GoKawiilJanuscape: Guest-to-Host Escape in KVM/x86
Abstract
This document describes the Januscape (CVE-2026-53359) vulnerability discovered and reported by Hyunwoo Kim (@v4bel). It is a KVM escape vulnerability that lets a guest escape to the host in a KVM/x86 environment. To the best of public knowledge, this is the first guest-to-host exploit research triggerable on both Intel and AMD rather than being limited to a single architecture.
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.).
In fact, Januscape was successfully used as a 0-day exploit in Google kvmCTF.
For the detailed technical information, see here.
Note After reporting this vulnerability to [email protected], the agreed embargo has ended, so the exploit is posted to oss-security and this Januscape document is published. For the disclosure timeline, see the technical detail document.
PoC Structure
Running the PoC inside a guest VM can trigger a host kernel panic. A full escape exploit that works in a controlled environment also exists, but it is not released at this time and is planned to be released in the very distant future.
On distributions like RHEL, /dev/kvm is world-writable ( 0666 ), so an unprivileged user could also turn this vulnerability into a reliable LPE to root. That said, doing so would be like paying gold for garbage, so I won't bother covering it.
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