Once you’re up and running with Sitecore 9, you’re likely going to want to setup Sitecore Rocks. Due to some security changes with Sitecore 9, Rocks will have a problem connecting to the instance.
Upon hitting the “Test” button in the Connection Manager, you’ll be greeted with the following:
If you go into the web.config and apply the following update (Do it in a transform, so it doesn’t make its way up to production), you should be good to go.
<location path="sitecore/shell/WebService"> <system.web> <authorization> <allow users="?,*" /> </authorization> </system.web> </location>
This should allow Rocks to connect to your instance.
Special thanks to Kern Nightingale on the forums for the insight into this!