Only Allow Post Author To Edit Own Post

I have a bit of a security issue I'm struggling to over come.

I allow users to edit the posts they create using a link with the following query string
"/?rwmb_frontend_field_post_id={{ post.ID }}"

This works great and populates the form with the content they want to edit.

HOWEVER...

You can just guess POSTID's in the url and edit other peoples entries too.

How do I lock this down so users can only edit what they have created?

Thanks everyone!