A serious vulnerability in the Linux kernel’s KVM virtualization component for ARM64 processors has been identified, potentially allowing guest virtual machines to access host memory on systems with nested virtualization enabled. The flaw, labeled as CVE-2026-89775, enables a guest to read and write host kernel memory, posing a risk of guest-to-host escape to execute code on the host machine.
Understanding the Linux Kernel Flaw
This vulnerability affects the mainline Linux kernel for ARM64 and has been addressed in versions 6.18.51, 7.2.5, and 7.3-rc1. Nested virtualization, which allows a guest to run its own hypervisor, is off by default on ARM64 systems. It requires Armv8.4 hardware with the FEAT_NV2 feature enabled. A plain ARM64 KVM host that does not activate this feature is not susceptible to the described attack.
The issue lies in how KVM manages nested virtualization on ARM64. When a guest structures its memory in a specific manner, a calculation error occurs, leading to skipped TLB invalidation, which leaves a page of freed host memory mapped and writable. This oversight allows the guest to manipulate the memory without triggering a hardware trap.
Potential Exploitation and Security Concerns
Security researcher Hyunwoo Kim, who discovered and reported the flaw on September 16, notes that a guest could exploit this vulnerability to escape its virtual environment and execute code on the host machine. As of now, no exploit code has been disclosed, and there is no evidence of the flaw being actively exploited in attacks.
The kernel’s records show the affected code is present from version 6.16, but the actual exploitable behavior begins in version 6.17. Additionally, on systems where /dev/kvm is accessible to all users, a local user could potentially exploit the flaw to gain root access, particularly in environments like Red Hat Enterprise Linux.
Fixes and Vendor Responses
Upstream, the flaw has been rectified in Linux 6.18.51, 7.2.5, and 7.3-rc1, with distributions releasing patches according to their schedules. For example, Red Hat acknowledges the issue in version 10 of its kernel, while Ubuntu has identified certain AWS, Azure, and GCP kernels as vulnerable. Meanwhile, Amazon Linux is working on a fix for its AL2023 kernel6.18 package.
For installations that cannot yet be patched, Red Hat reports a lack of viable workarounds. Notably, the attack vector targets only systems with nested virtualization enabled, which is not standard on ARM64. Vendors have rated the flaw’s severity between 7.8 and 9.3 out of 10, with the consensus being that while the impact is significant, the attack is local and not network-exploitable.
As of September 22, the flaw remains absent from the U.S. CISA’s catalog of exploited vulnerabilities, with its likelihood of exploitation rated below 1%. The flaw raises concerns about potential security risks for cloud providers, though leading services like Amazon Web Services and Google Cloud do not offer nested virtualization configurations for ARM instances, mitigating immediate risk exposure.
