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After the March 2021 HAFNIUM attacks, administrators needed to do two things in parallel: update vulnerable on-premises Exchange servers and investigate whether attackers had already gained access. Review any named mitigation settings as temporary historical measures—not as a substitute for current Microsoft servicing guidance or a compromise investigation. A successful update closes the vulnerability it addresses; it does not prove that a server is clean.
What the March 2021 attacks involved
Microsoft reported that it had observed multiple zero-day exploits targeting on-premises Exchange Server. In its initial reporting, Microsoft attributed the campaign with high confidence to HAFNIUM, which it assessed as a state-sponsored group operating from China. Microsoft later reported that other actors also began exploiting the vulnerabilities after disclosure, so HAFNIUM was not the only actor associated with the campaign.
The four vulnerabilities Microsoft named were CVE-2021-26855, CVE-2021-26857, CVE-2021-26858 and CVE-2021-27065. Microsoft reported RiskIQ telemetry indicating nearly 400,000 Exchange servers in the total universe on March 1, 2021. That is a historical count—not a current exposure estimate and not a count of servers known to be vulnerable or compromised.
Confirm which Exchange deployment is in scope
Microsoft’s March 2021 resource center said the vulnerabilities affected on-premises Exchange Server 2013, 2016 and 2019. It described the Exchange 2010 update as defense-in-depth and said Exchange Online was not affected by those March 2021 vulnerabilities. Organizations using a hybrid setup still needed to update their on-premises Exchange server, including one retained for management purposes.
These statements describe the 2021 vulnerabilities, not the present-day support or security status of any Exchange version. Before making changes, identify every on-premises server and check Microsoft’s current supported-version, servicing and security-update guidance for that deployment.
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Update servers and limit exposure while work is underway
Microsoft’s complete mitigation for the 2021 vulnerabilities was to install the applicable security updates. Its instructions at the time depended on the server’s Exchange version and cumulative update. Do not reuse 2021 build requirements as current instructions: confirm the applicable update and prerequisites in Microsoft’s current guidance.
Prioritize externally facing Exchange servers because they are reachable from the internet. If an update cannot be applied immediately, restricting untrusted access or isolating a server may reduce exposure, but Microsoft characterized such measures as partial protection and noted that they can affect functionality. Treat them as temporary risk reduction while arranging the update, not as proof that exploitation is impossible.
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For Exchange Server 2013, 2016 and 2019, Microsoft’s 2021 mitigation guidance described the following measures for situations in which updates could not be applied right away. They were bridges to patching, not durable security settings. Microsoft warned that they could affect server functionality and were not guaranteed to cover every possible way to exploit the vulnerabilities.
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| Mitigation | What to review |
|---|---|
| IIS URL Rewrite rule | Microsoft described a rule to filter malicious HTTPS requests. Confirm whether it was installed, whether it remains in place, and whether it should be removed or replaced in light of current Microsoft guidance and the server’s update status. |
| Disable Unified Messaging (UM) | Check whether UM was disabled as a temporary measure and whether the resulting loss of functionality was understood and accepted while the mitigation was active. |
| Disable the Exchange Control Panel (ECP) virtual directory | Check whether the ECP virtual directory was disabled and account for the administrative functionality affected by that change. |
| Disable the Offline Address Book (OAB) virtual directory | Check whether the OAB virtual directory was disabled and account for the functionality affected by that change. |
Do not assume that finding one of these settings means the server is fully protected. Establish whether the relevant security update was installed and assess the server separately for signs of prior access.
Know what the historical Microsoft tools did
Microsoft described the Exchange On-premises Mitigation Tool (EOMT) as a temporary mitigation tool. It checked for and mitigated CVE-2021-26855 and attempted to discover and remediate malicious files. The standalone ExchangeMitigations.ps1 script applied mitigations but did not perform the same additional scanning.
Those descriptions refer to the tools as documented during the 2021 response. Verify their present availability, instructions and behavior with Microsoft before using them; do not treat a mitigation script or its result as a replacement for supported updates or a full investigation.
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Investigate separately for exploitation and persistence
Applying an update or mitigation does not remove an attacker who already established access. Microsoft’s MSRC responder guidance on March 16, 2021 put the distinction plainly: “Applying the March 2021 Exchange Server Security Updates is critical to prevent (re)infection, but it will not evict an adversary who has already compromised your server.” This statement concerns those 2021 updates and incident response; it is not a blanket description of every later update.
Review Exchange product logs and scan for known web shells. Preserve relevant evidence, including timestamps and source IP addresses, before changes erase or alter it. Treat suspicious file access or a detected web shell as a reason to widen the investigation: Microsoft’s responder guidance advises checking for lateral movement and further compromise across the environment, rather than stopping at the Exchange server.
Escalate when findings indicate broader compromise
Consider engaging cybersecurity incident-response professionals if you detect or suspect advanced post-exploitation, such as credential dumps, lateral movement, or additional malware or ransomware. These findings can indicate that the incident extends beyond the original Exchange vulnerability and warrants investigation across the affected environment.
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