KEV Intelligence

Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-26855

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft / Microsoft Exchange Server 2016 Cumulative Update 19 · affected before publication

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Severity
CVSS 9.1 · Critical
Confidence
Confirmed
Exploit status
Exploited in the wild
EPSS
100.0%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-26855 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 2016 Cumulative Update 19 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 2016 Cumulative Update 19 Before publication Affected
Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 Before publication Affected
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 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
27 Apr 2025
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

Tags

cisa malware nuclei_scanner ransomware microsoft metasploit

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

The Shadowserver

Recorded 27 Apr 2025

Independent exploitation attestation added to the KEV Intelligence record.

Used in malware

The Shadowserver

Recorded 03 Nov 2021

Malware families have been linked to exploitation of this CVE.

Proof of concept available

GitHub

Recorded 06 Mar 2021

Public scanner or PoC coverage increases practical exploitability.

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
The Shadowserver 2026-06-01 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.

Virtual patch status

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

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Metasploit https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxylogon_rce.rb 28 Apr 2025
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-26855.yaml 25 Apr 2025

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.1 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C

EPSS

100.0%

Weaknesses

Server-Side Request Forgery (SSRF) CWE-918

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 9.1 Critical

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C

CVSS v2.0 7.5 High

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

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

ZephrFish/Exch-CVE-2021-26855

github · Created 2021-03-14 14:23:34 UTC · 27 stars · AI assessment 90%

CVE-2021-26855: PoC (Not a HoneyPoC for once!)

hackerschoice/CVE-2021-26855

github · Created 2021-03-11 20:51:48 UTC · 61 stars · AI assessment 90%

PoC of proxylogon chain SSRF(CVE-2021-26855) to write file by testanull, censored by github

alt3kx/CVE-2021-26855_PoC

github · Created 2021-03-10 05:21:19 UTC · 53 stars · AI assessment 90%

h4x0r-dz/CVE-2021-26855

github · Created 2021-03-09 16:54:39 UTC · 99 stars · AI assessment 90%

srvaccount/CVE-2021-26855-PoC

github · Created 2021-03-09 14:27:06 UTC · 17 stars · AI assessment 85%

PoC exploit code for CVE-2021-26855

hackerxj007/CVE-2021-26855

github · Created 2021-03-08 09:12:27 UTC · 5 stars · AI assessment 90%

CVE-2021-26855 exp

pussycat0x/CVE-2021-26855-SSRF

github · Created 2021-03-06 19:03:00 UTC · 20 stars · AI assessment 85%

This script helps to identify CVE-2021-26855 ssrf Poc

Microsoft Exchange Server SSRF Vulnerability

nuclei · Created Unknown

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. KEV confirmed by The Shadowserver

    Exploitation attested by an external source

  4. Metasploit module available

    Exploit module available

  5. Nuclei template available

    Scanner coverage available

  6. Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  7. First public exploitation report

    Exploit observed in malware

  8. Public PoC available

    Public proof-of-concept code published

  9. CVE published

    Vulnerability disclosed publicly

  10. 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.

View Pro pricing 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-2021-26855

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-26855",
  "confidence": "Confirmed",
  "cvss_score": 9.1,
  "cvss_estimated": false,
  "epss_score": 0.99999,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

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