An issue was discovered in Xen 4.8.x through 4.10.x allowing x86 PVH guest OS users to cause a denial of service (NULL pointer dereference and hypervisor crash) by leveraging the mishandling of configurations that lack a Local APIC.
https://xenbits.xen.org/xsa/advisory-256.html
https://www.debian.org/security/2018/dsa-4131