Hi,
I'm afraid nuBuilder does not have a REST API. You could write it yourself, ask someone to do it, or use an existing framework.
Attached is an example of a simple web service, which returns user data as JSON or XML.
Unzip the files and place the webservice folder in nuBuilder's root directory.
webservice.zip
Of course, you could extend this and add password authentication.
Screenshot of the web service portal:
webserivce.png
Result:
webservice_xml.png
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