Mior and Skyfall attacks expose data in AMD SEV-SNP virtual machines

Two newly disclosed attacks show how a compromised server host can undermine protections intended to keep AMD SEV-SNP virtual machines confidential. Mior can read and modify protected VM memory and forge an attestation measurement under specific conditions. Skyfall can recover selected VM data through a processor side channel. Both attacks were demonstrated in laboratory tests on AMD EPYC processors.

The findings matter to organizations that use confidential computing to protect workloads from cloud or infrastructure operators. The attacks require privileged control of the host environment, rather than access from an ordinary remote user. That makes host security and timely firmware maintenance central to reducing exposure. Customers renting confidential VMs cannot install server BIOS updates themselves and must rely on their cloud provider to update the underlying systems.

Mior targets SEV-SNP initialization

SEV-SNP uses hardware-enforced memory encryption and access controls to isolate a virtual machine from its host hypervisor. During startup, the processor’s Platform Security Processor (PSP) initializes the Reverse Map Table (RMP), which tracks memory-page ownership and permissions. Researchers from ETH Zurich reported that Mior interferes with this initialization, leaving part of the RMP—including an entry protecting the table itself—uninitialized.

In the researchers’ demonstration, the attack required control of the hypervisor and BIOS, also called UEFI, as well as a PCIe device. They used a graphics card to redirect PSP writes during startup. With the RMP protections altered, they enabled debugging on a confidential VM and used PSP debug functions to read and write its memory. The researchers reported success in all 1,000 attempts across two test runs. They also demonstrated that Mior could make an attestation report for a malicious VM carry the measurement of a harmless VM, undermining the report’s role in verifying which software started.

The tests used an EPYC 9135 processor, based on Zen 5. The researchers consider Zen 3 and Zen 4 potentially affected, but did not test those generations with Mior. They said the attack worked even with SEV-SNP’s optional ciphertext-hiding feature enabled. AMD tracks Mior as CVE-2026-0462 and rates it 5.9 on the CVSS 4.0 scale. Its listed impact covers SEV-SNP, not the earlier SEV and SEV-ES technologies. The researchers describe Mior as separate from earlier RMP-related attacks and say it also worked on firmware predating a fix for one of those attacks.

Skyfall uses processor timing leakage

Skyfall, disclosed by researchers from ETH Zurich and Google, builds on Transient Scheduler Attacks (TSA), a class of processor flaws that can expose information through instruction timing. The team found that when a hypervisor causes a VM startup to fail, some of the VM’s saved state may remain cached without normal cleanup. A malicious hypervisor can then use TSA to recover data, including saved register values.

In a test, the researchers recovered a disk-encryption key by arranging for it to be present in registers. They also demonstrated access to only three fixed cache lines within a targeted memory page, rather than the entire page. Skyfall was tested on an EPYC 7313 processor (Zen 3) and EPYC 9124 and 9334 processors (Zen 4); the team did not demonstrate the VM leak on Zen 5. Unlike Mior, Skyfall does not require BIOS tampering. AMD identifies the VM register-data exposure as CVE-2026-40671, with a CVSS 4.0 rating of 4.9.

The Skyfall researchers also reported recovering decrypted microcode updates and register values stored in System Management Mode (SMM) memory on their test processors. The SMM finding has a separate identifier, CVE-2026-18448, assigned by the EDK II project. Its bulletin attributes the issue to missing TSA mitigations in EDK II firmware code. The IEEE Symposium on Security and Privacy program committee, which accepted the Skyfall paper, noted that the work does not fully address what a realistic VM attack would require, including a free processor core, repeated failed VM starts and high-speed performance-counter reads.

Firmware fixes and cloud-provider responsibilities

AMD lists SEV firmware for Mior and microcode for Skyfall dated August 12, 2026. Platform firmware fixes are targeted for November for Skyfall and December for Mior across the affected EPYC 7003, 8004, 8005, 9004 and 9005 series. The advisories list the same Turin firmware package with different target months. AMD says customers should obtain BIOS updates from their server manufacturers. Fixes for embedded EPYC models are expected between January and April 2027; an Instinct MI300A Skyfall patch is dated September 21.

The Mior researchers say mitigative firmware is available, and the Skyfall team says updated microcode has been released. The advisories do not clarify whether those components protect systems independently or must be installed through platform BIOS updates, and they list no workaround. Server maker Supermicro has published BIOS fix information for some H12, H13 and H14 motherboards, while noting that BMC updates are also needed and that testing continues.

For enterprise teams, the immediate priority is to confirm whether hosted confidential workloads run on affected EPYC systems and obtain the provider’s firmware remediation status. Organizations operating their own servers should track vendor BIOS and related firmware releases, assess attestation requirements, and verify the relevant trusted computing base values where applicable. Neither AMD advisory states whether the flaws have been exploited in attacks.