Microsoft Releases Mitigations for Spectre-Like ‘Variant 4’ Attack

Updates released by Microsoft on Tuesday for its Windows operating system add support for a feature that should prevent attacks involving the recently disclosed speculative execution vulnerability known as “Variant 4.”

Researchers from several organizations warned in January that processors from Intel, AMD, ARM and other companies are affected by vulnerabilities that allow malicious applications to bypass memory isolation mechanisms and gain access to sensitive data. The flaws are tracked as Spectre (Variant 1 – CVE-2017-5753 and Variant 2 – CVE-2017-5715) and Meltdown (Variant 3 – CVE-2017-5754).

Last month, Intel, AMD, ARM, IBM, Microsoft and other major tech companies released updates, mitigations and advisories for two new variants of the speculative execution attack methods, namely Variant 3a and Variant 4.

Variant 4, which is similar to Spectre Variant 1, relies on a side-channel vulnerability known as Speculative Store Bypass (SSB) and it has been assigned the identifier CVE-2018-3639.

Microsoft has not identified any code patterns – in either its software or cloud services – that would allow Variant 4 attacks. However, the company announced on Tuesday – along with its monthly security updates – that it added support for Speculative Store Bypass Disable (SSBD) to Windows and Azure in an effort to completely eliminate the risk of attacks.

SSBD is designed to prevent a Speculative Store Bypass from occurring, but Microsoft noted that enabling the feature also requires microcode updates from Intel.

Microsoft has released updates that include the mitigation for Windows 10, Windows 7, Windows Server 2008, Windows Server 2016, and Windows Server versions 1709 and 1803. Support for SSBD has only been added for machines with Intel processors, but the company is working on updates for AMD devices as well. Systems powered by AMD CPUs will not require microcode updates.

When Variant 4 was disclosed, Intel announced that it had provided beta microcode updates to operating system vendors and equipment manufacturers to add support for SSBD.

However, Intel says the mitigation will be turned off by default and the company believes many will leave it that way.

Enabling SSBD may have some negative impact on performance, Microsoft and Intel said. Intel told customers last month that performance impact during its tests ranged between 2 and 8 percent.

Related: Intel Will Not Patch Spectre in Some CPUs

Related: Intel CPUs Vulnerable to New ‘BranchScope’ Attack

Related: Microsoft Releases More Microcode Patches for Spectre Flaw