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Comment by radiospiel | Hacker News Reader

I am somewhat excited about letsencrypt, but isn't ownership verification via DNS not a bit, hmm, strange? After all, a proper certificate should defend against MITM attacks; so if an attacker would be able to take control over the target's DNS, he could easily create a certificate, which looks legit for all intended purposes. Or do I miss something in that regard?

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I think you're right, however this is no different from how your average domain-verified certificate is issued (where you'd receive a confirmation mail to something like webmaster@example.com or serve a file given to you by the CA on a specific path). If an attacker manages to hijack your DNS, he could also modify your MX/A records and receive that mail/host the file.

You could argue it's even slightly better, since with this solution, DNS is the only attack surface, whereas with email verification a vulnerable mail or web server would be enough. That is, if DNS is their only supported verification mechanism (which I don't think is true.)

Ownership verification is already done via DNS by existing CAs. If an attacker has full control over your DNS, he can already change the DNS records and trick a CA into delivering him a genuine certificate.

This isn't really a MITM attack.

"If an attacker has full control over your DNS"

Shouldn't this be "over the CAs DNS resolver"?

/edit

well I guess if the attacker has control over your dns server, ofcours, but another vector is to spoof as the CAs resolver when the CA does a lookup for your domain.

Control of any name server used by the CA to resolve your hostname would be enough. That could be either the CAs DNS, any resolver in between (e.g. something like Google's Public DNS, which hopefully no real CA is using) or the authoritative name server of the domain. That is, unless you use DNSSEC.

What existing CA's offer this?

The ones i've used (godaddy, startssl, namecheap/ssls.com rapidssl/positivessl) all wanted email verification iirc.

EDIT: of course that's equivalent to verifying the MX. Never mind!

Email verification is worse - not only can it be attacked by modifying DNS, even a passive eavesdropper can generally get enough information to verify successfully.

Gandi offers three validation methods:

* DNS: you need to add a specific record to your DNS zone * File: you need to add a specific file to the root of a HTTP server listening on the domain name * Email: they send an email to admin@ on the domain

All three are pretty much ways to check that you are the person in control of the associated DNS.

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