I have enabled the Web Application Firewall in the Azure FrontDoor with the default policy with the detection mode. In the logs generated by the WAF, we can see the firewall is marking the reply url set in AAD with action as Block.
I believe the firewall is detecting this as threat. Since the url is required for the working of AD authentication, what can be done to ensure the security ? Or this can be ignored ?
Go to your WAF policy of Front Door WAF policy and click Managed rules. Collapse all and click the related policy and change action to Allow. Then refresh the WAF in front door, it will apply.
You could custom rules for WAF with Azure Front Door and refer to the disable rule in app gateway to fix false positives.
You shouldn't need the reply URL to properly as it's really only required to get the access token. If you're getting the access token and you don't need access to the reply url, this shouldn't be something you need to worry about.
If you would like to get to the reply url for purposes of your application, you could unblock it, but there shouldn't be any security issues assuming you know that the reply url is secure.
Per the docs : https://learn.microsoft.com/en-us/azure/active-directory/develop/reply-url
A redirect URI, or reply URL, is the location that the authorization server will send the user to once the app has been successfully authorized, and granted an authorization code or access token. The code or token is contained in the redirect URI or reply token so it's important that you register the correct location as part of the app registration process.
