Google has acknowledged a vulnerability in the Google Calendar app is being exploited by cybercriminals to inject fake and malicious items into Google Calendar.
Several Google Calendar phishing campaigns were detected over the summer of 2019 which were exploiting this flaw. The campaigns saw Google Calendar spam sent to large numbers of users, including invites to events and other requests and special offers that popped up on unsuspecting users’ screens.
These notifications contained links to webpages where users could find out more information about the events and special offers. If events were accepted, they would be inserted into users’ calendars and would trigger automatic notifications. The offers and invites would keep on appearing until the users’ clicked the link. Those links directed users to phishing pages where credentials were harvested.
Some of the scams required credit card information to be entered, others required the user to login using their Office 365 credentials. Links could also direct users to webpages where drive-by malware downloads take place.
Most people are aware of the threat of phishing emails, malicious text messages, and social media posts that harvest sensitive information, but attacks on calendar services are relatively unheard of. Consequently, many users will fail to recognize these notifications and calendar items as malicious, especially when they appear in a trusted app such as Google Calendar.
Unfortunately, these attacks are possible because in the default setting, anyone can send a calendar event to a user. That event will be inserted into the user’s calendar and will automatically trigger notifications, as is the case with legitimate events.
In addition to events, messages can include special offers, notifications of cash prizes, alerts about money transfers, and all manner of other messages to entice the user to click a malicious link and disclose sensitive information or download malware.
Google Calendar is not the only calendar service that is prone to these attacks. Apple users have also been targeted, as have users of other calendar apps.
How to Block Google Calendar Phishing Attacks
Recently, a Google employee acknowledged the increase in ‘calendar spam’ and confirmed action was being taken by Google to address the problem.
In the meantime, users can prevent these spam and phishing messages from appearing by making a change to the app settings. Users should navigate to Event Settings > Automatically Add Invitations, and select the option “No, only show invitations to which I’ve responded” and uncheck the “show declined events” option in View Options.
Businesses should also consider including Google Calendar phishing scams in their security awareness training programs to ensure employees are aware that phishing attacks are not limited to email, text message, telephone calls, and social media posts.