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Security advisories | Mar 04, 2021

Update Multiple 0-Day Exploits Targeting on Premise Exchange Versions


Exploitation of the Microsoft on premise Exchange vulnerabilities (announced March 2nd, 2021) is more widespread than previously reported. eSentire has observed reports of multiple APT groups exploiting these vulnerabilities in attack campaigns. Attacks targeting these vulnerabilities have been ongoing since at least January 2021. It is critical that Microsoft Exchange Server versions 2010, 2013, 2016, and 2019 are updated. All vulnerable servers are at a high risk of exploitation by advanced threat actor groups. Because exploitation occurred prior to patch release, internet-exposed Microsoft Exchange servers should be reviewed for any evidence of exploitation.

What we’re doing about it

  • eSentire teams have deployed detection content across our services in response to these attacks
    • esENDPOINT identifies suspicious Exchange processes and post-exploitation activity associated with known attacks
    • eSentire's BlueSteel engine identifies malicious PowerShell activity associated with this threat
    • esNETWORK identifies exploitation of CVE-2021-26857 and CVE-2021-26855
    • Additional detection methods across all services are under active development
  • In parallel to detection activity mentioned above, we are actively reviewing customer networks for Indicators of Compromise (IoCs)
  • MVS has local plugins available to identify all related vulnerabilities
    • MVS customers seeking assistance with their review or scans, please contact your MVS consultant or the eSentire Security Operations Center (SOC)
  • eSentire security teams continue to track this topic and additional detection measures are currently under review

What you should do about it

  • All affected versions of Microsoft Exchange should be prioritized for immediate patching
    • Specific guidance on updating these systems can be found here
    • Microsoft provides an Exchange Health Checker PowerShell script which can assist with security patching activities
  • esENDPOINT customers are advised to deploy endpoint agents to on-premise Exchange servers for ongoing monitoring
  • Microsoft has provided a PowerShell script which identifies known compromise behaviors on Exchange servers
    • In order to expedite threat identification, customers are encouraged to utilize this tool following completion of patching
    • In the event that the script reports positive identification of IoCs, customers should contact the eSentire SOC and refer to this advisory

Additional information

The eSentire Threat Intelligence team has observed reports which indicate more widespread exploitation of these vulnerabilities than initially believed. Customers are strongly advised to prioritize patching vulnerable Exchange servers exposed to the Internet. Additionally, customers are advised to work with the eSentire SOC to review these systems and remediate any identified threats resulting from these vulnerabilities.

eSentire teams are actively developing detection for this threat and performing retroactive queries across our customers in parallel. eSentire recommends customers ensure eSentire services are monitoring impacted systems and, out of an abundance of caution, review on-premise Exchange servers for evidence of exploitation per Microsoft’s guidance (Microsoft has automated these checks with the PowerShell script available here). Customers should coordinate with eSentire’s SOC on any positive identification from these activities.

Confirmed vulnerable on-premises Microsoft Exchange Servers:

  • Microsoft Exchange Server 2010
  • Microsoft Exchange Server 2013
  • Microsoft Exchange Server 2016
  • Microsoft Exchange Server 2019

Microsoft Exchange Server 2010 has reached End of Life (EoL) status but received emergency patches. Organizations are strongly recommended to discontinue the use of all EoL products in favor of actively supported alternatives.

Exploited Vulnerabilities:

CVE-2021-26855 (CVSS Score: 9.1/10) – Is a server-side request forgery (SSRF) vulnerability in Microsoft Exchange, which allows a threat actor to send an arbitrary HTTP request and authenticate as the Exchange server.

CVE-2021-26857 (CVSS Score: 7.8/10) – Is an insecure deserialization vulnerability in the Unified Messaging service and requires administrator privileges or the use of another vulnerability to exploit. This vulnerability gave HAFNIUM the controls to run code as SYSTEM on an exchange server.

CVE-2021-26858 / CVE-2021-27065 (CVSS Score: 7.8/10) – Are post-authentication arbitrary file write vulnerabilities in Microsoft Exchange and require either compromising an admin’s credentials or the use of CVE-2021-26855.

According to public reports, at least four separate APT groups have been confirmed to have exploited these vulnerabilities. Known threat actor groups include Hafnium, APT27 (ie. LuckyMouse), Tick (ie. Bronze Butler), and Calypso. The majority of victims are located in the United States, but limited activity was identified in 21 other countries. Specific victim details and attacker goals are currently not known. eSentire security teams will continue to track this event going forward.