Zapscape KVM Flaw Enables Guest-to-Host Escape on Linux Hosts

A newly disclosed KVM/x86 vulnerability, CVE-2026-64561 or Zapscape, allows a privileged L1 guest to break nested virtualization isolation and achieve guest-to-host escape on Linux systems. The bug is a use-after-free in KVM/x86 shadow MMU emulation tied to the recursive shadow-page zap path and stale-root check ordering during guest-triggered page fault handling, enabling corruption of host kernel shadow pages and potential host code execution. Researcher Hyunwoo Kim reported the flaw to the Linux kernel security team and published a proof-of-concept showing creation of a root-owned file on the host.

The issue affects both Intel and AMD hosts and is especially serious for environments that run untrusted guests with nested virtualization enabled, including multi-tenant public cloud infrastructure. On Intel, exploitation requires exposing both EPT page walk lengths 4 and 5 to the L1 guest, while no equivalent constraint was noted for AMD. The vulnerability affects Linux KVM versions from 5.9 onward across the disclosed commit range, and a fix has been merged upstream into mainline Linux, with users advised to deploy patched stable kernels or vendor-backported updates; no in-the-wild exploitation has been claimed.