CVE-2021-26427

Authenticated code execution from adjacent networks

Published Oct 13, 2021 · Updated Aug 3, 2024

Remote code execution in affected Microsoft Exchange Server builds allows remote authenticated users to execute arbitrary code from an adjacent network. Microsoft has not published the vulnerable component or the operation that mishandles attacker-controlled input. Exploitation requires low privileges and access to the same physical or logical network, and successful attacks can expose or alter data or interrupt service.

CVSS severity9.0
Critical
EPSS probability0.89%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Remote code execution in affected Microsoft Exchange Server builds allows remote authenticated users to execute arbitrary code from an adjacent network. Microsoft has not published the vulnerable component or the operation that mishandles attacker-controlled input. Exploitation requires low privileges and access to the same physical or logical network, and successful attacks can expose or alter data or interrupt service.

The record

CVE
CVE-2021-26427
Published
Oct 13, 2021
Updated
Aug 3, 2024
Vendor
Microsoft
Product
Microsoft Exchange Server 2016 Cumulative Update 21
Classifications
T1210
Attack vector
adjacent
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=15.01.0 <15.01.2308.015

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2016 Cumulative Update 21version=15.01.0 <15.01.2308.015
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.024
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 11version=15.02.0 <15.02.0986.009
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 10version=15.02.0 <15.02.0922.014
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 22version=15.0.0 <15.01.2375.012

Published CVSS scores

9.6NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
9.0MicrosoftCVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
5.8NVDAV:A/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1210Exploitation of Remote Services

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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