updating sharepoint item using REST API from outside of SharePoint

I need to update (CRUD) SharePoint list items from a stand alone application outside of SharePoint. This application can only use JavaScript to call SP. Deploying it as an app on SharePoint won't be possible as the client doesn't have the App framework set up properly. Can anyone please help if it is possible to Update SharePoint list items from a standalone webapp? and how would I go about handling authentication. Would greatly appreciate any help on this.

Thanks