CVE-2021-26866

System interruption via directory-junction deletion

Published Mar 11, 2021 · Updated Aug 19, 2026

Directory-junction abuse in Microsoft Windows Update Agent allows local users to delete directories and interrupt system operation. Windows Update Agent follows an attacker-created directory junction during deletion, causing it to remove an attacker-selected directory. Exploitation requires low-privileged code execution on the target, and deletion can leave the affected system unavailable.

CVSS severity7.1
High
EPSS probability1.15%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Directory-junction abuse in Microsoft Windows Update Agent allows local users to delete directories and interrupt system operation. Windows Update Agent follows an attacker-created directory junction during deletion, causing it to remove an attacker-selected directory. Exploitation requires low-privileged code execution on the target, and deletion can leave the affected system unavailable.

The record

CVE
CVE-2021-26866
Published
Mar 11, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Windows 10 Version 1507
Classifications
CWE-59, T1499
Attack vector
local
Privileges
authenticated

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

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Is a vulnerable build present?

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

  1. Affected versionversion=10.0.10240.0 <10.0.10240.18874

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Microsoft · Windows 10 Version 1507version=10.0.10240.0 <10.0.10240.18874
Microsoft · Windows Server 2019version=10.0.17763.0 <10.0.17763.1817
Microsoft · Windows 10 Version 1909version=10.0.0 <10.0.18363.1440
Microsoft · Windows 10 Version 1803version=10.0.0 <10.0.17134.2087
Microsoft · Windows Server 2016 (Server Core installation)version=10.0.14393.0 <10.0.14393.4283
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 10 Version 20H2version=10.0.0 <10.0.19043.867
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 10 Version 1607version=10.0.14393.0 <10.0.14393.4283
Microsoft · Windows 10 Version 2004version=10.0.0 <10.0.19043.867
Microsoft · Windows 10 Version 1809version=10.0.0 <10.0.17763.1817; version=10.0.17763.0 <10.0.17763.1817
Microsoft · Windows Server, version 1909 (Server Core installation)version=10.0.0 <10.0.18363.1440

Published CVSS scores

3.6NIST NVDAV:L/AC:L/Au:N/C:N/I:P/A:P
6.1NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
7.1Microsoft CorporationCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/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.

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No observations available

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

Weakness, pattern, technique

CWE-59Improper Link Resolution Before File Access ('Link Following')
T1499Endpoint Denial of Service

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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