Hackers exploit vulnerabilities that allow them to add spam events to a victim's calendars. All they need to know is the Gmail attached to a Google calendar to add an event.
The issue is that the default security settings in Google Calendar automatically allow an event invitation to be added to a calendar when the invitation is sent via Gmail. The event is added even if the recipient doesn't respond. If a user selects No, the event won't appear on their calendar. However, if you share your calendar with others, they will be able to see all the spam events even if you haven't responded.

How can you stop it?
Currently, when someone sends a calendar event invitation to a user’s Gmail address, the event is automatically added to the recipient’s calendar by default, unless the recipient has changed these settings. To prevent Google Calendar from automatically adding any invitations sent through Gmail, the default event settings setting should be set to “No, only show invitations I’ve responded to.” This way, a calendar won’t randomly show invitations sent by someone who knows Gmail . Defaulting to “no” will also stop anyone you share the calendar with from seeing invitations that have been declined.
How to Stop Gmail from Adding Events – Google Calendar Web Calendar
- Log in to Google Calendar
- Click the gear icon in the top right corner
- Select Settings
- In the “Event Settings” section
- Select No, only show invitations I've responded to
Finally, if you declined spam and don't want to see the removed calendar entries, mark each one as spam to remove them. To remove them, change the setting below.
- Log in to Google Calendar
- Click the gear icon in the top right corner
- Select View options
- In the “Event Settings” section
- Uncheck “Show declined events”

How to Stop Gmail from Adding Events – Google Calendar Web App
- Log in to Google Calendar
- Click the gear icon in the top right corner
- Select “Settings”
- In the “Event Settings” section
- Select “No” to automatically add invitations
Note: With this set off, Google Calendar will only show invitations that you have responded to. Google Calendar users should never respond to an invitation, as this could result in a spam message remaining as a deleted event on your calendar. Report the invitation as spam.

How to add events to Google Calendar – Google Calendar App
- Log in to Google Calendar
- Tap the hamburger menu in the top left corner of the app
- Scroll down and tap the gear icon at the bottom of the menu
- Select “Settings”
- Select “Events” from Gmail
- Toggle Events from Gmail to Off
How to prevent iPhone or iPad from adding events from email to Calendar
- Open the Settings app on your iPhone / iPad
- Scroll down and select Mail, Contacts, Calendars
- Scroll down to the CALENDARS section
- Toggle events detected in Mail to off
