CVE-2021-26328 in AMD
Summary
by MITRE • 01/11/2023
Failure to verify the mode of CPU execution at the time of SNP_INIT may lead to a potential loss of memory integrity for SNP guests.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 04/09/2025
The vulnerability identified as CVE-2021-26328 represents a critical flaw in the Secure Encrypted Virtualization (SEV) implementation within AMD processors, specifically affecting the SNP (Secure Nested Paging) initialization process. This weakness resides in the failure to properly validate the CPU execution mode during the SNP_INIT command sequence, which is fundamental to establishing secure virtual machine environments. The issue impacts systems utilizing AMD's SEV-SNP technology, which provides enhanced memory encryption and integrity protection for virtualized environments. The vulnerability stems from insufficient verification mechanisms that should ensure the CPU operates in the correct privileged mode before proceeding with the initialization of secure memory regions.
The technical flaw manifests when the SNP_INIT command is executed without proper validation of the CPU's execution context, potentially allowing malicious actors to manipulate the initialization process. This failure creates a scenario where the hypervisor or malicious guest code could bypass critical security checks that should verify the CPU is operating in the appropriate mode, typically requiring the CPU to be in a specific privileged state. The vulnerability is particularly concerning because it undermines the fundamental security model of SEV-SNP, where memory integrity protection relies on the assumption that initialization occurs in a controlled and verified execution environment. When the mode verification fails, it creates a potential attack vector that could compromise the memory encryption keys and allow unauthorized access to protected data within the virtual machine.
The operational impact of CVE-2021-26328 extends beyond simple data exposure, as it represents a potential pathway for privilege escalation and memory integrity violations within virtualized environments. Attackers who can exploit this vulnerability may be able to compromise the confidentiality and integrity of memory contents for SNP-enabled virtual machines, potentially gaining access to sensitive data, credentials, or system information that should remain protected. The implications are particularly severe in cloud computing environments where multiple tenants share the same physical hardware, as this vulnerability could enable one guest virtual machine to potentially access or corrupt memory contents of other guest VMs. This weakness directly violates the core principle of virtualization security where isolation between VMs should be maintained, making it a significant concern for enterprise and cloud security architectures.
Mitigation strategies for CVE-2021-26328 primarily involve updating firmware and hypervisor components to versions that properly implement mode verification during SNP initialization. System administrators should ensure that all AMD processors with SEV-SNP capabilities are running patched firmware versions that address the specific validation flaw. Additionally, implementing monitoring solutions that can detect anomalous SNP_INIT sequences or unauthorized mode transitions can provide early warning capabilities. Organizations should also consider implementing additional layers of security such as hypervisor hardening, network segmentation, and regular security assessments to reduce the attack surface. The vulnerability aligns with CWE-284 which addresses improper access control, and may be categorized under ATT&CK technique T1059 for command and scripting interpreter, as exploitation could involve manipulating execution contexts to bypass security checks. Security teams must prioritize patch management for affected systems and conduct thorough assessments of their virtualized environments to ensure proper implementation of SEV-SNP security features.