StackWarp Side-Channel Weakness Undermines AMD SEV-SNP Confidential VMs

Researchers at CISPA Helmholtz Center for Information Security disclosed StackWarp (CVE-2025-29943), a microarchitectural weakness affecting AMD Zen CPUs that can undermine the integrity guarantees of AMD SEV-SNP “confidential VM” protections. The attack model assumes a malicious insider with host/hypervisor control who can run a parallel hyperthread and exploit a previously undocumented hypervisor-side control bit to manipulate the protected guest’s stack pointer behavior, particularly when Simultaneous Multithreading (SMT) is enabled.

Reported impacts include the ability to recover sensitive data from SEV-SNP guests—such as cryptographic private keys—and to enable follow-on compromise scenarios like bypassing OpenSSH password authentication and privilege escalation within the VM. AMD issued patches (made available in July 2025) and later published a security bulletin rating the issue low severity, but the disclosure highlights ongoing risk that confidential computing isolation can be weakened by CPU-level behaviors; organizations running SEV-SNP should prioritize applying AMD’s updates and review SMT-related exposure in multi-tenant or high-trust boundary environments.