New Automated Certificate Management System supports more than 100 Custom Domains
Change effective on 13 August 2020
Beginning today, August 13, 2020, and continuing over the next several weeks, we will be rolling out an updated Automated Certificate Management (ACM) system. The change will occur automatically for all apps in both the Common Runtime as well as Private Spaces. There is nothing your need to do for this change to take effect.
If you want to find out if your app has been flagged in you can run:
heroku labs -a <your-app-name> | grep allow-multiple-sni-endpoints
With this new feature each Custom Domain will get its own ACM generated certificate. You are also able to manually upload a separate certificate for each domain.
Automated Certificate Management Dev Center Article provides details on how to configure certificates on your app.