Microsoft admits 8.5 million CrowdStruck machines estimate was lowballed

Promises to discourage use of kernel drivers – so they don't crash the world again

Microsoft has admitted that its estimate of 8.5 million machines crashed by CrowdStrike's faulty software update was almost certainly too low, and vowed to reduce infosec vendors' reliance on the kernel drivers at the heart of the issue.

Redmond posted an incident response blog on Saturday – titled "Windows Security best practices for integrating and managing security tools" – in which veep for enterprise and OS security David Weston explained how Microsoft measured the impact of the incident: by accessing crash reports shared by customers.

But of course, as Weston noted, not every Windows customer shares crash reports.

"It's worth noting the number of devices which generated crash reports is a subset of the number of impacted devices previously shared by Microsoft," he wrote. Which means the 8.5 million crashed machine estimate Redmond shared on July 20 for over a week was not entirely accurate. It was also advantageous to Microsoft, which was criticized for the fragility of its OS in the wake of the incident – especially in mainstream media, which often identified crashes caused by CrowdStrike as a Microsoft mess.

Weston's post justifies how Windows performed, on the grounds that kernel drivers like those employed by CrowdStrike can improve performance and prevent tampering with software in ways that enhance security.

He noted, however, that infosec vendors must rationalize those benefits against potential negative impacts on resilience.

"Since kernel drivers run at the most trusted level of Windows, where containment and recovery capabilities are by nature constrained, security vendors must carefully balance needs like visibility and tamper resistance with the risk of operating within kernel mode," he wrote.

Weston observed that security vendors can find the right balance.

"For example, security vendors can use minimal sensors that run in kernel mode for data collection and enforcement, limiting exposure to availability issues," he wrote. "The remainder of the key product functionality includes managing updates, parsing content, and other operations can occur isolated within user mode where recoverability is possible."

That arrangement, he suggested, "demonstrates the best practice of minimizing kernel usage while still maintaining a robust security posture and strong visibility."

Are you taking notes, CrowdStrike?

Weston also reminded readers that Redmond runs an industry forum called the Microsoft Virus Initiative (MVI) in which security vendors and the OS giant work together to "define reliable extension points and platform improvements, as well as share information about how to best protect our customers."

The Microsoft veep listed the many security-related enhancements Microsoft has made over the years, and revealed the software megalith now plans "to work with the anti-malware ecosystem to take advantage of these integrated features to modernize their approach, helping to support and even increase security along with reliability."

That work will involve four efforts, namely:

1. Providing safe rollout guidance, best practices, and technologies to make it safer to perform updates to security products;
2. Reducing the need for kernel drivers to access important security data;
3. Providing enhanced isolation and anti-tampering capabilities with technologies like recently announced VBS enclaves;
4. Enabling zero trust approaches like high integrity attestation which provides a method to determine the security state of the machine based on the health of Windows native security features.

Point two seems aimed at ensuring a CrowdStrike-like event becomes less likely in future.

Weston didn't explain how that reduced dependence will be delivered – some re-jigging of Windows will likely be needed to make it happen.

Microsoft and Windows have a long and inglorious history of security snafus. If Redmond's changes go awry, it won't have CrowdStrike to blame for any new problems. ®

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