Comodo was alerted at September 24 2016 07:11 BST to a report [1] of the
issuance by Comodo of a Server Authentication certificate [2] that
includes 'sb' as a SAN:dNSName.  sb is a gTLD.

To establish context, first we will explain our policy with regard to
the 'www' sub-domain, for which the common link to the base domain is
well known [3].
When we use an 'agreed-upon change to website' method to prove domain
control, we consider proof of control of 'www.<base_domain>' as also
proving control of '<base_domain>' (except where '<base_domain>' is a
public suffix).
We don't give any other sub-domain this treatment, only 'www'.
We believe that the currently enforced and audited (pre-ballot 169) BRs
permit us to do this under section method 7.

When an applicant requests a certificate for the FQDN '<base_domain>',
we also add the FQDN 'www.<base_domain>' into the certificate, and
vice-versa.  This has been a fairly common practice amongst CAs for
several years.

The applicant requested a certificate for ''.  Although our code
correctly identified that the base domain ('sb') was a public suffix,
there was an error of logic that failed to prevent the certificate from
being issued in this case.

We investigated the matter promptly, identified the root cause, and
within a few hours we had deployed a bugfix to correct the error of
logic, so that issuance of a certificate where the Additional FQDN is a
public suffix is now blocked as intended.

The bugfix we deployed on September 24 had actually been prepared
several weeks earlier, after we'd been alerted (by [4]) to a similar
occurrence: we had issued 3 certificates [5] for '' (a valid
registered domain) that also included 'tc' (a public suffix) as a dNSName.
Since '' and 'tc' both belong to the same entity, we took the view
that the cert had not been misissued and that an incident report was not
warranted.  We also took the view that this flaw did not require an
urgent hotfix.  Unfortunately, scheduled deployment of the bugfix did
not occur before the 'sb' certificate [2] was issued.
Today we performed an exhaustive search of all the server authentication
certificates we've issued since November 1 2015, and as a result we
found just one further certificate [6] in which we'd included a public
suffix ( due to this bug.







Rob Stradling
Robin Alden
Comodo CA Ltd.

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