CVE-2021-26877Network attack vector

Remote code execution via unvalidated TXT lengths

Published Mar 11, 2021 · Updated Aug 19, 2026

Remote code execution in Microsoft Windows DNS Server allows remote attackers to run code through a crafted Dynamic Update request. In dns.exe, File_PlaceStringInFileBuffer() trusts a TXT record length larger than its data length and performs an out-of-bounds heap read of up to 255 bytes. The DNS role and Dynamic Update must be enabled on a writable primary zone; insecure zones permit unauthenticated access, while secure AD-integrated zones require a domain-joined machine.

CVSS severity9.8
Critical
EPSS probability16.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Remote code execution in Microsoft Windows DNS Server allows remote attackers to run code through a crafted Dynamic Update request. In dns.exe, File_PlaceStringInFileBuffer() trusts a TXT record length larger than its data length and performs an out-of-bounds heap read of up to 255 bytes. The DNS role and Dynamic Update must be enabled on a writable primary zone; insecure zones permit unauthenticated access, while secure AD-integrated zones require a domain-joined machine.

The record

CVE
CVE-2021-26877
Published
Mar 11, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Windows Server 2019
Classifications
T1210
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 19, 2026Record 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=10.0.17763.0 <10.0.17763.1817

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Windows Server 2019version=10.0.17763.0 <10.0.17763.1817
Microsoft · Windows Server 2016 (Server Core installation)version=10.0.14393.0 <10.0.14393.4283
Microsoft · Windows Server 2012 (Server Core installation)version=6.2.9200.0 <6.2.9200.23298
Microsoft · Windows Server 2012version=6.2.9200.0 <6.2.9200.23298
Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19043.867
Microsoft · Windows Server 2016version=10.0.14393.0 <10.0.14393.4283
Microsoft · Windows Server 2008 R2 Service Pack 1version=6.1.7601.0 <6.1.7601.24566
Microsoft · Windows Server 2008 R2 Service Pack 1 (Server Core installation)version=6.1.7601.0 <6.1.7601.24566
Microsoft · Windows Server 2012 R2 (Server Core installation)version=6.3.9600.0 <6.3.9600.19968
Microsoft · Windows Server 2019 (Server Core installation)version=10.0.17763.0 <10.0.17763.1817
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19043.867
Microsoft · Windows Server 2008 Service Pack 2version=6.0.6003.0 <6.0.6003.21070
Microsoft · Windows Server 2012 R2version=6.3.9600.0 <6.3.9600.19968
Microsoft · Windows Server 2008 Service Pack 2 (Server Core installation)version=6.0.6003.0 <6.0.6003.21070
Microsoft · Windows Server, version 1909 (Server Core installation)version=10.0.0 <10.0.18363.1440

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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)
Vectornetwork
Privilegesunauthenticated
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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo