CVSSv3: None yet
The “Januscape” vulnerability has been present in the Linux kernel since 2010. Patches have been released for the upstream kernel in commit 81ccda30b4e8.
Januscape is a use-after-free vulnerability in the shadow MMU emulation of KVM/x86. It can trigger the bug with guest-side actions alone to corrupt the host kernel’s shadow page, and it can threaten the guest-host isolation of KVM/x86 hosts that accept untrusted guests and expose nested virtualization, particularly multi-tenant x86 public clouds (GCP, AWS, etc.).
This vulnerability does not impact QEMU directly.
For those playing along at home, this is the same researcher who discovered DirtyFrag.