Re: [Acme] Trust and security in DNS challenge validation automation

[Ilari Liusvaara <[e-mail]>]

On Tue, Jan 16, 2018 at 12:12:00PM +0200, Joona Hoikkala wrote:
> 
> While ACME tries to promote automation of the challenge validation, the
> landscape of said automation looks rather grim. DNS server software and
> service providers rarely provide means to limit the privileges of
> credentials used to update DNS zones.
> 
> This leads to users being forced to use and save their credentials often
> equipped with inflated privileges to every machine that needs to acquire
> certificates using DNS challenges, effectively meaning that when one of
> such machines gets compromised so does the whole DNS zone, or multiple
> zones in some cases.

It should be noted that currently compromise of the automation itself
is catastrophic. Even if you do not run the automation as root nor
give it read access to TLS keys, which prevents trivially stealing your
TLS keys. And even if you do not give it unneeded validation authority.

This is because the authority of automation necressarily includes
obtaining certificates for arbitrary keys, so if attacker can not
extract your key, they can just substitute their own.

HPKP could address attacks like this. Unfortunately, it is being
deprecated, with "replacement" being Certificate Transparency.
Unfortunately, CT does not help against attacks like this, because
after the certificate has been issued, it is too late as revocation
does not work.

I earlier had idea of Public Key Pinning with CAA records. It
would be much safer than HPKP (because if keys get lost, they
can be rather quickly changed) and could actually help against the
issue (as CAA is proactive, not reactive like CT). I should post a
draft about it...


-Ilari