On 09/11/2014 08:46 PM, Alexey Klyukin wrote:
On Mon, Sep 8, 2014 at 8:04 PM, Heikki Linnakangas
<hlinnakan...@vmware.com> wrote:
2. I still wonder if we should follow the RFC 6125 and not check the Common
Name at all, if SANs are present. I don't really understand the point of
that rule, and it seems unlikely to pose a security issue in practice,
because surely a CA won't sign a certificate with a bogus/wrong CN, because
an older client that doesn't look at the SANs at all would use the CN
anyway. But still... what does a Typical Web Browser do?


At least Firefox and Safari seem to follow RFC strictly, according to
some anecdotical evidences (I haven't got enough time to dig into the
source code yet):

http://superuser.com/questions/230136/primary-common-name-in-subjectaltname
https://bugzilla.mozilla.org/show_bug.cgi?id=238142

Chrome seem to follow them, so the major open-source browsers are
ignoring the Common Name if the SubjectAltName is present.
Still, I see no win in ignoring CN (except for the shorter code), it
just annoys some users that forgot to put the CN name also in the SAN
section, so my 5 cents is that we should check both.

Hmm. If that's what the browsers do, I think we should also err on the side of caution here. Ignoring the CN is highly unlikely to cause anyone a problem; a CA worth its salt should not issue a certificate with a CN that's not also listed in the SAN section. But if you have such a certificate anyway for some reason, it shouldn't be too difficult to get a new certificate. Certificates expire every 1-3 years anyway, so there must be a procedure to renew them anyway.

- Heikki



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