ZAP has no problems scanning applications running on localhost, however there are a couple of things you need to be aware of. By default ZAP listens on port 8080. If your app also listens on 8080 then you’ll need to change one of them to listen on a different port - it’s probably easier to change ZAP using the Options Local Proxies screen, remember to change your browser’s proxy settings as well: Configuring Proxies.
