A tiny sop to Win10 1903 users bitten by the KB 4512941 Cortana/SearchUI.exe redlining bug

Microsoft has finally acknowledged – by tweet, no less – what many have known since Friday: The “optional, non-security” August patch for Win10 version 1903, KB 4512941, can redline your PC. Thanks, Cortana.

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Franck V. (CC0)

On Friday, Microsoft released KB 4512941, the long-anticipated second August cumulative update for Win10 version 1903 – which is to say, the latest patch for the latest version of Windows. Within a few hours, people were complaining on various online forums that installing the update immediately triggered excessive CPU use. I wrote about that on Friday night.

Cortana’s SearchUI.exe routine can send one “core” processing unit through the roof. Various reports place the redlining at anywhere from 40% to 100% utilization of the single core, resulting in a constant drain of 20% or more on the entire PC. 

In Win10 1903, when you search your computer, you use Cortana. So the native Search function also breaks down. 

Although the second August cumulative update for 1903 is billed as “optional,” it’s a key update precisely because the first August cumulative update broke Visual Basic, Visual Basic for Applications, and VBScript. Patches in July and August also broke Windows Sandbox, the Preboot Execution Environment and MIT Kerberos – all of which are supposed to be corrected by this second August patch.

But if you install the second August patch, KB 4512941, there’s a decent chance your machine will break out in hives. 

Most damning, Mayank Parnmar at Windows Latest reported on Saturday:

It’s important to note that Microsoft actually tested KB4512941 with Windows Insiders in the Release Preview Ring for more than a week before shipping the update to the general public.

According to some posts on Feedback Hub, reports of high CPU usage were submitted multiple times by testers earlier this week, but the reports appear to have been ignored because they weren’t upvoted enough.

Nobody knows exactly which machines will get hit, but Günter Born has significant evidence that it’s all related to a bad Cortana cache. Says Born:

One could now simply say: Ok, I clear the cache and it’s fixed. Unfortunately this won’t help, because the cache will not be rebuilt if it is deleted or a new cache is created.

We heard exactly nothing from Microsoft about the bug until Tuesday afternoon – four days after complaints started to appear. The acknowledgment came in a short tweet from the @WindowsUpdate account:

We are currently investigating an issue where users are reporting high CPU usage linked to SeachUI.EXE [sic] after installing the optional update on August 30 (KB4512941). We will provide an update in an upcoming release.

Even now, on Wednesday morning, five days after the initial complaints, Microsoft hasn’t acknowledged the problem in the KB article, which says:

Known issues in this update

Microsoft is not currently aware of any issues with this update.

Nor has MS mentioned anything in the Release Information Status page, which was designed specifically to highlight bugs like this.

So we get a tweet and a promise that the bug will be fixed sometime “in an upcoming release." Be still my beating heart.

There’s some talk about solving the redlining problem by setting the Registry key, HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Search\BingSearchEnabled to 1. But there’s also considerable evidence that approach doesn’t work in all situations.

Meanwhile, we’re stuck between Scylla and Charybdis. You need to get the first August cumulative installed to block DejaBlue, which remains a potent threat although it hasn’t been exploited as yet. You need to get the second August cumulative update installed to fix the bugs created (or perpetuated) in the first cumulative update. But the second cumulative update may hobble your machine.

Windows as a Service.

Join us for a late summer roast on AskWoody.com. Just don’t tell me that Win10 1903 is ready for prime time, OK?

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