CVE-2025-40300: 
Linux Kernel vulnerability analysis and mitigation

VMSCAPE (CVE-2025-40300) is a newly discovered Spectre-based vulnerability that affects the Linux kernel, specifically targeting insufficient branch predictor isolation between guest virtual machines and userspace hypervisors like QEMU. The vulnerability was disclosed on September 11, 2025, by researchers at ETH Zurich (The Register).

VMSCAPE represents the first practical Spectre Branch Target Injection (BTI) attack that enables a malicious guest VM to leak arbitrary memory from an unmodified hypervisor without requiring code modifications. The vulnerability specifically affects AMD Zen 1-5 processors and Intel Coffee Lake processors. On AMD Zen 4 processors, researchers achieved data exfiltration rates of 32 bytes per second. The attack exploits incomplete isolation of branch prediction state across virtualization boundaries, particularly between guest user processes and host user processes (GBHackers).

The vulnerability enables attackers to extract cryptographic keys and other sensitive infrastructure secrets from cloud environments. On AMD Zen 4 processors, researchers demonstrated successful extraction of disk encryption and decryption keys within approximately 18 minutes. The complete attack chain, including initial reconnaissance and secret location identification, requires about 1,092 seconds to execute fully (The Register).

Linux kernel maintainers have implemented mitigation patches based on Indirect Branch Prediction Barrier (IBPB) on VM exits. The patches introduce minimal performance overhead in typical cloud scenarios, with only 1% impact on compute-intensive workloads and up to 51% overhead for I/O-heavy operations. To mitigate this issue, it is recommended to implement IBPB each time the kernel returns to QEMU (Red Hat).

AMD has announced they will issue a Security Brief acknowledging the potential vulnerability, while Intel has stated that existing mitigations on their processors can be used to address this issue. Intel engineers are working with Linux to ensure appropriate mitigations are applied to Linux userspace hypervisor software (The Register).