Beware: Hackers Hijack Calendar Subscriptions for Attacks
▼ Summary
– Threat actors are exploiting digital calendar subscriptions to deliver malicious content through third-party servers that can add events directly to users’ devices.
– Malicious calendar subscriptions are often hosted on expired or hijacked domains, enabling large-scale social engineering attacks.
– BitSight’s research identified 347 suspicious calendar domains that were contacted by approximately four million unique IP addresses daily, with the highest concentration in the US.
– The risks include phishing, malware distribution, JavaScript execution, and attacks exploiting emerging technologies like AI assistants through harmful calendar files.
– Calendar subscriptions represent an overlooked security blind spot, with less robust defenses compared to email, creating vulnerabilities in personal and corporate security.
A growing cybersecurity threat involves hackers manipulating digital calendar subscriptions to deliver malicious content directly to users’ devices. This method exploits the legitimate functionality that allows third parties, such as retailers or sports organizations, to automatically add events and send notifications through calendar services. Recent findings from BitSight reveal that cybercriminals are setting up deceptive systems to trick individuals into subscribing to these harmful calendar feeds.
These malicious calendar subscriptions often operate through expired or hijacked domains, making them effective tools for large-scale social engineering campaigns. Once a user subscribes, attackers can push calendar files containing dangerous elements like phishing links, malware-laden attachments, or even scripts that execute automatically. The potential harm extends from traditional phishing and malware distribution to more advanced threats leveraging JavaScript or emerging technologies like AI assistants.
BitSight’s investigation began after sinkholing a single domain linked to a calendar distributing German public and school holidays. Sinkholing is a cybersecurity practice that redirects malicious traffic to a controlled server for analysis. Researchers noted that this domain alone attracted 11,000 unique IP addresses daily, raising immediate suspicions about its true purpose.
Expanding their probe, the team uncovered an additional 347 suspicious domains hosting calendars for various themes, including FIFA 2018 events and the Islamic Hijri calendar. Collectively, these domains were accessed by roughly four million unique IP addresses every day, with the highest concentration of users located in the United States.
Analysis of sync requests in the sinkhole indicated that these were not new subscriptions but background synchronization attempts from calendars users had previously subscribed to. This means that anyone controlling an expired or hijacked domain could respond to these sync requests with manipulated calendar files, inserting new, harmful events directly into users’ calendars.
It is important to clarify that this research does not point to a vulnerability within platforms like Google Calendar or iCalendar. Instead, the security risk stems from the use of third-party calendar subscriptions. While companies such as Apple and Google have strengthened security across their ecosystems, BitSight’s report emphasizes that calendar-based threats represent an emerging risk that may not yet be fully mitigated.
The report concludes that awareness and protective measures around calendar subscriptions need significant strengthening, especially when compared to more established defenses for email systems. This security gap poses a serious risk to both individual users and corporate networks, highlighting the need for improved vigilance and updated security protocols surrounding calendar sync services.
(Source: InfoSecurity Magazine)
