This Hyper-V vulnerability could plague Microsoft users for some time

New details have emerged about a critical remote code execution vulnerability in Hyper-V that was discovered by cybersecurity researchers and patched in May 2021.

The vulnerability, tracked as CVE-2021-28476 was reported to Microsoft by Guardicore Labs’ Ophir Harpaz and SafeBreach Labs’ Peleg Hadar, and was assigned a CVSS score of 9.9.

“Hyper-V is Azure’s hypervisor; for this reason, a vulnerability in Hyper-V entails a vulnerability in Azure, and can affect whole regions of the public cloud. Triggering denial of service from an Azure VM would crash major parts of Azure’s infrastructure and take down all virtual machines (VM) that share the same host,” note the researchers in a new joint blog post.

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The vulnerability was found using an in-house developed fuzzer dubbed hAFL1, which the researchers will detail in next month’s Black Hat USA 2021 conference.

Businesses are slow to patch

The bug originates in Hyper-V's network switch driver named vmswitch, and affects all versions of Windows from Windows 7 upwards, including Windows 10, as well as Windows Server 2008 through to Windows Server 2019. 

The researchers note that the vulnerability was first spotted in August 2019, which suggests to them that the bug might have been in production for over a year before it was discovered and patched.

To exploit the vulnerability, the attacker must have access to a guest VM through which they can send a specially crafted packet to the Hyper-V host to trigger mayhem.

While Microsoft has ensured that the Azure service is safe from this issue, Harpaz told BleepingComputer that it is fairly common for vulnerabilities to remain unpatched for years on machines in enterprise networks.

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