Hello I love you won't you tell me your name?

I am not sure what I can do with Cassy to make her realize that there are such things as strangers. That child has no fear of anyone at all. Every person is her best friend and she is not afraid to let them know. She will run up to complete strangers and wrap her arms around them and tell them "You are my best friend" I have tried explaining that she can not do that, it does not matter what I say she just can not grasp it. I have so many fears of her running off with a stranger. She is quick in the blink of an eye boom she can be gone just like that, I do my best to keep her right with me hold her hand, but sometime it just takes a second. I can not put her in the stroller or carriage because that causes an extremely bad melt down and I am not sure how I feel about "leashes" for children. I guess I just have be more careful and not take her out into public places as much =/