Ensure the web app has 'Client Certificates (Incoming client certificates)' set to 'On'

Description:

Client certificates allow for the app to request a certificate for incoming requests. Only clients that have a valid certificate will be able to reach the app.

Rationale:

The TLS mutual authentication technique in enterprise environments ensures the authenticity of clients to the server. If incoming client certificates are enabled, then only an authenticated client who has valid certificates can access the app.

Utilizing and maintaining client certificates will require additional work to obtain and manage replacement and key rotation.

From Azure Portal

1. Login to Azure Portal using https://portal.azure.com
2. Go to 'App Services'
3. Click on each App
4. Under the Settings section, Click on 'Configuration', then 'General settings'
5. Set the option 'Client certificate mode' located under Incoming client certificates to 'Require'

From Azure CLI

To set Incoming client certificates value for an existing app, run the following command:

az webapp update --resource-group --name --set clientCertEnabled=true