The client in OpenSSH before 7.2 mishandles failed cookie generation for untrusted X11 forwarding and relies on the local X11 server for access-control decisions, which allows remote X11 clients to trigger a fallback and obtain trusted X11 forwarding privileges by leveraging configuration issues on this X11 server, as demonstrated by lack of the SECURITY extension on this X11 server.
https://security.gentoo.org/glsa/201612-18
https://lists.debian.org/debian-lts-announce/2018/09/msg00010.html
https://cert-portal.siemens.com/productcert/pdf/ssa-412672.pdf
https://bugzilla.redhat.com/show_bug.cgi?id=1298741
https://anongit.mindrot.org/openssh.git/commit/?id=ed4ce82dbfa8a3a3c8ea6fa0db113c71e234416c
http://www.securityfocus.com/bid/84427
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html
http://www.openssh.com/txt/release-7.2
http://rhn.redhat.com/errata/RHSA-2016-0741.html