Prevent ICS files from being automatically added to my calendar

Question

This has been asked before, but, as far as I can tell, the suggested solution does not work. And this is becoming a security issue, as scammers are attaching ICS files to email and getting an entry on my calendar, which is VERY irritating.

The supposed solution is to "go to the File tab, select Options, choose the Calendar section, and click on the AutoAccept/Decline button. Uncheck the box that says "Automatically process meeting requests and responses to meeting requests and polls." That "solution" does not work. There is also a registry edit on a key called DisableResponseToMeetingInvites at HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar, which looks dangerous (It's unclear exactly what this is doing and I have to be able to respond to legitimate ICS files from clients).

The point is that this is allowing scammers access to my calendar. It's only an entry that is tentative because it is not auto-accepted, but who wants scammers putting ANYTHING on your calendar?! Users need a way to set the processing of these files as fully manual. NOTHING should appear on my calendar until I have opened that file and accepted the meeting request. Is there a reliable way to accomplish this?

Answer 1

Hello R. Campbell,

Thank you for posting to Microsoft Q&A. 

I understand how irritating it is when a "tentative" entry appears on your calendar from a scammer without your consent, especially when you are trying to maintain a professional and clean schedule. 

Currently, Outlook does not have a native "manual only" setting for ICS files because a background process called the "Sniffer" automatically places invites on your calendar to prevent scheduling conflicts. While it is tempting to use registry to stop this, doing so often breaks the calendar's ability to sync and update legitimate meetings from your actual clients.

To help you stay secure while keeping your workflow intact, I recommend using a Client-Side Intercept Rule to filter these unwanted entries.

Instead of trying to disable the system, we can tell Outlook to redirect any meeting request that doesn't come from a known contact. 

1. Open Classic Outlook and go to File > Manage Rules & Alerts. 
2. Click New Rule... and select Apply rule on messages I receive. 
3. For the condition, check the box for which is a meeting invitation or update. 
4. Click Next, and for the action, select move it to the specified folder (choose your Junk Email folder or a dedicated "Meeting Review" folder). 
5. Click Next to get to the Exceptions. 
6. Check the box for except if sender is in specified Address Book (click "specified" and choose your Contacts). 
7. Finish and name the rule (e.g., "Block Unknown Meetings"). 

By moving the email to the Junk folder before the system processes it, you prevent the tentative entry from touching your calendar. Meanwhile, your legitimate clients in your Contacts will bypass the rule and appear on your calendar as usual. 

I know this is a workaround rather than a direct switch, but it is the most reliable way to protect your calendar within the current Exchange architecture. I truly hope this helps you regain control over your schedule and keep those scammers at bay! 

Please let me know if you have any other questions. 

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Answer 2

Outlook does not provide a setting that fully prevents all .ICS-based invitations from creating any calendar entries until manually opened and accepted.

Available controls only cover:

1. Automatic processing of meeting requests and responses in Outlook for Windows
   • The setting described (File > Options > Mail > Tracking > Automatically process meeting requests and responses to meeting requests and polls) controls whether Outlook auto-processes responses and some request handling, but it does not block all ICS-based calendar items from appearing.
   • Related behavior for keeping responses out of the Inbox and auto-updating tracking is also controlled under the Tracking section, but again this does not stop initial calendar entries from being created.
2. Automatic processing and “Events from email” in Outlook on the web
   • In Outlook on the web, under Settings > Mail > Calendar > Events from email, there are options such as:
     • “Don’t add events to my calendar from email”
     • “Automatically add events to my calendar from email”
   • These control events that are detected in email (e.g., travel, reservations), not specifically ICS meeting invitations, and do not provide a granular “never create any calendar item from ICS until I explicitly accept” mode.
3. Attachment blocking and security prompts
   • Outlook’s attachment security can block or warn on certain file types, and there are registry-based methods and Exchange admin controls to change which attachments are blocked. However, the documented guidance focuses on blocking potentially unsafe attachments or allowing additional types, not on treating ICS files as a class that must never create calendar entries.
   • Recent security updates add prompts when opening ICS files in some scenarios, but they do not introduce a “no calendar entry until manual accept” global switch.

Because of these limitations, there is no supported, reliable configuration in Outlook that:

• Allows normal handling of legitimate meeting requests, and
• Simultaneously guarantees that no ICS-based item ever appears on the calendar until explicitly opened and accepted.

Mitigation options within documented behavior are therefore limited to:

• Adjusting automatic processing and tracking options to reduce automatic handling of meeting responses.
• Using Outlook on the web’s Events from email settings to prevent non-invite events from being auto-added where applicable.
• Relying on general attachment and account security practices (e.g., being cautious with unsolicited ICS attachments, and using standard Outlook/Exchange security features) rather than a dedicated “ICS never auto-add” control.

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References:

• Calendar settings in Outlook on the web
• Change how Outlook processes read receipts and meeting responses
• Outlook blocked access to the following potentially unsafe attachments
• Outlook prompts security notice opening .ICS files after installing protections for Microsoft Outlook Information Disclosure Vulnerability released: Dec 12, 2023