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DNS-Persist-01: A New Model for DNS-Based Challenge Validation

letsencrypt.org

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Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#71

Post by cyberax »

tptacek wrote: post #64
cyberax wrote: post #60

Earlier quoted context omitted.

When you shoot yourself in the foot with DNSSEC, you typically end up with a non-working setup. The biggest problem is that DNS replies are often cached, so fixes for the mistakes can take a while to propagate. With Let's Encrypt you typically can fix stuff right away if something fails.

When you shoot yourself in the foot with DNSSEC, your entire domain falls of the Internet, as if it had never existed in the first place. It's basically the worst possible case failure and it's happened to multiple large shops; Slack being the most notorious recent example.

Yes, and it'd be great if DNSSEC added an "advisory" signature level. So it can be deployed without doing a leap of faith.

But let's not pretend that WebPKI is perfect. More than one large service failed at some point because of a forgotten TLS certificate renewal. And more than one service was pwned because a signing key leaked. Or a wildcard certificate turned out to be more wildcard than expected.

I understand the failures of DNSSEC and DNS in general. And we need to do something about it because it's really showing signs of its age as we continue to pile on functionality onto it.

I don't have an idea for a good solution for everything, but I just can't imagine us piling EVERYTHING onto WebPKI either.

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#72

Post by IgorPartola »

Am I just stupidly missing something or does this in theory allow anyone who controls a DNS server for my domain or anyone who controls traffic between LE and the DNS server for my domain to get a TLS certificate they can use to impersonate my domain?

I suppose the same is true for DNS-01 but this would make it even easier because the attacker can just put up their LE account instead of mine into the DNS response and get a certificate.

At this point why not just put my public cert into a DNS record and be done with it?

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#73

Post by bombcar »

IgorPartola wrote: post #72

Am I just stupidly missing something or does this in theory allow anyone who controls a DNS server for my domain or anyone who controls traffic between LE and the DNS server for my domain to get a TLS certificate they can use to impersonate my domain? I suppose the same is true for DNS-01 but this would make it even easier because the attacker can just put up their LE account instead of mine into the DNS response and…

Yes, anyone who controls your DNS can get a TLS certificate from anyone who offers them - because, uh, they control your DNS!

Try to figure out a way to block me from getting a TLS certificate if I can modify your DNS.

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#74

Post by radiator »

amluto wrote: post #65
9dev wrote: post #57

Earlier quoted context omitted.

For the love of god, switch to a DNS provider with an API. Whatever legacy behemoth you’re working with doesn’t justify a gap this wide.

Name one that doesn’t have an AWS-style per-query cost. (There might well be a nice one, but I haven’t found it yet.)

Hetzner DNS

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#75

Post by bombcar »

newsoftheday wrote: post #26
jsheard wrote: post #23

Earlier quoted context omitted.

And to elaborate, the reasons you might want to use a DNS challenge are to acquire wildcard certificates, or to acquire regular certificates on a machine or domain which isn't directly internet-facing. If neither of those apply to you then the regular HTTP/TLS methods are fine.

OK I was sort of thinking that might be the case but wanted to make sure in case I had to start prepping now, thanks. We use no wildcard domains today, maybe down the road.

Wildcard domains are a great way to get certs for all your "internal systems" with only having to expose one (or a bit of one on DNS) to the Internet at large.

This is going to greatly simplify some of my scripts.

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#76

Post by toast0 »

amluto wrote: post #65
9dev wrote: post #57

Earlier quoted context omitted.

For the love of god, switch to a DNS provider with an API. Whatever legacy behemoth you’re working with doesn’t justify a gap this wide.

Name one that doesn’t have an AWS-style per-query cost. (There might well be a nice one, but I haven’t found it yet.)

If it's for a business, I would contact them to see if they have a commercial offering, but I think the Hurricane Electric Free DNS might actually fit.

https://dns.he.net/

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#77

Post by bombcar »

csense wrote: post #21

To get a Let's Encrypt wildcard cert, I ended up running my own DNS server with dnsmasq and delegating the _acme-challenge subdomain to it. Pasting a challenge string once and letting its continued presence prove continued ownership of a domain is a great step forward. But I agree with others that there is absolutely no reason to expose account numbers; it should be a random ID associated with the account in Let's En…

Your account ID is exposed in the certificate generated; what's the real difference?

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#78

Post by gurjeet »

IgorPartola wrote: post #72

Am I just stupidly missing something or does this in theory allow anyone who controls a DNS server for my domain or anyone who controls traffic between LE and the DNS server for my domain to get a TLS certificate they can use to impersonate my domain? I suppose the same is true for DNS-01 but this would make it even easier because the attacker can just put up their LE account instead of mine into the DNS response and…

If you don't trust your DNS provider to _not_ do malicious acts against you, you shouldn't be in that relationship.

If someone can perform MITM attack between LetsEncrypt and a DNS server, we've got bigger problem than just certificate issuance.

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#79

Post by echoangle »

IgorPartola wrote: post #72

Am I just stupidly missing something or does this in theory allow anyone who controls a DNS server for my domain or anyone who controls traffic between LE and the DNS server for my domain to get a TLS certificate they can use to impersonate my domain? I suppose the same is true for DNS-01 but this would make it even easier because the attacker can just put up their LE account instead of mine into the DNS response and…

If I control your DNS, I can also just do the HTTP Acme challenge. If you control the DNS, it’s basically your domain anyways.

Re: DNS-Persist-01: A New Model for DNS-Based Challenge Validation

#80

Post by tialaramex »

inahga wrote: post #62
bob1029 wrote: post #37

I've changed my mind about the short lived cert stuff after seeing what is enabled by IP address certificates with the HTTP-01 verification method. I don't even bother writing the cert to disk anymore. There is a background thread that checks to see if the current instance of the cert is null or older than 24h. The cert selector on aspnetcore just looks at this reference and blocks until its not null. Being able to d…

You should persist certs somewhere. Otherwise your availability is heavily tied to LE’s uptime.

Technically, because Let's Encrypt always publishes all requested certificates to the logs (this isn't mandatory, it's just easier for most people so Let's Encrypt always does this) your tool can go look in the logs to get the certificate. You do need to know your private key, nobody else ever knew that so if you don't have that then you're done.
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