Last night Microsoft pushed out a patch for folks running the latest release of Windows 10 (version 1607). Unless you’ve taken steps to block updates, your PC probably installed it overnight. But there’s a problem: Nobody can say for sure what the patch does. There’s no documentation – or more accurately, there’s a surfeit of documentation, with at least two credible, conflicting descriptions of KB 3211320.

It’s likely that KB 3211320 is a servicing stack update – a new version of the software that handles Windows patches. But there’s also a credible description in Microsoft’s own posts that says KB 3211320 is a critical patch for Microsoft Edge, identified as MS17-001.

Yes, Gildna, new KB 3211320 is

The explanation I find most credible can be found in the Microsoft Update Catalog. , then click on the link for the version of the patch that pertains to you to see the usual Update Details. Click on the tab marked Package Details (see screenshot) and the patch is clearly marked as superseding both KB 3199209 and KB 3199986. Those are the Windows 10 1607 servicing stack updates released on Oct. 18 and Oct. 27, respectively.

. Microsoft has a history of pushing Win10 servicing stack updates without any warning and only a dribble of recognition long after the fact.

Poster Enthousiast on My Digital Life and independently confirmed its components.

Of course, as of this writing there’s no KB article for KB 3211320. You’re expected to install it because, you know, Microsoft.

Paradoxically, Microsoft also lists KB 3211320 as a Critical update for Microsoft Edge running on Windows 10 1607, as part of MS17-001. It’s an Important / Elevation of Privilege bug that, according to the documentation, requires a restart. As many of you can attest, the KB 3211320 we got last night doesn’t require a restart.

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Thanks to ch100 on AskWoody and Enthousiast on MDL.