Take the dog for walks and bring high quality food rewards (like steak). Give the rewards while at a distance from strangers. The dog will tell you how much distance you should keep between the strangers and the two of you. (i.e. when the dog starts to react, you're too close; the idea is to communicate that strangers are nothing to be scared of. This is done by giving food to the dog in the presence of strangers. Depending on how reactive the dog is, you may have to keep a lot of distance at first. Keep doing this for several sessions, getting closer to strangers over many sessions, at your dog's pace.
If the dog is too reactive to even take food rewards, then you need to go back to confidence building, or try from an even further distance from strangers.
Best of luck.