@datakick helped me, to investigate my issue. It's not related to @vsn, I believe. In my case 'ph_simpleblog' module had a severe security hole. I must say, that I haven't updated this module for years. I would expect, that there were fixes as the issue of this module was actually officially known. Btw the dev of this module is now working for prestashop company. I am sure, he wouldn't make this error again, but I it reminded me on a few things:
It can hit everyone (I use only a handful external modules) -> make sure you have uptodate backups.
Update your store (with core updater bleeding edge) and modules regularly
Delete all stuff (especially modules) you don't need.
Make sure, you have a contact person, who can help you in such a situation. Such a finding will lead to a lot of stress. It's very important, to have an expert, who knows, what is going on and whom you can trust. @datakick is the obvious choice for all tb users.