Microsoft is investigating an issue that triggers Outlook security alerts when trying to open .ICS calendar files after installing December 2023 Patch Tuesday Office security updates.
Microsoft 365 users affected by this issue report seeing dialog boxes warning them that "Microsoft Office has identified a potential security concern" and that "This location may be unsafe" when double-clicking ICS files saved locally. Read More..
"This behavior is not expected when opening .ICS files. This is a bug and will be addressed in a future update.”