Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-26855

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server 2013 Cumulative Update 21 · affected before 15.00.1395.012

Watch CVE-2021-26855 Browse KEV feed
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. 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

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Affected
Microsoft Exchange Server 2013 Cumulative Update 21
Before 15.00.1395.012
Microsoft Affected
Microsoft Exchange Server 2013 Cumulative Update 22
Before 15.00.1473.006
Microsoft Affected
Microsoft Exchange Server 2013 Cumulative Update 23
Before 15.00.1497.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 10
Before 15.01.1531.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 11
Before 15.01.1591.018
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 12
Before 15.01.1713.010
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 13
Before 15.01.1779.008
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 14
Before 15.01.1847.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 15
Before 15.01.1913.012
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 16
Before 15.01.1979.008
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 17
Before 15.01.2044.013
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 18
Before 15.01.2106.013
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 19
Before 15.01.2176.009
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 8
Before 15.01.1415.010
Microsoft Affected
Microsoft Exchange Server 2016 Cumulative Update 9
Before 15.01.1466.016
Microsoft Affected
Microsoft Exchange Server 2019
Before 15.02.0221.018
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 1
Before 15.01.1979.006
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 2
Before 15.02.0397.011
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 3
Before 15.02.0464.015
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 4
Before 15.02.0659.012
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 5
Before 15.02.0595.008
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 6
Before 15.02.0529.013
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 7
Before 15.02.0721.013
Microsoft Affected
Microsoft Exchange Server 2019 Cumulative Update 8
Before 15.02.0792.010
View vendor advisory (opens in new tab)
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 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

The Shadowserver

Recorded 27 Apr 2025

A trusted third party reported exploitation.

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 Previdian Pro API.

Learn about Pro API access
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
User-Agents
Callback hosts
0
0
0

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

Virtual patch status

No Previdian 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

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

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%

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

Weaknesses

Server-Side Request Forgery (SSRF) CWE-918

Recent mention · The Hacker News

Three Threat Groups Target Russian Enterprises With Backdoors, Ransomware, and Wipers

Enterprises in Russia have emerged as the target of three threat activity clusters tracked as NightEagle, Hacking Cat, and Toy Ghouls, according to multiple reports from Kaspersky. The cybersecurity vendor said it has identified attacks mounted by NightEagle (aka APT-Q-95), a...

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

All Mentions

Recent mention · The Hacker News

Three Threat Groups Target Russian Enterprises With Backdoors, Ransomware, and Wipers

The Hacker News · 16 Sep 2026

Enterprises in Russia have emerged as the target of three threat activity clusters tracked as NightEagle, Hacking Cat, and Toy Ghouls, according to multiple reports from Kaspersky. The cybersecurity vendor said it has identified attacks mounted by NightEagle (aka APT-Q-95), a threat actor known to be active since at least 2023, that involve new techniques for persistence and lateral movement.

Recent mention · Tenable Blog

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

Tenable Blog · 27 May 2026

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 dead: Vulnerability prioritization based on exploitation risk offers a path forward. A directed graph model linking 600+ threat actors to vulnerabilities in 7,800 customer environments reveals that 68% of organizations carry at least one CVE previously exploited by a named adversary, and 321 tracked threat groups can...

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. 04:21 UTC

    KEV confirmed by CVE

    Exploitation attested by an external source

  2. 14:21 UTC

    KEV confirmed by Tenable Blog

    Exploitation attested by an external source

  3. 00:00 UTC

    KEV confirmed by The Shadowserver

    Exploitation attested by an external source

  4. 15:02 UTC

    Metasploit module available

    Exploit module available

  5. 00:00 UTC

    Nuclei template available

    Scanner coverage available

  6. 00:00 UTC

    Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  7. 00:00 UTC

    First public exploitation report

    Exploit observed in malware

  8. 19:03 UTC

    Public PoC available

    Public proof-of-concept code published

  9. 23:55 UTC

    CVE published

    Vulnerability disclosed publicly

  10. 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-2021-26855

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-26855",
  "confidence": "Confirmed",
  "cvss_score": 9.1,
  "cvss_estimated": false,
  "epss_score": 0.99996,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}