Changes to Azure Certificates and HPKP
An email landed in my inbox this morning from Microsoft Azure regarding HTTP Public Key Pinning, a subject I have posted about at some length recently. If you don’t know what HPKP is or how it is used, refer back to some of my previous posts on the subject.
A normal HPKP implementation would see you configure your website to pin your own public certificate. Whilst I would advise against it because you have no ownership or control over the certificates, it would be entirely possible to pin the Microsoft Azure Websites certificates using HPKP to your site. The email from Microsoft this morning was an advisory that Microsoft is changing the certificate it uses.
If you are using HPKP and think there is a chance you may have pinned the Microsoft certificates, I would strongly advise you to read the Microsoft Knowledge Base article at https://blogs.technet.microsoft.com/kv/2017/04/20/azure-tls-certificates-changes/?WT.mc_id=azurebg_email_Trans_33716_1407_SSL_Intermediate_Cert_Change for more information.
If you are unsure if you are using HPKP or if you are unsure of which public keys you have pinned, I would suggest you use the Qualys SSL Test site as this will report the certificates in use with HPKP and whether it is enabled.