2026-07-21 · view entry permalink →
HOLLOWGRAPH: a Cavern-framework backdoor that turns a compromised Microsoft 365 calendar into a Graph-API dead-drop C2
Group-IB has published a technical profile of HOLLOWGRAPH, a NativeAOT-compiled .NET DLL it "attribute[s], with high confidence, to the Cavern backdoor framework" (Group-IB, 2026-07-20) — the modular C2 that Check Point Research previously tied to the Iran-MOIS-linked Cavern Manticore actor and that this pipeline has tracked since 2026-07-09. The interest for defenders is not the actor but the command-and-control design, which is built entirely on trusted Microsoft cloud services and leaves almost no attacker-owned network footprint. Infosecurity Magazine corroborates the reporting (Infosecurity Magazine, 2026-07-20).
HOLLOWGRAPH implements only two operations, get and send, and never beacons to attacker infrastructure. Instead it treats a compromised Microsoft 365 mailbox's calendar as a two-way dead-drop over the Graph API. To exfiltrate, the implant encrypts a file with hybrid RSA-OAEP + AES-256-GCM (separate key pairs per direction), creates a calendar event dated far in the future — 2050-05-13, in a fixed 22:00–23:00 UTC window — so the mailbox owner is unlikely to notice, uploads the ciphertext as event attachments, and renames the event subject to an operator-recognisable tag. To receive tasking, it queries the same calendarView window, filters events by subject, downloads the attachment planted by the operator, and decrypts it. A separate, unencrypted channel refreshes the four Entra ID (Azure AD) values the implant needs — tenant ID, client ID, client secret and target mailbox — by DNS tunneling: length- and data-encoded queries against an attacker domain resolved as IPv6 AAAA records and reassembled into fixed-size chunks.
The kill chain, described for reasoning about detection: the implant authenticates as an application/service identity to Microsoft Graph and drives calendar operations programmatically — the Graph-API calendar dead-drop is bidirectional web-service command-and-control and the credential refresh rides DNS as an application-layer channel; the calendar payloads are encrypted end-to-end; and the implant operates against cloud-account credentials rather than an on-host identity. Group-IB is explicit about the attribution ceiling: it "cannot confidently attribute this activity to any previously identified threat actor," assessing only a low-confidence technical overlap with the Iranian-nexus Lyceum sub-group — so this is a Cavern-framework component, not a confirmed named-actor campaign. Telemetry to date is narrow: 12 infected systems, roughly 3 actively communicating, all evidence pointing to Israeli organisations, with activity observed between 3 June and 9 July 2026.
Group-IB Threat Intelligence team has identified HOLLOWGRAPH, a new malware sample that we attribute, with high confidence, to the Cavern backdoor framework
we cannot confidently attribute this activity to any previously identified threat actor.