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

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.

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Source: Palo Alto