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CVE-2021-26858

Microsoft Exchange Server Remote Code Execution

7.8 HIGHActively exploited (CISA KEV)Used in ransomwareEPSS 93.7%
  1. Published2 Mar 2021
  2. Added to CISA KEV3 Nov 2021
  3. Remediation due3 May 2022
Actively exploited (CISA KEV)

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Added to KEV: 3 Nov 2021Federal remediation due: 3 May 2022Used in ransomware

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS 3.1 breakdown

Attack vectorLocal
Attack complexityLow
Privileges requiredNone
User interactionRequired
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected products

Microsoft Exchange Server2013 · 2016 · 2019

Remediation

No fixed version is recorded in the NVD data. Check the vendor advisory for the latest guidance.

References

Frequently asked

Is CVE-2021-26858 being actively exploited?

Yes. CVE-2021-26858 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, added 3 Nov 2021. It has been used in ransomware campaigns.

How severe is CVE-2021-26858?

CVE-2021-26858 has a CVSS 3.1 base score of 7.8, rated high. EPSS estimates a 93.7% probability of exploitation.

Which products are affected by CVE-2021-26858?

Affected products include Microsoft Exchange Server.

How do I fix CVE-2021-26858?

Apply the vendor's patch or advisory guidance, then verify no affected versions remain across your devices.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2021-26858 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. Because it is on the CISA KEV list, Secuno flags any affected asset as top-priority automatically. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.