Oh yeah, it'd be great if Google could MITM half the SSL on the internet...
Let's Encrypt doesn't have any more ways of MITMing people using their certs than any other CA - that is, they _could_ do it by generating rogue certs, but that's no different than what Google can already do since they're a CA as well. Plus, certificate transparency logs should make it visible if they ever do so.
0: Barring weird cases I've seen of some companies letting you generate a cert entirely on their website, letting you download the private key once it's done. Which is bad practice for the reason you're talking about right now, since by then you have no assurance that they haven't kept a copy of that private key for later use.
It's a very valid attack, although minimal. To say they don't have any way of MITM'ing a connection is wrong even if it's unlikely.
Chrome and Safari currently validates that a certificate has been published in the publicly available transparency logs as part of considering it valid.
Either Google doesn't publish the certificate in the logs and it's not valid, or they publish it and people are able to see the misissuance.
It's not foolproof, but it makes the attack even less likely.
I totally agree, it's why I qualified it with "just by virtue of them being the one validating the cert for a certain website" and later on adding that they could do so in other ways, like the one you're suggesting. Reading it again makes me realize that it could be understood that way, though, sorry if I wasn't clear enough. Sometimes not being a native English speaker betrays me a little bit. =)
https://en.wikipedia.org/wiki/DNS_Certification_Authority_Au...
If a CA is incompetent or malevolent it would just ignore CAA records or not check them at all.
It would be a serious bug if a web browser for example went "Hey this site has a cert from Bob's CA but the CAA records for the domain say only Alice's CA is to issue" and rejected the certificate from Bob's CA. The CAA notice is about allowing new issuances right now but maybe last week when I got this certificate from Bob's CA I didn't set that CAA record so that was fine.
It would be valid (maybe not a brilliant idea, but valid) to set CAA to refuse all issuance, changing it only for a few minutes once a week while you do all your certificate changes.
Google also controls a major public DNS resolver (8.8.8.8) and the most popular browser.
But hey, y'know, they're the good guys or whatever so I'm sure it's fine. (Probably. Right up until the moment when it's not.)
Cloudflare (MiTM [0] or not) is on record that 10% (and not 66%) of the all internet traffic now flows through their networks.
How exactly would Google MITM half the SSL on the internet by virtue of issuing certificates via ACME?
The private key never leaves the subject's system (the system hosting a website for example). Google would never have access to the private key for which it would issue the public key certificate.
Further, if Google abuses its power by issuing a fake certificate for another website and uses that to MITM all traffic to that website, all browsers and systems would remove the offending CA certificates from their trust store immediately. Look what happened to DigiNotar.
Not sure how that is relevant. DigiNotar was a trusted root CA in all major browsers. So if an attacker managed to get a fake certificate issued by DigiNotar, they could attack 100% of the users visiting the website for which the fake certificate was issued.
In fact, they did issue fake certificates by accident due to a security breach. As soon as the error was caught, their CA certificates were removed from all browsers. They went bankrupt! That's how serious this business of issuing certificates is.
For this reason alone, having a major browser dev as a CA is not a good idea, regardless of how much or little you trust google.