There is a GnuTLS security advisory [0] regarding "an issue that affects
validation of certificates using OCSP responses, which can falsely
report a certificate as valid under certain circumstances."

I updated GnuTLS on core-updates to 3.5.4, the latest release of the 3.5
series.

For master, the naive approach of cherry-picking the patch [1] did not
work; the test 'system-prio-file' fails consistently with that change. I
could instead try grafting the updated version.

What do you think? The authors seem to think it's a relatively minor
issue [2], since exploiting it requires an attacker to compromise the
certificate authority.

[0]
http://gnutls.org/security.html#GNUTLS-SA-2016-3
http://lists.gnutls.org/pipermail/gnutls-devel/2016-September/008146.html

[1]
https://gitlab.com/gnutls/gnutls/commit/964632f37dfdfb914ebc5e49db4fa29af35b1de9

[2]
http://lists.gnutls.org/pipermail/gnutls-devel/2016-September/008148.html