[Acme] SNI extension for tls-alpn-01 challenge in draft-ietf-acme-ip-05

[Corey Bonnell <[e-mail]>]

Hello,

Draft-ietf-acme-ip-05 specifies that for the tls-alpn-01 challenge, an SNI value with the in-addr/ipv6.arpa domain name corresponding to the iPAddress being validated MUST be specified. I have a concern that this requirement suffers the same problem that rendered tls-sni-01 insecure: namely, an arbitrary user on a shared hosting provider could upload an arbitrary certificate for the required .ip-addr/ipv6.arpa domain, thus circumventing any security provided by the mandatory SNI extension.

The mandatory ALPN extension prevents this from being exploited to obtain fraudulent certificates, but given how trivially the SNI requirement can be defeated in the same manner as for tls-sni-01, I don’t believe that requiring SNI provides any security value and is not necessary. If the intent for requiring the SNI extension is to convey to the TLS server that an IP address is being validated, couldn’t that similarly be accomplished by *not* specifying any SNI extension at all? Tls-apln-01 (for dNSNames) requires that a SNI value be specified, so TLS servers could differentiate between challenge requests for dNSNames and iPAddress based on the presence (or absence) of the SNI extension.


Thanks,

Corey