Microsoft on Saturday issued an out-of-band Windows security update that disabled a patch the company released earlier this month to protect personal computers from possible attacks leveraging one of the “Spectre” vulnerabilities.

The weekend release was Microsoft’s response to an announcement seven days ago by Intel, which told customers of all stripes – from computer makers to end users – to stop deploying the firmware updates it had offered after disclosures of the Spectre and Meltdown flaws. According to Intel, the new firmware “may introduce [a] higher-than-expected [number of] reboots and other unpredictable system behavior” on Broadwell and Haswell processors. Those silicon families were introduced in 2015 and 2013, respectively.

Microsoft reacted to that disturbing news by voiding mitigations for one of the three areas of vulnerability posed by Spectre and Meltdown.

“Our own experience is that system instability can in some circumstances cause data loss or corruption,” Microsoft confirmed in the support document accompanying the surprise update. “While Intel tests, updates and deploys new microcode, we are making available an out-of-band update today, KB4078130, that specifically disables only the mitigation against CVE-2017-5715 – ‘Branch target injection vulnerability.’ In our testing this update has been found to prevent the behavior described.”

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