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HOLLOWGRAPH

tool · tool:hollowgraph-malware

NativeAOT .NET backdoor Group-IB links with high confidence to the Cavern C2 framework; abuses the Microsoft Graph API to turn a compromised M365 mailbox calendar into a two-way dead-drop (far-future events, hybrid RSA-OAEP + AES-256-GCM attachments) with DNS-tunneled Microsoft Entra ID credential refresh. Narrowly targets Israeli organisations; observed 3 June - 9 July 2026 (Group-IB, 2026-07-20).

Coverage
1
first 2026-07-21 → last 2026-08-12
Latest activity
2026-08-12
Group-IB details HOLLOWGRAPH, a .NET implant using a victim's own M365 calendar as two-way C2 over the Graph…
Peak priority
notable
1 notable
Targets
public-sector
sectors: public-sector, technology · regions: middle-east
Sources cited
5
4 hosts

Defender insights

What each entry about HOLLOWGRAPH tells a defender to do, newest first.

2026-07-21NOTABLEGroup-IB details HOLLOWGRAPH, a .NET implant using a victim's own M365 calendar as two-way C2 over the Graph API, with DNS-tunneled Entra credential refresh

Latest update · triage

Relationships explore in graph

Typed, source-stated connections from the entity registry; each edge cites the entry whose reporting establishes it.

variant of

Story timeline

  1. 2026-07-21HOLLOWGRAPH: a Cavern-framework backdoor that turns a compromised Microsoft 365 calendar into a Graph-API dead-drop C2
    deep-diveGroup-IB details HOLLOWGRAPH, a .NET implant using a victim's own M365 calendar as two-way C2 over the Graph API, with DNS-tunneled Entra credential refresh

Hunting pivots

Affected products
Microsoft 365Microsoft Entra IDMicrosoft GraphMicrosoft Graph APIMicrosoft Outlook
Tags
cloudespionageidentityiran-nexusnation-state
ATT&CK techniques (9 across 5 tactics)

9 techniques observed across 1 entry about this entity, derived from entry metadata and body evidence, never asserted without a published entry behind it · pinned to MITRE ATT&CK v20.0 · compare on the matrix · Navigator layer (JSON)

  • Initial AccessValid Accounts: Cloud Accounts
  • PersistenceValid Accounts: Cloud Accounts
  • Privilege EscalationValid Accounts: Cloud Accounts
  • StealthObfuscated Files or Information · Masquerading: Match Legitimate Resource Name or Location · Valid Accounts: Cloud Accounts
  • Command and ControlFallback Channels · Application Layer Protocol: DNS · Web Service: Bidirectional Communication · Ingress Tool Transfer · Dynamic Resolution · Encrypted Channel

Initial Access TA0001

T1078.004Valid Accounts: Cloud Accounts×1

Valid accounts in cloud environments may allow adversaries to perform actions to achieve Initial Access, Persistence, Privilege Escalation, or Defense Evasion. Cloud accounts are those created and configured by an organization for use by users, remote support, services, or for administration of resources within a cloud service provider or SaaS application. Cloud Accounts can exist solely in the cloud; alternatively, they may be hybrid-joined between on-premises systems and the cloud through syncing or federation with other identity sources such as Windows Active Directory.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

Persistence TA0003

T1078.004Valid Accounts: Cloud Accounts×1

Valid accounts in cloud environments may allow adversaries to perform actions to achieve Initial Access, Persistence, Privilege Escalation, or Defense Evasion. Cloud accounts are those created and configured by an organization for use by users, remote support, services, or for administration of resources within a cloud service provider or SaaS application. Cloud Accounts can exist solely in the cloud; alternatively, they may be hybrid-joined between on-premises systems and the cloud through syncing or federation with other identity sources such as Windows Active Directory.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

Privilege Escalation TA0004

T1078.004Valid Accounts: Cloud Accounts×1

Valid accounts in cloud environments may allow adversaries to perform actions to achieve Initial Access, Persistence, Privilege Escalation, or Defense Evasion. Cloud accounts are those created and configured by an organization for use by users, remote support, services, or for administration of resources within a cloud service provider or SaaS application. Cloud Accounts can exist solely in the cloud; alternatively, they may be hybrid-joined between on-premises systems and the cloud through syncing or federation with other identity sources such as Windows Active Directory.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

Stealth TA0005

T1027Obfuscated Files or Information×1

Adversaries may attempt to make an executable or file difficult to discover or analyze by encrypting, encoding, or otherwise obfuscating its contents on the system or in transit. This is common behavior that can be used across different platforms and the network to evade defenses.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1036.005Masquerading: Match Legitimate Resource Name or Location×1

Adversaries may match or approximate the name or location of legitimate files, Registry keys, or other resources when naming/placing them. This is done for the sake of evading defenses and observation.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1078.004Valid Accounts: Cloud Accounts×1

Valid accounts in cloud environments may allow adversaries to perform actions to achieve Initial Access, Persistence, Privilege Escalation, or Defense Evasion. Cloud accounts are those created and configured by an organization for use by users, remote support, services, or for administration of resources within a cloud service provider or SaaS application. Cloud Accounts can exist solely in the cloud; alternatively, they may be hybrid-joined between on-premises systems and the cloud through syncing or federation with other identity sources such as Windows Active Directory.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

Command and Control TA0011

T1008Fallback Channels×1

Adversaries may use fallback or alternate communication channels if the primary channel is compromised or inaccessible in order to maintain reliable command and control and to avoid data transfer thresholds.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1071.004Application Layer Protocol: DNS×1

Adversaries may communicate using the Domain Name System (DNS) application layer protocol to avoid detection/network filtering by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1102.002Web Service: Bidirectional Communication×1

Adversaries may use an existing, legitimate external Web service as a means for sending commands to and receiving output from a compromised system over the Web service channel. Compromised systems may leverage popular websites and social media to host command and control (C2) instructions. Those infected systems can then send the output from those commands back over that Web service channel. The return traffic may occur in a variety of ways, depending on the Web service being utilized. For example, the return traffic may take the form of the compromised system posting a comment on a forum, issuing a pull request to development project, updating a document hosted on a Web service, or by sending a Tweet.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1105Ingress Tool Transfer×1

Adversaries may transfer tools or other files from an external system into a compromised environment. Tools or files may be copied from an external adversary-controlled system to the victim network through the command and control channel or through alternate protocols such as ftp. Once present, adversaries may also transfer/spread tools between victim devices within a compromised environment (i.e. Lateral Tool Transfer).

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1568Dynamic Resolution×1

Adversaries may dynamically establish connections to command and control infrastructure to evade common detections and remediations. This may be achieved by using malware that shares a common algorithm with the infrastructure the adversary uses to receive the malware's communications. These calculations can be used to dynamically adjust parameters such as the domain name, IP address, or port number the malware uses for command and control.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

T1573Encrypted Channel×1

Adversaries may employ an encryption algorithm to conceal command and control traffic rather than relying on any inherent protections provided by a communication protocol. Despite the use of a secure algorithm, these implementations may be vulnerable to reverse engineering if secret keys are encoded and/or generated within malware samples/configuration files.

Evidence: 2026-07-21/hollowgraph-m365-calendar-graph-api-c2-cavern · ATT&CK page ↗

Entries about HOLLOWGRAPH (1)

2026-07-21 · view entry permalink →

NOTABLEupdatedNATOB2

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.

Group-IB Threat Intelligence 2026-07-20

If Microsoft Graph authentication or tenant validation fails, the module attempts to retrieve replacement connection settings through DNS AAAA responses.

The new module shares several behavioral patterns with previously reported OilRig tooling, including the use of Microsoft-hosted services, attachment-based command exchange, and a secondary mechanism for restoring access to a cloud C2 channel.

Kaspersky (Securelist / GReAT) 2026-07-21

Project CAV3RN is a modular espionage framework used against targets in Israel.

The main finding is a complex C2 module that uses DNS A-record responses to choose between direct HTTPS and a Google Apps Script relay for each transaction. The same DNS infrastructure can validate and replace the relay deployment ID, allowing the operator to rotate the Google channel.

Kaspersky Securelist (GReAT) 2026-08-11
Updaterun 2026-07-22T0409Z-intelaffected_productsentitiesevidencesourcestechniquesbody

The HOLLOWGRAPH entry documented an Iran-linked backdoor that used Microsoft Graph and far-future Outlook calendar events as its command-and-control channel. Kaspersky GReAT has now published independent analysis of the same toolset (which Check Point tracks as "Cavern Manticore") detailing a new communication module (AzureCommunication.dll) that replaces the earlier HTTP/WebSocket component with Microsoft Graph, exchanging RSA-OAEP-SHA256 + AES-256-GCM-encrypted commands and results as attachments inside far-future Outlook calendar events (a fixed 2050-05-13 window) keyed to a controller-generated agent ID (Kaspersky, 2026-07-21; Check Point Research, 2026-07-06).

The new element beyond prior reporting is a resilience layer: when Graph authentication or tenant validation fails, the module recovers replacement connection settings (TenantId, ClientId, ClientSecret, UserEmail) via DNS AAAA responses from attacker-controlled nameservers, encoding length markers and 14-byte chunks in specially formatted subdomains. On attribution, Kaspersky retains its low-confidence assessment that Project CAV3RN is associated with OilRig (APT34) (a link it first drew in a previous report) noting the new module shares behavioural patterns with previously reported OilRig tooling (Microsoft-hosted-service C2, attachment-based command exchange, a secondary cloud-C2 recovery mechanism) while explicitly identifying no direct code reuse or infrastructure overlap (Kaspersky, 2026-07-21). Treat the OilRig association as an analytic lead, not a settled attribution.

Updaterun 2026-08-12T0411Z-intelevidencesourcestechniquesbody

Kaspersky's GReAT team published a further instalment on Project CAV3RN on 2026-08-11, describing it as "a modular espionage framework used against targets in Israel" and expanding on two earlier publications (Kaspersky Securelist, 2026-08-11). The prior entry here covered the framework's DNS-based C2 fallback and Kaspersky's low-confidence association with OilRig. The delta is a channel-selection design that is worth carrying into detection engineering regardless of who operates it.

Kaspersky states: "The main finding is a complex C2 module that uses DNS A-record responses to choose between direct HTTPS and a Google Apps Script relay for each transaction. The same DNS infrastructure can validate and replace the relay deployment ID, allowing the operator to rotate the Google channel" (Kaspersky Securelist, 2026-08-11). The mechanics are specific enough to hunt on. The communication module is a 64-bit DLL compiled with .NET 8 NativeAOT. Before polling for commands or sending a result, it issues an A-record query for a name built from a short random nonce concatenated with a numeric error state, then a hex-encoded client identifier, under a fixed operator-controlled domain. One exact address is treated as a rejection; otherwise the module reads the fourth octet of the answer and maps it, in combination with the current error state, onto direct HTTPS, the Apps Script relay, an exception, or closing the transaction with no channel at all. A recovered Apps Script deployment ID is written back to the module's on-disk configuration, while other configuration changes pushed by the operator stay in memory. The two channels differ in shape as well as destination. On the direct-HTTPS path the module contacts a configured attacker-controlled address whose endpoint is gated on a custom client-identifier HTTP header, returning a failure response to requests without it and an encoded tasking body to requests carrying it. On the Apps Script path the module instead POSTs a JSON envelope to the deployment URL, with the upstream method and the headers to replay (the same client-identifier value among them) carried as fields inside that JSON body rather than as headers on the request to Google. Tasking comes back base64-encoded and XORed either way.

The second new component is an inter-component broker, a 64-bit Visual C++ DLL that masquerades as the RNP OpenPGP library through a set of rnp_* exports, with one of those exports starting the broker. At startup it creates its control structure, initialises a message dispatcher and scans the host directory for DLLs, grouping candidates by their CompanyName resource and loading the highest-versioned member of each group that exposes four specific named exports. It rescans that directory every second, so a component can be added or upgraded without restarting the host, but only by dropping a higher-versioned DLL under a new path, because replacing a file in place is not detected (Kaspersky Securelist, 2026-08-11).

Triage: high-volume DNS lookups under a single parent domain are also how legitimate telemetry agents, CDN clients and some licence checks behave, so the query volume alone is not the signal. The discriminators the described mechanism supports are the label structure (a short changing nonce plus a stable hex-encoded identifier per host, rather than a service-shaped name) and the tight temporal coupling, with one lookup preceding each outbound connection rather than a periodic refresh independent of traffic. Note what is not available as a discriminator on the relay path: the custom client-identifier travels inside the JSON body of a TLS POST to a legitimate Google endpoint, so it is not visible to header inspection or to anything short of TLS interception at the proxy.

threat21 Jul 04:43Zmulti-sourceOpen finding →
Sources: Group-IB Threat Intelligence · Infosecurity Magazine · Kaspersky (Securelist / GReAT) · Check Point Research · Kaspersky Securelist (GReAT)

Co-occurring entities

Derived: referenced by the same focused operational entries (weekly summaries and report roundups don't count); ×N counts the shared entries.

Where this entity is cited

  • Deep dive1

Source distribution

  • securelist.com2 (40%)
  • group-ib.com1 (20%)
  • infosecurity-magazine.com1 (20%)
  • research.checkpoint.com1 (20%)
All cited sources (5)