On March 2, 2021, the Microsoft Threat Intelligence Center (MSTIC) released details on an active state-sponsored threat campaign leveraging four zero-day vulnerabilities to attack on-premises instances of Microsoft Exchange Server. In the attacks Microsoft observed, the threat actor used these vulnerabilities to access on-premises Exchange servers, which enabled access to email accounts and allowed 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 zero-day vulnerabilities disclosed on March 2, 2021 are:

• CVE-2021-26855 is a server-side request forgery (SSRF) vulnerability in Exchange that allows an 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.
  

Microsoft has released out-of-band patches for all four vulnerabilities as of March 2, 2021.

Microsoft Exchange customers should apply the latest updates on an emergency basis and take steps to harden their Exchange instances. We strongly recommend that organizations monitor closely for suspicious activity and indicators of compromise (IOCs) stemming from this campaign. Rapid7 has a comprehensive list of IOCs available here.

For further information on the HAFNIUM-attributed threat campaign and related IOCs, see Microsoft’s blog here: https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers/

Affected products

• Exchange Server 2013
  
• Exchange Server 2016
  
• Exchange Server 2019
  

Security updates are available for the following specific versions of Exchange:

• Exchange Server 2010 (for Service Pack 3 – this is a Defense in Depth update)
  
• Exchange Server 2013 (CU 23)
  
• Exchange Server 2016 (CU 19, CU 18)
  
• Exchange Server 2019 (CU 8, CU 7)
  

Exchange Online is not affected.

Technical analysis and guidance

Microsoft said on March 2 that the attacks they observed were “limited and targeted.” Rapid7 detection and response teams, however, have been detecting indiscriminate exploitation of Exchange servers since February 27, 2021, including but not limited to the attacker behaviors below:

• Attacker Tool – China Chopper Webshell Executing Commands
  
• Attacker Technique – ProcDump Used Against LSASS
  

More information on attacker behaviors and a timeline of widespread Exchange exploitation is available here.

We strongly recommend that organizations both apply all updates for on-premises Exchange Server installations on an emergency basis and examine their environments for signs of compromise.