Chinese state-backed threat actor APT41 has been discovered leveraging Google Calendar as a command-and-control (C2) channel in a sophisticated cyber campaign, according to Google’s Threat Intelligence Group (TIG). The team has since dismantled the infrastructure and implemented defenses to block similar future exploits.
The campaign began with a previously breached government website — though TIG didn’t disclose how it was compromised — which hosted a ZIP archive. This file was distributed to targets via phishing emails.
Once downloaded, the archive revealed three components: an executable file and a dynamic-link library (DLL) disguised as image files, and a Windows shortcut (LNK) masquerading as a PDF. When users attempted to open the phony PDF, the shortcut activated the DLL, which then decrypted and launched a third file containing the actual malware, dubbed ToughProgress.
Upon execution, ToughProgress connected to Google Calendar to retrieve its instructions, embedded within event descriptions or hidden calendar events. The malware then exfiltrated stolen data by creating a zero-minute calendar event on May 30, embedding the encrypted information within the event's description field.
Google noted that the malware’s stealth — avoiding traditional file installation and using a legitimate Google service for communication — made it difficult for many security tools to detect.
To mitigate the threat, TIG crafted specific detection signatures, disabled the threat actor’s associated Workspace accounts and calendar entries, updated file recognition tools, and expanded its Safe Browsing blocklist to include malicious domains and URLs linked to the attack.
Several organizations were reportedly targeted. “In partnership with Mandiant Consulting, GTIG notified the compromised organizations,” Google stated. “We provided the notified organizations with a sample of TOUGHPROGRESS network traffic logs, and information about the threat actor, to aid with detection and incident response.”
Google did not disclose the exact number of impacted entities.
