An issue was discovered in Xen through 4.9.x allowing x86 HVM guest OS users to obtain sensitive information from the host OS (or an arbitrary guest OS) because intercepted I/O operations can cause a write of data from uninitialized hypervisor stack memory.
www.securityfocus.com/bid/101496
www.securitytracker.com/id/1039568
lists.debian.org/debian-lts-announce/2017/11/msg00027.html
lists.debian.org/debian-lts-announce/2018/10/msg00009.html
security.gentoo.org/glsa/201801-14
support.citrix.com/article/CTX228867
www.debian.org/security/2017/dsa-4050
xenbits.xen.org/xsa/advisory-239.html