[cabfpub] Ballot 202 - Underscore and Wildcard Characters

Erwann Abalea Erwann.Abalea at docusign.com
Thu Jul 20 11:02:28 MST 2017

> Le 20 juil. 2017 à 16:52, Ryan Sleevi <sleevi at google.com> a écrit :
> On Thu, Jul 20, 2017 at 10:16 AM, Erwann Abalea
> <Erwann.Abalea at docusign.com> wrote:
>> Bonjour,
>> Looking back in time on the list for a reason to allow for underscore in
>> SAN:dNSName, I found basically 2 potential reasons:
>> - allow things such as « _sip._tls.xxxxxxx.com »
>> - provide certificates for names that are not internet routable, such as «
>> m_staging9.xxxxxxx.com »
>> The first option is avoided here (and it’s fine), because this should go
>> into an SRV-type entry (or in the SAN:URI for this example).
>> The text of the proposed ballot opens the path to having underscores into
>> the the « xxxxxx » portion. This path is closed later by the validation of
>> domain authorization or control (except if method is used).
>> Text in disallows FQDNs that contain a label within the class of
>> R-LDH labels than don’t start with « xn-- ». IIUC, such domains are still
>> allowed under some TLDs that don’t support IDNA (such as « in--valid.am », «
>> in--valid.gov », or « in--valid.co.uk »). I just validated that Safari,
>> Firefox, and Chrome (on MacOSX) are happy to connect to
>> http://in--valid.mydomain.com (for a properly owned my domain.com).
>> Is that the intention of the ballot to disallow CAs to provide certificates
>> for such domains?
> Yes.
> See the terminology discussion in
> https://tools.ietf.org/html/rfc5890#section-2.3.1 (as well as
> Figure-1) . In effect, this prohibits CAs from issuing for Reserved
> LDH Labels - except for those explicitly noted (IDNA).

I was just reading it.

So valid names will be composed only of:
 - LDH labels that are correct A-labels (the (2) box)
 - NR-LDH labels (the box on top right)
 - a subset of NON-LDH labels: only the underscore will be authorized and only where an hyphen can be present; will « xn__something » be authorized?

Using censys.io and the following expression returns some currently valid certificates that contain a fake A-label while being valid names:
parsed.extensions.subject_alt_name.dns_names:/(.*\.)?[^xX][^nN]--.*/ and tags: "trusted"

With the following expression, there’s a list of registered domains that contain a double hyphen at 3rd and 4th position while not starting with xn:

Such certificates would then be considered as misissued.

> The existing RFCs already prohibit issuance to non-LDH labels, by
> virtue of RFC 5280 ( - "When the subjectAltName extension
> contains a domain name system label, the domain name MUST be stored in
> the dNSName (an IA5String). The name MUST be in the "preferred name
> syntax", as specified by Section 3.5 of [RFC1034] and as modified by
> Section 2.1 of [RFC1123].")

The names returned by the upper-mentioned requests only contain valid LDH labels.

On the other hand, the proposed ballot would clearly rule out a lot of currently valid certificates that shouldn’t exist (containing a label starting with an underscore).

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