CVE-2021-26401 in AMD
Summary
by MITRE • 03/11/2022
LFENCE/JMP (mitigation V2-2) may not sufficiently mitigate CVE-2017-5715 on some AMD CPUs.
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Analysis
by VulDB Data Team • 05/18/2025
CVE-2021-26401 represents a critical mitigation gap in the implementation of software-based defenses against speculative execution side-channel vulnerabilities, specifically targeting the mitigation of CVE-2017-5715 also known as Spectre Variant 2. The vulnerability affects certain AMD processor architectures where the LFENCE/JMP mitigation technique fails to provide adequate protection against the specific attack vectors that exploit the vulnerability. This issue stems from the complex interaction between hardware microarchitecture and software mitigation strategies, particularly when dealing with the branch target injection attack pattern that allows attackers to extract sensitive information from the processor's speculative execution engine.
The technical flaw manifests in AMD processors that implement the LFENCE/JMP mitigation pattern as a workaround for the Spectre Variant 2 vulnerability. This mitigation technique relies on inserting LFENCE instructions followed by JMP operations to prevent the processor from speculatively executing code that could lead to information disclosure. However, certain AMD CPU models fail to properly enforce the memory ordering guarantees required by this mitigation, allowing attackers to bypass the protection mechanisms. The vulnerability occurs because the processor's microarchitecture does not consistently maintain the required memory ordering semantics between the LFENCE instruction and subsequent branch operations, creating a window where speculative execution can still access sensitive data.
The operational impact of CVE-2021-26401 is significant for organizations utilizing affected AMD processors in environments where sensitive data processing occurs. Attackers can potentially exploit this vulnerability to perform information disclosure attacks against systems running vulnerable software configurations, particularly in cloud environments where multiple tenants share the same physical hardware. The vulnerability affects systems that rely on the LFENCE/JMP mitigation approach, which has been widely deployed across enterprise environments as a temporary workaround while hardware-level mitigations are implemented. This weakness particularly impacts server environments, virtualization platforms, and systems handling confidential data where the risk of side-channel attacks is heightened.
Organizations should implement immediate mitigations including updating firmware and microcode to the latest versions provided by AMD, ensuring that all systems are running patched firmware that properly addresses the LFENCE/JMP mitigation behavior. The mitigation strategy should include disabling or carefully managing the use of software mitigations that rely on LFENCE/JMP patterns when operating on affected AMD processors, and instead relying on hardware-based mitigations or alternative software approaches such as retpoline or other branch target injection mitigations. System administrators should also monitor for any signs of exploitation attempts and implement additional network-level monitoring to detect potential abuse of this vulnerability. This vulnerability aligns with CWE-119 which deals with improper access to memory locations and relates to ATT&CK technique T1059.005 for command and scripting interpreter and T1557.001 for local network mapping.