There are a few reasons Azure App Services (Azure Web Apps, Azure Function Apps etc…) may get a 403 response. Other than the code itself returning this, Access Restrictions or IP Restrictions could be at play (and you can mitigate with this knowledge).
The site has IP Restrictions in place
This will result in a message like:
Error 403 – Forbidden
The web app you have attempted to reach has blocked your access.
Using the f12 tools in the browser and inspecting the network traffic it will say: Status Code: 403 Ip Forbidden
A possible way to resolve this is to see what ‘Access Restrictions’ are configured:
and what has been denied and allow that denied range or IP. You can see the rejected IP addresses in the ‘Diagnose and Solve Problems’ blade of you Azure App Service:
Also consider the source. So for example if you expect a different IP address to hit your web app, you may need help from your internal networking folks. Is it going through a VPN Gateway? Try “whatismyip” type tools to discover the IP if the problem client.