It was discovered in gnutls before version 3.6.7 upstream that there is an uninitialized pointer access in gnutls versions 3.6.3 or later which can be triggered by certain post-handshake messages.
https://usn.ubuntu.com/3999-1/
https://security.netapp.com/advisory/ntap-20190502-0005/
https://security.gentoo.org/glsa/201904-14
https://gitlab.com/gnutls/gnutls/issues/704
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2019-3836
https://access.redhat.com/errata/RHSA-2019:3600
http://lists.opensuse.org/opensuse-security-announce/2019-05/msg00017.html