SSL Certificate Warning – Office 365 / Exchange Online and CloudFlare

Symptoms

• Your device connects to your Office 365 / Exchange Online email account
• The device pops up a security or SSL certificate warning complaining of:
  • an invalid name
  • “Cannot Verify Server Identity”
  • “A secure connection could not be established with the server”
  • or equivalent message
• When you view the certificate details, you’re being issued an SSL certificate in the name of CloudFlare (cloudflare-dns.com)
• You now scratch your head and wonder why CloudFlare is issuing certificates when you’re trying to talk to Microsoft Office 365

Cause and Remedy for SSL Certificate Warnings

CloudFlare is nothing to do with Office 365. So the question is, why does CloudFlare return a certificate when you were trying to communicate with Microsoft?

CloudFlare provide DNS services. Without technical detail, this is basically what makes the internet work, in the way most people understand and use it. DNS ensures that all your internet traffic reaches the correct destinations.

CloudFlare offers a service called DNS Proxying. In simple terms, it intercepts all requests to your domain (e.g your website). It then forwards them on to your web server if they look legitimate. So, any malicious attacks involving a denial-of-service for example (a big flood of traffic designed to stop your website from working) do NOT hit your website directly. They are hoovered up by CloudFlare and blocked.

If your nameservers are with CloudFlare (ask your IT people), you should log into CloudFlare and check to see if the autodiscover “A” record is being proxied. If so, CloudFlare is intercepting any requests by your device when it attempts to use this vital Microsoft autodiscover process, and passes it through to Microsoft. CloudFlare acts as a proxy.

This explains why you get a mismatch on the certificate and how CloudFlare is mysteriously getting in the way, even though we’re trying to talk to Microsoft. To be fair, it seems that most of the time it’s seamless and works fine anyway. But if you get repeated password prompts in Outlook or any kind of SSL certificate / security warning about an invalid certificate or mismatched name, this is likely to be the cause.

Since you don’t care if Microsoft receives a DoS attack, as it’s not your problem, it’s fine to turn off proxying for the autodiscover record on its own. This means that when your device attempts to retrieve email, it talks to the autodiscover service. But Cloudflare doesn’t intercept that request and hence you don’t get the SSL warning.