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X.509 name constraints and potential interpretation conflict
From: Florian Weimer <fweimer () redhat com>
Date: Mon, 12 Aug 2013 15:05:48 +0200

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 <fweimer () redhat com>


Original Text
   DNS name restrictions are expressed as host.example.com.  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, www.host.example.com would satisfy the constraint but
   host1.example.com 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.

When used with TLS and HTTP (according to RFC 2818), section, Name Constraints, is technically a NOP that doesn't constraint the CA that has this attribute because RFC 2818 mandates processing of the common name attribute in the subject distinguished name. Consequentially, the constraint can be bypassed by issuing a certificate without a subject alternative name. The fix is to apply the DNS name restrictions to the relevant parts of the subject distinguished name, too, as implemented here:


Florian Weimer / Red Hat Product Security Team

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