Is CVE-2021-26858 actively exploited?

Yes. Previdian tracks CVE-2021-26858 as a known exploited vulnerability. Confidence is confirmed.

What exploitation activity has Previdian observed?

Previdian has not yet recorded sensor-observed exploitation attempts for CVE-2021-26858. Our sensors continue to monitor for activity targeting this CVE.

Which versions are affected?

CVE-2021-26858 affects Microsoft Exchange Server 2013 Cumulative Update 21 and 23 other products. Previdian lists 24 version rows; examples include 15.00.0 to < 15.00.1395.012, 15.00.0 to < 15.00.1473.006, and 15.00.0 to < 15.00.1497.012.

Yes. CVE-2021-26858 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-26858

Q: What is CVE-2021-26858?

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

Q: How should organizations remediate it?

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

Q: Is it in CISA KEV?

Yes. CVE-2021-26858 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server 2013 Cumulative Update 21 · affected before 15.00.1395.012

Watch CVE-2021-26858 Browse KEV feed

Severity: CVSS 7.8 · High
Confidence: Confirmed
Exploit status: Exploited in the wild
EPSS: 93.7%
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. Previdian tracks this CVE as a known exploited vulnerability. Confidence is confirmed. Listed in CISA KEV. Also confirmed by third-party sources.

Who is affected?

Microsoft Exchange Server 2013 Cumulative Update 21 affected before 15.00.1395.012.

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

Vendor Product Affected Status

Microsoft Affected

Microsoft Exchange Server 2013 Cumulative Update 21

Before 15.00.1395.012

Affected

Microsoft Affected

Microsoft Exchange Server 2013 Cumulative Update 22

Before 15.00.1473.006

Affected

Microsoft Affected

Microsoft Exchange Server 2013 Cumulative Update 23

Before 15.00.1497.012

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 10

Before 15.01.1531.012

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 11

Before 15.01.1591.018

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 12

Before 15.01.1713.010

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 13

Before 15.01.1779.008

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 14

Before 15.01.1847.012

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 15

Before 15.01.1913.012

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 16

Before 15.01.1979.008

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 17

Before 15.01.2044.013

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 18

Before 15.01.2106.013

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 19

Before 15.01.2176.009

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 8

Before 15.01.1415.010

Affected

Microsoft Affected

Microsoft Exchange Server 2016 Cumulative Update 9

Before 15.01.1466.016

Affected

Microsoft Affected

Microsoft Exchange Server 2019

Before 15.02.0221.018

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 1

Before 15.01.1979.006

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 2

Before 15.02.0397.011

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 3

Before 15.02.0464.015

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 4

Before 15.02.0659.012

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 5

Before 15.02.0595.008

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 6

Before 15.02.0529.013

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 7

Before 15.02.0721.013

Affected

Microsoft Affected

Microsoft Exchange Server 2019 Cumulative Update 8

Before 15.02.0792.010

Affected

View vendor advisory

Published: 02 Mar 2021
Exploitation Reported: 03 Nov 2021
Attack vector: Local
Complexity: Low
Privileges: None
User interaction: Required

CVE References

CVE Record CVE.org · CVE Record https://www.cve.org/CVERecord?id=CVE-2021-26858
Microsoft Exchange Server Remote Code Execution Vulnerability msrc.microsoft.com · Vendor Advisory https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26858

Exploitation evidence

Why Previdian 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

A trusted third party reported exploitation.

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 Previdian Pro API.

Learn about Pro API access

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
User-Agents
Callback hosts: 0; 0; 0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No Previdian 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

Potential damage if exploited. Separate from whether attackers are using it.

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

EPSS

93.7%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Recent mention · Tenable Blog

27 May 2026

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

12 Aug 2026
04:21 UTC

KEV confirmed by CVE

Exploitation attested by an external source
02 Jun 2026
14:21 UTC

KEV confirmed by Tenable Blog

Exploitation attested by an external source
03 Nov 2021
00:00 UTC

Added to CISA KEV

Listed in the CISA Known Exploited Vulnerabilities catalog
03 Nov 2021
00:00 UTC

First public exploitation report

Exploit observed in malware
02 Mar 2021
23:55 UTC

CVE published

Vulnerability disclosed publicly
08 Feb 2021
00:00 UTC

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.

Start 14-day Pro trial Read API docs

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

Sample API response

{
  "cve_id": "CVE-2021-26858",
  "confidence": "Confirmed",
  "cvss_score": 7.8,
  "cvss_estimated": false,
  "epss_score": 0.93651,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability

Tags

CVE References

Exploited in the wild

Used in malware

Known exploited vulnerability sources

Observed signals

Scanner coverage

Virtual patch status

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

All CVSS Scores

KEV confirmed by CVE

KEV confirmed by Tenable Blog

Added to CISA KEV

First public exploitation report

CVE published

CVE ID reserved

Automate this intelligence