This change adds a feature flag, TLSSNIRevalidation. When it is enabled, Boulder
will create new authorization objects with TLS-SNI challenges if the requesting
account has issued a certificate with the relevant domain name, and was the most
recent account to do so*. This setting overrides the configured list of
challenges in the PolicyAuthority, so even if TLS-SNI is disabled in general, it
will be enabled for revalidation.
Note that this interacts with EnforceChallengeDisable. Because
EnforceChallengeDisable causes additional checked at validation time and at
issuance time, we need to update those two places as well. We'll send a
follow-up PR with that.
*We chose to make this work only for the most recent account to issue, even if
there were overlapping certificates, because it significantly simplifies the
database access patterns and should work for 95+% of cases.
Note that this change will let an account revalidate and reissue for a domain
even if the previous issuance on that account used http-01 or dns-01. This also
simplifies implementation, and fits within the intent of the mitigation plan: If
someone previously issued for a domain using http-01, we have high confidence
that they are actually the owner, and they are not going to "steal" the domain
from themselves using tls-sni-01.
Also note: This change also doesn't work properly with ReusePendingAuthz: true.
Specifically, if you attempted issuance in the last couple days and failed
because there was no tls-sni challenge, you'll still have an http-01 challenge
lying around, and we'll reuse that; then your client will fail due to lack of
tls-sni challenge again.
This change was joint work between @rolandshoemaker and @jsha.
8153b919be