A newly disclosed Linux kernel flaw lets a guest virtual machine read and write memory belonging to its host on ARM64 systems with nested virtualization enabled, a class of bug that can end in a full guest-to-host escape. The vulnerability, CVE-2026-89775, sits in the kernel’s KVM code for ARM64 processors and was reported by researcher Hyunwoo Kim, who disclosed it on 16 September. Red Hat’s advisory describes the root cause as a memory-translation calculation that misfires when the system’s memory management unit is inactive, leaving a freed page of host memory mapped and writable by the guest. The bug is fixed upstream in Linux 6.18.51, 7.2.5, and 7.3-rc1, though distributions are shipping the patch on different timelines.
The blast radius depends heavily on configuration. Nested virtualization is off by default on ARM64 and needs specific newer hardware to enable, so a standard ARM64 KVM host sits outside the primary attack path. Kim also documented a second route: on any system where an unprivileged user can open the /dev/kvm device, a local user can build a guest and use the same flaw to gain root, no nested virtualization required. Red Hat flags this as relevant to Red Hat Enterprise Linux 10, where that device is open to all users by default; RHEL 6 through 9 are unaffected. Ubuntu’s 26.04 kernels, including its cloud images for AWS, Azure, and GCP, are vulnerable.
The original insight is that these two attack paths carry very different urgency, and treating this as one undifferentiated “KVM bug” risks both over- and under-reacting. Cloud operators running nested virtualization for multi-tenant workloads face a real host-escape risk and should patch on the upstream timeline. Everyone else’s exposure hinges on a narrower question: whether unprivileged users can open /dev/kvm at all, a permissions setting worth auditing on any multi-user Linux host regardless of this specific CVE. No exploit code has been published and there is no evidence of active exploitation. CyberTech has tracked how unevenly patch timelines move across vendors for the same underlying flaw, and this bug’s split status, some kernels fixed, others exposed, is a familiar version of that pattern.
Source: Red Hat
