CVE-2025-40300

Published Sep 11, 2025

Last updated 21 days ago

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Automated description summarized from trusted sources.

CVE-2025-40300, also known as VMSCAPE, is a Spectre-based vulnerability that affects AMD Zen CPUs (Zen 1 through Zen 5) and Intel Coffee Lake processors. It exploits incomplete branch predictor isolation in cloud environments, allowing a malicious guest user to potentially leak secrets from the hypervisor in the host domain. This attack is the first of its kind to enable a malicious guest VM to leak arbitrary memory from an unmodified hypervisor without requiring code modifications. The VMSCAPE attack targets the KVM/QEMU virtualization stack, demonstrating how attackers can extract cryptographic keys and other sensitive infrastructure secrets. Researchers have shown that VMSCAPE can leak memory data at a rate of 32 bytes per second, potentially extracting disk encryption keys in approximately 18 minutes on AMD Zen 4 processors. The vulnerability stems from the incomplete isolation of branch prediction state across virtualization boundaries, specifically between guest and host user processes. Linux kernel developers have released patches to mitigate VMSCAPE by adding an Indirect Branch Prediction Barrier (IBPB) on VMEXIT.

Description: In the Linux kernel, the following vulnerability has been resolved: x86/vmscape: Add conditional IBPB mitigation VMSCAPE is a vulnerability that exploits insufficient branch predictor isolation between a guest and a userspace hypervisor (like QEMU). Existing mitigations already protect kernel/KVM from a malicious guest. Userspace can additionally be protected by flushing the branch predictors after a VMexit. Since it is the userspace that consumes the poisoned branch predictors, conditionally issue an IBPB after a VMexit and before returning to userspace. Workloads that frequently switch between hypervisor and userspace will incur the most overhead from the new IBPB. This new IBPB is not integrated with the existing IBPB sites. For instance, a task can use the existing speculation control prctl() to get an IBPB at context switch time. With this implementation, the IBPB is doubled up: one at context switch and another before running userspace. The intent is to integrate and optimize these cases post-embargo. [ dhansen: elaborate on suboptimal IBPB solution ]
Source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
NVD status: Awaiting Analysis

Hype score: Not currently trending

References

Sources include official advisories and independent security research.

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