Microsoft has detected multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks.

In the attacks observed, the threat actor used these vulnerabilities to access on-premises Exchange servers which enabled access to email accounts and allowed the installation of additional malware to facilitate long-term access to victim environments.

Microsoft Threat Intelligence Center (MSTIC) attributes this campaign with high confidence to HAFNIUM, a group assessed to be state-sponsored and operating out of China, based on observed victimology, tactics, and procedures.

The vulnerabilities recently being exploited were CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065, all of which were addressed in today’s Microsoft Security Response Center (MSRC) release – Multiple Security Updates Released for Exchange Server.

Who is HAFNIUM?

HAFNIUM primarily targets entities in the United States across a number of industry sectors, including infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs.

HAFNIUM has previously compromised victims by exploiting vulnerabilities in internet-facing servers, and has used legitimate open-source frameworks, like Covenant, for command and control. Once they’ve gained access to a victim network, HAFNIUM typically exfiltrates data to file sharing sites like MEGA.

In campaigns unrelated to these vulnerabilities, Microsoft has observed HAFNIUM interacting with victim Office 365 tenants. While they are often unsuccessful in compromising customer accounts, this reconnaissance activity helps the adversary identify more details about their targets’ environments.

HAFNIUM operates primarily from leased virtual private servers (VPS) in the United States.

Technical Details

Microsoft is providing the following details to help customers understand the techniques used by HAFNIUM to exploit these vulnerabilities and enable more effective defense against any future attacks against unpatched systems.

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CVE-2021-26855 is a server-side request forgery (SSRF) vulnerability in Exchange which allowed the attacker to send arbitrary HTTP requests and authenticate as the Exchange server.

CVE-2021-26857 is an insecure deserialization vulnerability in the Unified Messaging service. Insecure deserialization is where untrusted user-controllable data is deserialized by a program. Exploiting this vulnerability gave HAFNIUM the ability to run code as SYSTEM on the Exchange server. This requires administrator permission or another vulnerability to exploit.

CVE-2021-26858 is a post-authentication arbitrary file write vulnerability in Exchange. If HAFNIUM could authenticate with the Exchange server then they could use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.

CVE-2021-27065 is a post-authentication arbitrary file write vulnerability in Exchange. If HAFNIUM could authenticate with the Exchange server then they could use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.

Attack Details

After exploiting these vulnerabilities to gain initial access, HAFNIUM operators deployed web shells on the compromised server. Web shells potentially allow attackers to steal data and perform additional malicious actions that lead to further compromise. One example of a web shell deployed by HAFNIUM, written in ASP, is below:

Following web shell deployment, HAFNIUM operators performed the following post-exploitation activity:

  • Using Procdump to dump the LSASS process memory:
  • Using 7-Zip to compress stolen data into ZIP files for exfiltration:
  • Adding and using Exchange PowerShell snap-ins to export mailbox data:
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  • Using the Nishang Invoke-PowerShellTcpOneLine reverse shell:
  • Downloading PowerCat from GitHub, then using it to open a connection to a remote server:

HAFNIUM operators were also able to download the Exchange offline address book from compromised systems, which contains information about an organization and its users.