Qemu before version 2.9 is vulnerable to an improper link following when built with the VirtFS. A privileged user inside guest could use this flaw to access host file system beyond the shared folder and potentially escalating their privileges on a host.
https://security.gentoo.org/glsa/201704-01
https://lists.gnu.org/archive/html/qemu-devel/2017-02/msg04347.html
https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg06225.html
https://lists.debian.org/debian-lts-announce/2018/09/msg00007.html
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2016-9602
http://www.securitytracker.com/id/1037604