A newly discovered espionage implant has been using a hijacked Microsoft 365 calendar as its command channel, planting operator instructions and smuggling out stolen files as attachments on calendar events dated to the year 2050.
Group-IB, which named the malware HollowGraph, says the approach moves tasking and stolen data through legitimate Microsoft Graph API traffic, so the activity looks like ordinary Microsoft 365 chatter and network controls keyed to attacker-owned destinations have nothing to flag.
The implant is a .NET DLL that supports just two commands, get and send, and it never reaches out to an attacker-owned server for payloads. Instead, it treats the compromised mailbox’s calendar as a two-way dead drop.
To pull tasking, it queries the calendar for an event the operator has planted, buried at 2050-05-13, so the mailbox owner is unlikely to ever scroll to it, and reads its instructions from an attached file. To exfiltrate, it does the reverse: encrypts the stolen file, creates its own far-future event, and uploads the data as one or more attachments.
Everything moving through the calendar is wrapped in hybrid RSA and AES-256 encryption, with separate key pairs for incoming tasking and outgoing data.
A second, cruder channel keeps that Graph access alive. Over DNS, HollowGraph refreshes the app’s Entra ID (Azure AD) login details: its tenant ID, client ID, client secret and target mailbox. It decodes those values from IPv6 AAAA records sent back by an attacker domain, cloudlanecdn[.]com, then writes them to logAzure.txt, a file dressed up as a routine log. Those are stored client-credentials, not access tokens, and unlike the calendar traffic this channel runs in the clear.
Group-IB ties HollowGraph to Cavern with high confidence, on shared command syntax and matching internal tasking. Cavern is a modular backdoor framework that Check Point documented earlier this month and attributed to an Iranian Ministry of Intelligence and Security-linked actor it calls Cavern Manticore, a cluster it says overlaps with the known Iranian groups MuddyWater and Lyceum.
But the firm link is to the code, not the crew. Group-IB will not name the operator behind this campaign: “Based on the evidence currently available, we cannot confidently attribute this activity to any previously identified threat actor,” it wrote, noting only a low-confidence overlap with Lyceum, a subgroup of the Iranian actor OilRig. The compromised exfiltration mailbox belongs to an Israeli organization, but Group-IB treats that as victim geography, not attribution.
Group-IB found the implant on at least 12 machines, only about three of them actively talking to the attacker during the analysis window, with victim traffic running from 3 June to 9 July 2026. It reads that small, selective footprint as targeted espionage rather than opportunistic crime, even though the technique behind it could be reused far more widely than this one campaign.
There is no Microsoft software vulnerability here and no patch to deploy. HollowGraph rides a compromised account and the Graph API’s normal functionality, which is exactly why it is hard to spot. The work is on identity and application permissions plus monitoring, not a patch.
What to watch for
Group-IB’s detection advice leans on the malware’s own habits, and the sharpest signals live in the calendar itself. Hunt for events that carry:
- a far-future date, specifically
2050-05-13; - a subject that is a bare GUID, or that matches the operator’s
Event ID:orBoss{..}ID{..}naming; - attachments named
File{n}.txt.
On the identity side, the firm recommends restricting and auditing the client-credential OAuth apps that can reach Graph and alerting on newly created client secrets, plus the usual Entra ID hygiene of Conditional Access, credential rotation, and anomalous-token detection.
For detection, audit Microsoft Graph and mailbox activity for application-driven calendar changes: events created, attachments uploaded, or subjects renamed by an app rather than a person. Watch DNS too, for unusually frequent AAAA queries and long, high-entropy subdomains aimed at a single domain. Hunting for cloudlanecdn[.]com and the logAzure.txt config file is the fast first pass, and the full indicator set, including file hashes, is in Group-IB’s report.
Hiding command-and-control in trusted Microsoft services is not new; attackers have already run it through Outlook inboxes, draft folders and OneDrive. Events parked in 2050, out where no one looks, are simply the latest spot defenders had no reason to inspect. The operator still has no name, and victim traffic was flowing as recently as 9 July, which makes those far-future calendar entries worth a look now.
