Wow, I don't think I would've even considered such an attack...

DNSSEC, HSTS and Certificate Pinning would've made it more difficult to abuse this, but I guess it would've been pretty easy to get valid SSL certificates for all your favourite .io domains.

Let's try to play malicious party here:

Phase A: First set up a simple DNS forwarder playing by the rules and answering requests as we should (as to not get any unwanted attention). Gather usage statistics.

Phase B: Crawl the list of most-used domains to see if there are any valuable targets without HTTPS (port 443 is closed). Alternatively/additionally see if there are API subdomains used by software other than browsers (of which a few won't have annoying features like Cert Pinning - golang's DNS resolver for example afaik doesn't do DNSSEC). Pick some medium to high level targets where the attack might go undetected for at least some time.

Phase C: MitM time! Get certificates for the target domain(s) of your choice and get to work. Start with only a few percent of the requests to not draw too much attention (and to avoid the majority of their traffic coming from a single IP (range) all of a sudden) Obfuscate the attack by acting like a third party app or something simply doing requests for their users.

Congratulations on finding the vulnerability (and thanks for looking for that kinda stuff in the first place).

HSTS requires the site is HTTPS with a valid cert. If you own all .io, you can use LetsEncrypt to get that for free. They now even support Wildcard Certs! :-) That said, you would have to choose your targets carefully and/or load balance your requests to LetsEncrypt. There is a rate limit. There are browser plugins that can tell you if a cert just changed, assuming you have been to that site prior.

Then there is Public Key Pinning. This would be great, but I suspect the number of big companies implementing this are low. I don't have numbers, but you can test your favorite sites in Qualys[1] or using testssl.sh[2] that only depends on openssl and bash.

You could proxy all requests to the real root servers for .io and only become authoritative for the ones you wish to target.

Given the small number of zones, I think a modest server could keep up, or you could balance the load on a bunch of VM's. It may take a while for anyone to notice. I am curious actually, how many fellow geeks have nagios/sensu alerts that would tell them if the root server IP's changed.

All of this said, there are BGP attacks you can do that accomplish the same thing for any TLD and the IP's wouldn't even have to change. Only more advanced monitoring tools that keep an eye on route path might notice, but probably would not alert anyone.

[1] https://www.ssllabs.com/ssltest/index.html [2] https://github.com/drwetter/testssl.sh