CVE-2026-46076: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1

Synopsis

A virtualization logic flaw in the Linux kernel's KVM nested SVM (nSVM) subsystem allows a guest virtual machine running as L2 to make hypercalls as if it were the L1 hypervisor when L1 has not intercepted VMMCALL and nested TLB flush is enabled. An attacker with a low-privilege account who can execute code inside an L2 guest can trigger the flaw locally, without any network access or victim interaction. Successful exploitation allows the attacker to read limited host or hypervisor data, tamper with virtualization state, and crash the affected VM or hypervisor layer, causing a significant availability disruption. Patched-image rebuilds at versions 6.12.86, 6.18.27, and the identified upstream commits are available on HarborGuard for environments running an affected kernel version.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityNot required

  The attacker needs an existing shell or process on the host (or inside a guest VM); no network access to the target is required.

• AuthenticationRequired

  Any low-privilege account inside the L2 guest is sufficient to execute the VMMCALL instruction that triggers the flaw.

• Victim interactionNot required

  No interaction from another user or administrator is needed; the attacker executes the exploit entirely on their own.

• Attack complexityDetail

  Attack complexity is low: the exploit is reliable and requires no special race conditions, memory-layout knowledge, or other environmental factors beyond meeting the nested-SVM configuration conditions.

Blast Radius

• Reads limited hypervisor or host memory regions exposed through the unintended hypercall path, potentially disclosing confidential virtualization state.
• Modifies virtualization-layer state by issuing hypercalls that should have been reserved for the L1 hypervisor, potentially corrupting TLB or VM control structures.
• Crashes the affected guest VM or the hypervisor layer handling the malformed hypercall, causing sustained service disruption for all workloads running on that host.
• Undermines the isolation boundary between nested guest (L2) and hypervisor (L1), weakening the security guarantees of any multi-tenant or nested-virtualization deployment.