The remote Debian host is missing a security-related update.
This update provides mitigations for the 'lazy FPU' vulnerability affecting a range of Intel CPUs, which could result in leaking CPU register states belonging to another vCPU previously scheduled on the same CPU. For additional information please refer to https://xenbits.xen.org/xsa/advisory-267.html
Upgrade the xen packages. For the stable distribution (stretch), this problem has been fixed in version 4.8.3+xsa267+shim4.10.1+xsa267-1+deb9u8.