Additionally, based on the infrastructure and victims’ location, Group-IB experts suggest that Israel is the likely target of this malware.
The HollowGraph malware component is designed to self-register in the Microsoft Graph API using the credentials stolen from the Microsoft 365 mailbox. It stores the configuration data in the logAzure.txt file, which contains the Microsoft Entra ID information, client credentials, mailbox addresses of the victims, domains controlled by the attackers, and keys required to communicate with the C2 infrastructure.
The attackers have taken measures to ensure that this file is not suspicious; specifically, it is placed in the known location used by Microsoft Entra ID and has the standard log file name.
The malware communicates with its C2 server using the Microsoft 365 calendars. Specifically, HollowGraph creates events scheduled on May 13, 2050, and uses their titles and attachments to exchange data with the attackers. There are two types of such events: GET and SEND. The first one is used to retrieve the encrypted commands by extracting the contents of the event’s title.
In turn, the SEND type of events is used to exfiltrate the stolen data by adding it as an attachment.
Researchers note that the mailbox calendars are used as a “dead drop” to store the data; hence, HollowGraph does not use traditional C2 servers to avoid detection. It implements a strong encryption scheme to protect the command and data exfiltration channels and uses a combination of RSA and AES_256_GCM encryption algorithms. The RSA public key is embedded into the calendar event, whereas the HollowGraph malware uses the AES key to encrypt the data.
Besides the Microsoft Graph API, HollowGraph also uses the Domain Name System (DNS) to communicate with its C2 servers. Specifically, the malware resolves the domains controlled by the attackers to extract the IPv6 AAAA records, which contains the updated Microsoft Entra ID credentials required to maintain persistence on the compromised mailboxes. Similar to the information stored in the logAzure.txt file, these credentials include the Microsoft Entra ID tenant, client ID and secret, and the mailbox information.
All of these credentials are extracted from the DNS responses and stored in the malware configuration.
Group-IB researchers conclude that HollowGraph is a sophisticated backdoor that utilizes various cybersecurity technologies to compromise targeted Microsoft 365 mailboxes and remain undetected for as long as possible. While the technical capabilities of this malware component overlaps with the ones attributed to the Lyceum Iranian nation-state actor, the researchers are not certain about its origin.
Nevertheless, Group-IB experts note that there is a high likelihood that HollowGraph belongs to the Cavern framework used by Lyceum.
To detect and prevent similar attacks, the cybersecurity experts recommend that organizations monitor the Microsoft Graph API activity and Microsoft 365 audit logs for any suspicious activities related to the creation of the calendar events. Specifically, defenders should pay attention to the events created by applications using the Microsoft Graph API scheduled far in the future, with the suspicious subjects and attachments.
The researchers also recommend that organizations add the cloudlanecdn[.]com domain to their threat intelligence platforms and continuously monitor their Microsoft 365 environments for any unauthorized OAuth client credential applications. In addition, Group-IB experts note that Microsoft Entra ID Conditional Access policies and outbound DNS traffic should be reviewed to detect and block similar恶意 activities, such as DNS tunneling.
This report highlights the importance of the growing threat landscape in cloud environments caused by the increasing reliance on the collaborative software in enterprise networks.
Malware components like HollowGraph demonstrate that the attackers do not limit themselves to traditional network security tools and can use the trusted infrastructure to launch attacks against various organizations. In particular, the attackers utilize the cloud infrastructure as a part of their mitigation strategy. In turn, the defenders should shift their focus from traditional network perimeter security to inspecting individual hosts and applications for detecting malicious activities.
