : I subscribe !

Hello Danny,

This is most likely due to your ACL configuration and it has nothing to do with any "hacking activity". This is what happens commonly on most sites. The "Registered" user group is configured to allow posting and publishing and the site allows anyone to register on the site.

The quickest way to fix this, is to only trust selective user groups by giving them the permissions to publish blog posts. Alternatively, if your site doesn't allow user's to register, disable registration altogether in the User's configuration area.