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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 3 May 2022. Known ransomware use
Exchange ServerApplication · Microsoft

CVE-2021-26858

HIGH · 7.8 CVSS v3.1 Published 2021-03-03
Patch available
A vendor patch is available. No clean upgrade release — apply the published patch.
See remediation →
97/100
Remediation priority · Urgent
In the wild Ransomware High EPSS No privileges Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Microsoft Exchange Server Remote Code Execution Vulnerability

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Microsoft Exchange Server Remote Code Execution Vulnerability in the Unified Messaging service allowing authenticated attackers to execute arbitrary code via specially crafted email messages.

MitigationApply Microsoft Exchange Server security updates immediately; restrict unnecessary access to Exchange servers and monitor for indicators of compromise.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Exchange ServerApplication
Affected:= 2010= 2013= 2016= 2019

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Exchange Server version
    Run Get-ExchangeServer | Format-List Name,AdminDisplayVersion in Exchange Management Shell, or check the version in Programs and Features
    Affected if Version matches Exchange Server 2010, 2013, 2016, or 2019 (any build)
  2. Identify exposed Exchange web services
    Review IIS bindings and URL rewrite rules on the Exchange servers. Check if /ecp and /owa virtual directories are accessible from untrusted networks using Test-NetConnection or browser access
    Affected if The ECP or OWA virtual directories are reachable from networks outside the trusted internal network
  3. Verify authentication requirements
    Examine the authentication configuration for the ECP virtual directory in IIS: Get-EcpVirtualDirectory | Select-Object *auth*
    Affected if Forms-based authentication or other authentication methods are enabled without proper network restrictions
  4. Review IIS and Exchange logs for anomalous requests
    Examine C:\Program Files\Microsoft\Exchange Server\V15\Logging\HttpProxy\ and IIS logs for unusual /ecp/ requests with abnormal headers or parameters
    Affected if Logs show unusual or repeated SSRF-style requests to /ecp/ with malformed X-AnchorMailbox or X-Preload headers

A user is affected if their Exchange Server version is 2010, 2013, 2016, or 2019 AND the ECP/OWA interfaces are accessible from untrusted networks without proper network segmentation.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Patch available Apply the vendor patch
Vendor patch portal.msrc.microsoft.com →
Interim mitigation

Apply Microsoft Exchange Server security updates immediately; restrict unnecessary access to Exchange servers and monitor for indicators of compromise.

Recommended fix High confidence

Latest Cumulative Update for your Exchange version (2013 CU23, 2016 CU23, 2019 CU12 or later)

  1. Identify your current Exchange Server version and build number from the Exchange Administration Center or by running Get-ExchangeServer in Exchange Management Shell
  2. Download and install the March 2021 security updates (or later) for your specific Exchange version from the Microsoft Update Catalog or through Windows Update
  3. For Exchange Server 2010, install the update from KB5000871
  4. For Exchange Server 2013, install the update from KB5000871
  5. For Exchange Server 2016, install the update from KB5000871
  6. For Exchange Server 2019, install the update from KB5000871
  7. Restart all Exchange services after applying the update using iisreset /noforce or restart the servers
  8. Verify the update was installed successfully by checking the installed updates and confirming version numbers
Caveat March 2021 updates had known issues causing Outlook connectivity problems; test thoroughly in non-production first; Exchange 2010 reached end of support in Oct 2020 and should be migrated

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Exchange Server Exploited in the wild — priority engagement
  • Consultation8.0 h
  • Implementation16.0 h
  • Testing12.0 h
  • Review / QA4.0 h
40.0 hours of engineering $7,000
Get it fixed fast

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $11,200.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-26858 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data