KEV Intelligence

Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft / Microsoft Exchange Server 2019 · affected before publication

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Severity
CVSS 7.8 · High
Confidence
Confirmed
Exploit status
Exploited in the wild
EPSS
89.5%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-26858 is an unauthenticated Microsoft Exchange Server Remote Code Execution Vulnerability.

Is it exploited?

Yes. KEV Intelligence tracks this CVE as a known exploited vulnerability. Confidence is confirmed.

Who is affected?

Microsoft / Microsoft Exchange Server 2019 affected before publication.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Vendor Product Affected Status
Microsoft Microsoft Exchange Server 2019 Before publication Affected
Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 2 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 13 Before publication Affected
Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 3 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 14 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 4 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 15 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 5 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 6 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 16 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 17 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 7 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 18 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 Before publication Affected
Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 8 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 1 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 9 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 Before publication Affected
Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 Before publication Affected
Published
02 Mar 2021
Exploitation Reported
03 Nov 2021
Attack vector
Local
Complexity
Low
Privileges
None
User interaction
Required

Tags

microsoft malware ransomware cisa

CVE References

Exploitation evidence

Why KEV Intelligence marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

CISA

Recorded 03 Nov 2021

Independent exploitation attestation added to the KEV Intelligence record.

Used in malware

CISA

Recorded 03 Nov 2021

Malware families have been linked to exploitation of this CVE.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the KEV Intelligence Pro API.

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Source Added
CISA First 2021-11-03 00:00 UTC
Tenable Blog 2026-06-02 14:21 UTC
CVE 2026-08-12 04:21 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
0
User-Agents
0

Raw values available in Pro and Enterprise.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No KEV Intelligence virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

No detection artifacts or sensor request patterns are available for this CVE yet.

Check back as sensor telemetry and scanner integrations are updated.

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

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

EPSS

89.5%

Recent mention · Tenable Blog

Inside the customer environment: Where threat actors, vulnerabilities, and exposed assets intersect

Tenable Research has developed a graph-based model linking 600+ threat groups to real-world customer exposures. It reveals which vulnerabilities sit at the intersection of severity, active exploitation, and organizational risk.Key takeawaysThe "patch everything" strategy is...

Read full advisory

All CVSS Scores

CVSS v3.1 7.8 High

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

CVSS v2.0 6.8 Medium

AV:N/AC:M/Au:N/C:P/I:P/A:P

Timeline

From disclosure to observed exploitation

  1. KEV confirmed by CVE

    Exploitation attested by an external source

  2. KEV confirmed by Tenable Blog

    Exploitation attested by an external source

  3. Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  4. First public exploitation report

    Exploit observed in malware

  5. CVE published

    Vulnerability disclosed publicly

  6. CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-26858

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-26858",
  "confidence": "Confirmed",
  "cvss_score": 7.8,
  "cvss_estimated": false,
  "epss_score": 0.89509,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

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