Threat Assessment: Active Exploitation of Four Zero-Day Vulnerabilities in Microsoft Exchange Server

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Executive Summary

On Mar. 2, 2021, Volexity reported in-the-wild-exploitation of four Microsoft Exchange Server vulnerabilities: CVE-2021-26855, CVE-2021-26857, CVE-2021-26858 and CVE-2021-27065.

As a result of these vulnerabilities being exploited, adversaries can access Microsoft Exchange Servers and allow installation of additional tools to facilitate long-term access into victims' environments. There has also been a report of multiple threat actors leveraging these zero-day vulnerabilities, meaning post-exploitation activity may vary depending on the purpose of the different threat actors.

These vulnerabilities affect the following Microsoft Exchange Server versions:

• Microsoft Exchange 2013.
• Microsoft Exchange 2016.
• Microsoft Exchange 2019.

Microsoft has released an emergency out-of-band security update to patch these vulnerabilities. We strongly advise immediately updating all Microsoft Exchange Servers to the latest available patched versions released by Microsoft.

Due to the surge of this malicious activity, we’ve created this threat assessment for overall threat awareness. Full visualization of the techniques observed and their relevant courses of action can be viewed in the Unit 42 ATOM Viewer.

If you think you may have been impacted, please email crypsis-investigations@paloaltonetworks.com or call 855.875.4631 to get in touch with the Crypsis Incident Response team.

Activity Overview: Exploits of Microsoft Exchange Server Vulnerabilities

There have been reports of activity from several threat actors exploiting four zero-day vulnerabilities affecting Microsoft Exchange Servers. When these vulnerabilities are chained together, threat actors are able to exploit and gain access to Microsoft Exchange servers.

For this attack to be successful, the adversary would first need to identify an on-premises Microsoft Exchange Server that is able to receive untrusted connections from an external source on port 443. If an adversary is successful in securing a connection, they can then exploit CVE-2021-26855 to authenticate themselves as a Microsoft Exchange server. This can be followed by the exploitation of CVE-2021-26857, CVE-2021-26858 and CVE-2021-27065 post-authentication, allowing the adversary to gain remote access.

If access is achieved, the exploited vulnerabilities will allow threat actors to execute commands remotely, which can include uploading a China Chopper webshell to establish persistence in the compromised system. The webshell would then allow the adversary to steal data and execute additional malicious actions, such as downloading remote files or network service scanning.

When the webshell is deployed, post-exploitation activity typically occurs in the system, most commonly the use of Procdump to dump Local Security Authority Subsystem Service (LSASS) process memory to obtain credentials. There have also been instances in which threat actors downloaded additional tools, such as Nishang and Powercat, to support their actions. Following the extraction of credentials and data from the compromised system, threat actors often use compression utilities such as 7zip or Winrar to compress and stage the stolen data for exfiltration. It’s worth noting that since different groups have leveraged these vulnerabilities, post-exploitation activities may vary depending on the actor behind the attack.

Microsoft has shared ways to determine whether your system has been compromised by this activity. There is also an emergency out-of-band security update to patch these vulnerabilities.

Courses of Action

This section documents the relevant tactics, techniques and procedures (TTPs) observed being used by adversaries leveraging CVE-2021-26855, CVE-2021-26857, CVE-2021-26858 and CVE-2021-27065, and maps them directly to Palo Alto Networks product(s) and service(s) which can protect against them. It also further instructs customers on how to ensure their devices are configured correctly.

<sup>Table 1. Courses of Action for observed techniques used by multiple threat actors leveraging zero-day Microsoft Exchange Server vulnerabilities.
†These capabilities are part of the NGFW security subscriptions service.
* These analytic detectors will trigger automatically for Cortex XDR Pro customers.</sup>

Conclusion

Due to the alarming activity of threat actors exploiting these zero-day vulnerabilities against vulnerable Microsoft Exchange Servers, we strongly advise immediately updating all Microsoft Exchange Servers to the latest available patched versions released by Microsoft.

As of the time of writing, based on signatures and indicators that have been observed, Palo Alto Networks customers are protected across our product ecosystem, with specific protections deployed in the following products and subscriptions:

• Next-Generation Firewall:
  • Threat Prevention. (Deploy Content Pack 8380 which detects the four 0-day vulnerabilities)
  • URL Filtering.
• Cortex XDR.
• Cortex XSOAR.

It is imperative for customers to employ best practices in order to ensure Palo Alto Networks products are configured in a manner best suited for your protection.

Additional Resources

• Hunting for the Recent Attacks Targeting Microsoft Exchange
• HAFNIUM targeting Exchange Servers with 0-day exploits
• Operation Exchange Marauder: Active Exploitation of Multiple Zero-Day Microsoft Exchange Vulnerabilities
• Mass Exploitation of Exchange Server Zero-Day CVEs: What You Need to Know