A domain cleanup issue was discovered in the C xenstore daemon (aka cxenstored) in Xen through 4.9.x. When shutting down a VM with a stubdomain, a race in cxenstored may cause a double-free. The xenstored daemon may crash, resulting in a DoS of any parts of the system relying on it (including domain creation / destruction, ballooning, device changes, etc.).
https://www.debian.org/security/2017/dsa-4050
https://lists.debian.org/debian-lts-announce/2018/10/msg00009.html
http://xenbits.xen.org/xsa/advisory-233.html