The remote Debian host is missing a security-related update.
This update provides mitigations for the Spectre v4 variant in x86-based micro processors. On Intel CPUs this requires updated microcode which is currently not released publicly (but your hardware vendor may have issued an update). For servers with AMD CPUs no microcode update is needed, please refer to https://xenbits.xen.org/xsa/advisory-263.html for further information.
Upgrade the xen packages. For the stable distribution (stretch), this problem has been fixed in version 4.8.3+xsa262+shim4.10.0+comet3-1+deb9u7.