Good news. On Sunday, March 19, 2023, Microsoft confirmed that the warning message that Microsoft Defender displayed when unzipping the Sibelius 2023.3 installer was a false positive and have MARKED IT SAFE in the latest security intelligence in Microsoft Defender.
You are advised to update to Microsoft Defender version 1.385.456.0 and above. This is usually done automatically, but to check this, go to Windows Security > Virus & threat protection and click "Check for updates".
Thank you for your patience while we investigated with Microsoft this weekend to clarify the validity of the warning and integrity of our installer. We will keep you updated on any news as soon as we can.