Cronjob via HTTP

Hey there,
I want to run a Cronjob every day, to update one of my mysql tables. I read a lot about it, but there's a problem:

I can run Cronjobs only through a HTTP request or a script.
(site.com/Cronjob/update or /path/to/file.php)

I set up a controller, called Cronjob, to handle my stuff. But I don't want to give access to this controller, so that everyone can execute it.
How would I do this?

I do not have access to cli, so I can not simply check if it is a cli_request..^^

Thanks for helping  

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