Those who are conscious of security issues may ask,
"Can you guarantee an editor will not steal my work?"
Well, can anyone guarantee that you will not be mugged on your way to the grocery store? Obviously not!
I do not personally know of any editor who has stolen an author's words or idea. I have heard rumors that it has happened, but I have never seen anything even remotely like evidence. Not even a lawsuit.
There is a piece of advice circulating in writing circles that you should send yourself a copy of your manuscript through registered mail. You never open the envelope until you sue the person who steals your work. Then you open it in court in front of a judge as proof that you wrote the material when you said you did. I have never done that. I have more faith in the people I work with.
There is another way to protect yourself.
Hire only a reputable editor.
Check out my credentials here, you will see not only that I join organizations with high ethical standards for their members, but also that I am trusted with positions of leadership within those organizations.
Security is an important of the editing services I offer. Check out the home page for others.