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Date: Tue, 20 Aug 2013 18:40:35 +0200
From: Ludwig Nussel <>
To: Florian Weimer <>
Subject: Re: X.509 name constraints and potential interpretation

Florian Weimer wrote:
> NSS CA roots are widely reused, but the implementation deviates from
> RFC 5280 in such a way that NSS can safely accept additional root
> certificates as long as they have name constraints.  I think this is a
> bug in RFC 5280, and the fix in NSS is sound, but it could still
> result in surprising behavior if the root store is used unfiltered
> with TLS implementations that lack this bug fix.
> For reference, here is the RFC 5280 errata I submitted:
> --------------------------------------
> Type: Technical
> Reported by: Florian Weimer <>
> Section:
> Original Text
> -------------
>     DNS name restrictions are expressed as  Any DNS
>     name that can be constructed by simply adding zero or more labels to
>     the left-hand side of the name satisfies the name constraint.  For
>     example, would satisfy the constraint but
> would not.
> Corrected Text
> --------------
> [Add this to the paragraph]
>     If an implementation extracts DNS names from the subject
>     distinguished name, DNS name restrictions MUST be applied
>     to these names as well.

Do you have an idea in mind how to do that in practice? E.g with
openssl? Checking name constraints and the logic of verifying server
identifiers is at different layers there. Ie openssl makes sure
certificate name constraints on subjAltNames are applied but the special
interpretation of the CN as host name is left to the applications

An alternative approach would be to disallow that legacy CN
interpretation as host name in chains that use name constraints.


  (o_   Ludwig Nussel
SUSE LINUX Products GmbH, GF: Jeff Hawn, Jennifer Guild, Felix Imend├Ârffer, HRB 16746 (AG N├╝rnberg)

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