Azure CDN: Intermittent Availability Issues — Ongoing Investigation

Problem description

I am experiencing intermittent availability issues with my Azure CDN profile (Front Door Standard/Premium) that started around August 31, 2026. Some users report that the web application is unreachable, while others can access it successfully. A Resource Health event on the same day indicated potential service problems. I am seeking clarification on the cause of these issues and how to resolve them.

Environment

Azure CDN profile on a Front Door Standard/Premium SKU, deployed in West Europe region; origin group configured for a Linux-based web application; origin host: Linux web app endpoint in West Europe; no custom hostnames or IP-based origins noted.

What I've already tried

I have checked Azure Resource Health and observed a resource event indicating potential CDN issues. I have opened a support case providing details such as origin group name, origin host FQDN, and outage timing. No manual diagnostic logs or metrics were attached. Automated diagnostics suggested a Domain Fronting issue, with errors like HTTP 421 (SSLMismatchedSNI). I have also reviewed DNS and certificate configurations.

Current status

I am seeking assistance to understand the root cause of these intermittent issues, particularly whether they are related to recent platform changes, certificate configurations, or other infrastructure factors. I want to know what Microsoft has observed and what steps can be taken to prevent future occurrences.