[Buildroot] [PATCH 1/3] package/mbedtls: add BR2_PACKAGE_MBEDTLS_X509_UNSUPPORTED_CRITICAL_EXTENSION
Yann E. MORIN
yann.morin.1998 at free.fr
Fri Apr 24 11:45:59 UTC 2020
On 2020-04-24 13:26 +0200, Nicola Di Lieto spake thusly:
> On Fri, Apr 24, 2020 at 11:07:10AM +0200, Yann E. MORIN wrote:
> > - an X.509 parser that encounters an extension marked 'critical' when
> > parsing a certificate, and that does not recognise that extension,
> > *must* reject that certificate.
> >
>
> I wouldn't be so sure that it must be done at parsing stage though.
Well, "parsing" is maybe not the correct word, but as far as I
understand wikipedia on X.209 [0]:
A certificate-using system must reject the certificate if it
encounters a critical extension that it does not recognize,
or a critical extension that contains information that it cannot
process.
[0] https://en.wikipedia.org/wiki/X.509#Structure_of_a_certificate
So, as mbedtls does not know what to do with such a critical extension,
allowing it to just ignore it is a violation of the X.509 spec.
> OpenSSL
> and GnuTLS parse the certificate without problems and then fail at
> validation stage. OpenSSL even has a "-ignore_critical" command line switch
> to ignore critical extensions:
>
> https://man.openbsd.org/openssl.1#ignore_critical
But that is different: this is a command line argument, which is
obviously not the default. Enabling X509_UNSUPPORTED_CRITICAL_EXTENSION
will make that the default behaviour, which is unsound.
Regards,
Yann E. MORIN.
> I honestly think the approach of mbedTLS to critical extensions is blunt to
> say the least. And just as a side note, mbedTLS *will* still happily
> generate, sign and export a certificate with an unsupported critical
> extension, even when it's built without that damn feature. Don't ask me how
> I know... if you don't believe me, look at
> mbedtls_x509write_crt_set_extension, called on line 1167 in ualpn.c
>
> >
> >I think we should refuse to use mbedtls with uacme.
>
> I agree, at least until mbedTLS changes its approach.
>
> Regards
> Nicola
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