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When submitting a SSL certificate signing request (to a CA) for the public key of

myrandomdomain.com

is it possible to add some target domain, e.g.

security.stackexchange.com

to the "Subject Alternative Name" of the CSR? If this is possible, and I was able to control a network node between visitors of security.stackexchange.com and their browsers, and send my own responses, then does that mean the browser would simply accept my fraudulent communications? I understand certificate pinning could solve this, but other than certificate pinning, is this attack possible?

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    TLDR: NO. You must prove 'ownership' of all domain names (or wildcards) in the cert, both Subject.CN and SAN. What exactly 'ownership' means depends on the CA; for CAs trusted on the Internet and especially the web (like Stack), see CABforum.org Baseline Requirements 3.2 (and Extended Validation requirements 9.2.2 9.2.3 11.7.1). Note public CAs have been issuing SAN for about a decade even though most people paid no attention, so if there were a problem it would have happened years ago. – dave_thompson_085 Jul 7 '17 at 5:10
  • ... In contrast there was a problem with people getting certs for local names and addresses (like 192.168/16) and using them to trick people on other intranets; CAB phased this out over roughly the past two years, and if you search you'll find lots of Qs like 'why can't I get a cert for a private name/addr' and 'what can I do about SSL/TLS to a private name/addr'. – dave_thompson_085 Jul 7 '17 at 5:17
  • @dave_thompson_085. thanks, if you wanted to copy these comments into an answer, I would accept it. It makes sense and restores a little more faith in the PKI. – the_velour_fog Jul 7 '17 at 6:09

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