Gandalf_The_Grey

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As part of Patch Tuesday, Microsoft released Windows 10 Cumulative Updates KB4571756, KB4574727, and KB4570333 for versions 2004, 1903 and 1909 and 1803. Apparently, BitDefender Antivirus is reportedly flagging KB4574727 as a Trojan “Trojan.Ciusky.Gen.13” and thus, preventing the Windows 10 Septemeber update installation.

The issue has been reported by affected users on Answers and BitDefender forums and on social media sites. Generally, users rely on Windows Update to download and install Cumulative Updates. If they face any issue while downloading or installing the update, they visit Microsoft Update Catalog for manual download.

Windows Update downloads the cumulative Update to the Software Distribution folder and installs the update after that. BitDefender is blocking update installation by flagging download as Trojan.
One user reported on the BitDefender forum confirms installing the update manually by downloading from the Microsoft Update catalog also produces the same result.

Definitely, this is a false alarm from BitDefender Total Security. According to reports, some users went on to uninstall the update following the threat detection.
 

Gandalf_The_Grey

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Seems to be solved by Bitdefender now according to a comment on that article:
N/A says

September 9, 2020 at 9:21 pm

Encountered same issue, so I paused updates yesterday. But after updating BitDefender today, and then trying to update Windows, it allowed Windows to update.
 
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