Zapscape Flaw in KVM Breaks Nested Virtualization Isolation Allowing L1 Guest Root Code Execution on Linux Host
The Zapscape vulnerability (CVE-2026-64561) affects KVM/x86 in the Linux kernel and enables an attacker with kernel privileges inside an L1 virtual machine to escape to the host and execute code as root. The flaw occurs in the shadow MMU when handling page faults for nested guests, specifically due to an incorrect order of stale root condition checks that leads to a use-after-free condition. This weakens the isolation between the host and L1 guests precisely when nested virtualization is enabled for potentially untrusted tenants. A public proof-of-concept demonstrates the escape by creating a file named /Zapscape owned by root on the host. The issue impacts Linux kernels starting from version 5.9, with fixes already merged into stable branches including 6.6.148, 6.12.101, 6.18.42, 7.1.6 and 7.2 rc5. On Intel systems the attack requires EPT page walk lengths 4 and 5 to be exposed to the L1 guest, while no equivalent condition is documented for AMD. Organizations are advised to apply patches promptly or disable nested virtualization for untrusted workloads.
HispasecAug 7, 2026Translated from Spanish