Does ArcGIS Online support UNC paths as hyperlinks?

Answer

No, ArcGIS Online does not support Universal Naming Convention (UNC) paths as hyperlinks. It is not possible to hyperlink to UNC paths in an ArcGIS Online pop-up window.

Note:
As an alternative, use URLs (http://) in the ArcGIS Online pop-up window as hyperlinks. There are two ways to configure URLs in an ArcGIS Online pop-up window:

•  Create a folder in the 'wwwroot' folder, and configure Directory Browsing with Internet Information Services (IIS) Manager. Refer to the solution provided in the following article, Problem: Unable to access hyperlinks in ArcGIS Viewer for Silverlight application pop-up, for more information.
•  Use Custom Attribute Display when configuring pop-ups. Refer to ArcGIS Blog: Pop-ups: text element essentials .