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On March 31, 2021, the Cybersecurity and Infrastructure Security Agency (CISA) gave federal agencies five days to identify and investigate vulnerable on-premises Microsoft Exchange servers. The requirement applied to operational servers that had been connected to the internet at any time since January 1, 2021, according to contemporaneous reporting. It did not apply to Exchange Online or Microsoft 365 cloud email services.
What the five-day deadline required
CISA’s March 31 supplemental direction added an urgent identification and investigation requirement to Emergency Directive 21-02. Agencies had to find in-scope Exchange servers and check them for evidence of compromise—not simply report whether they had installed an update. SecurityWeek reported that the lookback covered operational servers connected to the internet at any point from January 1, 2021. CISA’s supplemental direction and SecurityWeek’s April 1, 2021 report describe the requirement.
The five days were a federal-agency deadline set in 2021, not a general deadline imposed on every Exchange administrator. CISA’s alert also urged organizations beyond the federal civilian agencies covered by the directive to take protective action.
Which Exchange servers were affected
The issue concerned Microsoft Exchange Server deployed on premises. CISA said the vulnerabilities were not known to affect Exchange Online or Microsoft 365 cloud email services. A hybrid organization should therefore distinguish its cloud mailboxes from any on-premises Exchange servers still operating or exposed to the internet; the cloud-service qualification does not establish that an organization has no vulnerable on-premises server.
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The March 3, 2021 Emergency Directive followed reports of active exploitation of four vulnerabilities:
- CVE-2021-26855: An unauthenticated server-side request forgery (SSRF) flaw in Exchange Control Panel that could let an attacker authenticate as the server and access mailboxes.
- CVE-2021-26857: An insecure deserialization vulnerability that could enable code execution as SYSTEM after authentication.
- CVE-2021-26858 and CVE-2021-27065: Post-authentication arbitrary file-write vulnerabilities.
CISA warned that successful exploitation could provide persistent access, expose files and mailboxes, steal credentials, and undermine trust and identity across an enterprise network. See Emergency Directive 21-02 and CISA Alert AA21-062A.
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What agencies were told to do
CISA’s response combined discovery, investigation, remediation, and temporary containment. The tools named in its supplemental direction were Microsoft’s Test-ProxyLogon.ps1 script and Microsoft Safety Scanner (MSERT).
- Identify in-scope servers. Find operational on-premises Exchange servers, including any that were internet-connected during the January 1, 2021 lookback period reported at the time.
- Investigate for compromise. Run Test-ProxyLogon.ps1 and MSERT as directed, then review relevant indicators and logs. Finding and investigating a server was distinct from patching it.
- Collect forensic evidence if compromise is suspected. CISA’s alert called for forensic triage that could include memory, registry hives, Windows event logs, and web logs. Preserve and collect evidence in a way that supports incident investigation.
- Apply Microsoft’s available out-of-band patches. Patch promptly even if the investigation finds no malicious activity; CISA’s alert explicitly said organizations with no activity should apply available patches immediately and implement the alert’s mitigations.
- Restrict external access only as a temporary measure when needed. CISA advised external-access restrictions as interim mitigation where immediate patching was not possible. They do not replace patching or determine whether an attacker already gained access.
The supplemental direction and alert provide the official context for the tools and response actions: supplemental direction and Alert AA21-062A.
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What Exchange administrators should take from the incident
For administrators reviewing the Hafnium-era attacks, the key distinction is between exposure, compromise investigation, and remediation. An internet-facing server could have been in scope for identification; that fact alone does not prove it was breached. Conversely, installing a patch does not by itself establish that earlier access did not occur. If indicators or suspicious activity are found, treat the issue as a potential incident and conduct forensic triage rather than relying on patch status alone.
The directive’s legal requirement applied to the federal agencies it covered. State and local governments, critical-infrastructure operators, and private organizations were not made subject to that federal-agency deadline by the directive, but CISA’s alert provided security guidance for organizations operating affected on-premises Exchange servers.
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Timeline
- March 2, 2021: Microsoft and partners disclosed the Exchange zero-day campaign and related vulnerabilities.
- March 3, 2021: CISA issued Emergency Directive 21-02 and Alert AA21-062A.
- March 31, 2021: CISA issued supplemental direction requiring additional investigation, forensic triage, and hardening actions.
- April 1, 2021: SecurityWeek reported the five-day identification requirement and the internet-exposure lookback to January 1, 2021.
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