I can't for the life of me find how to configure client certificate validation with Heroku SSL endpoint. Is is supported at all? Is there an alternative add-on that does support it?
Heroku's knowledgebase says they do not support certificate-based TLS authentication: https://help.heroku.com/3NTWIMNK/does-heroku-support-client-side-ssl-connections-a-k-a-two-way-authentication
So it would appear that server side verification of client certificates isn't supported by heroku right now if you rely upon their web server. I.e. having a server hosted on heroku check the client certificate provided by the connecting client. This requires a check at the web server level.
However I did come across this for nodejs: https://www.npmjs.org/package/client-certificate-auth
which would lead me to believe that for nodejs is should be possible, since you have full access to the http(s) server. I haven't tried it though