Researchers from Microsoft Threat Intelligence Center (MSTIC) identified FoggyWeb, a new custom malware utilized by the Nobelium APT group to distribute further payloads and steal critical information from Active Directory Federation Services (AD FS) servers.
FoggyWeb is a post-exploitation backdoor utilized by the APT group to remotely exfiltrate the setup databases of affected Active Directory Federation Services (AD FS) servers, as well as the decrypted token-signing and token-decryption certificates. It also enables threat actors to download and execute additional elements.
The analysis published by Microsoft stated, “Once NOBELIUM obtains credentials and successfully compromises a server, the actor relies on that access to maintain persistence and deepen its infiltration using sophisticated malware and tools. NOBELIUM uses FoggyWeb to remotely exfiltrate the configuration database of compromised AD FS servers, decrypted token-signing certificate, and token-decryption certificate, as well as to download and execute additional components.”
“Use of FoggyWeb has been observed in the wild as early as April 2021.”
The hackers load FoggyWeb from the encrypted file Windows.Data.TimeZones.zh-PH.pri using the version.dll DLL. The version.dll is loaded by the AD FS service executable 'Microsoft.IdentityServer.ServiceHost.exe' via the DLL search order hijacking approach, which involves the core Common Language Runtime (CLR) DLL files.
To decrypt the backdoor directly in memory, the loader employs a proprietary Lightweight Encryption Algorithm (LEA) function. The backdoor sets up HTTP listeners for actor-defined URIs in order to intercept GET/POST requests to the AD FS server that match the custom URI patterns.
Microsoft researchers offered the following advice to companies that have been affected or are suspected of being under attack by the group:
- Examine your on-premises and cloud infrastructure, including configuration, per-user and per-app settings, forwarding rules, and any other modifications made by the actor to retain their access.
- Remove user and app access, evaluate each's settings, and re-issue fresh, strong credentials in accordance with established industry best practices.
- To prevent the exfiltration of secrets via FoggyWeb, use a hardware security module (HSM), as explained in Securing AD FS servers.
The NOBELIUM APT is the threat actor behind the SolarWinds supply chain assault, which included various implant families such as the SUNBURST backdoor, TEARDROP malware, GoldMax malware, Sibot, and GoldFinder backdoors.
NOBELIUM focuses on government agencies, non-governmental organizations (NGOs), think tanks, military, information technology service providers, health technologies and research, and telecommunications providers.
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