Threat actors have been increasingly exploiting Microsoft Graph API to facilitate stealthy communications with their command-and-control (C&C) infrastructures, effectively evading detection. By using Microsoft’s cloud services as a host, these actors manage to blend their malicious traffic with legitimate traffic, making it difficult for targeted organizations to identify suspicious activities. This trend has been noted by the Symantec Threat Hunter Team, who observed that several nation-state-aligned hacking groups have adopted this technique since January 2022.
The abuse of Microsoft Graph API dates back to at least June 2021 with the discovery of a custom implant named Graphon. This implant was part of an activity cluster known as Harvester and was specifically designed to communicate with Microsoft infrastructure, thus hiding its malicious activities within legitimate service traffic. This method’s effectiveness has led to its adoption by various notorious groups such as APT28, REF2924, Red Stinger, Flea, APT29, and OilRig, signaling a significant shift in how sophisticated threat actors operate.
A recent example of this technique was detected by Symantec in Ukraine, where a previously undocumented malware called BirdyClient (also known as OneDriveBirdyClient) was deployed. This malware utilized a DLL file that mimicked a legitimate DLL associated with the application Apoint and connected to Microsoft Graph API to use OneDrive as a C&C server for uploading and downloading files. The exact distribution method of the DLL file remains unclear, as does the full scope of the attackers’ objectives and identities.
The increasing reliance on Microsoft Graph API by attackers is partly driven by the API’s ability to camouflage malicious communications with legitimate traffic to widely used cloud services, which are less likely to arouse suspicion. This strategy not only hides the attackers’ activities but also provides them with a secure and low-cost infrastructure, as basic accounts for services like OneDrive are free. The trend underscores the challenges organizations face in securing their networks against sophisticated cyber threats that exploit trusted relationships and widely adopted technologies.
