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High severity · published Jan 13, 2026
CVE-2026-20856
Improper input validation in Windows Server Update Service allows an unauthorized attacker to execute code over a network
8.1
CVSS
1.1%
EPSS
No
On KEV
Description
Improper input validation in Windows Server Update Service allows an unauthorized attacker to execute code over a network.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)1.1% · higher than 66% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
W1
Windows 10 1607 · Microsoft
before 10.0.14393.8783; before 10.0.14393.8783 · source: NVD
W1
Windows 10 1809 · Microsoft
before 10.0.17763.8276; before 10.0.17763.8276 · source: NVD
W1
Windows 10 21h2 · Microsoft
before 10.0.19044.6809 · source: NVD
W1
Windows 10 22h2 · Microsoft
before 10.0.19045.6809 · source: NVD
W1
Windows 11 23h2 · Microsoft
before 10.0.22631.6491 · source: NVD
W1
Windows 11 24h2 · Microsoft
before 10.0.26100.7623 · source: NVD
W1
Windows 11 25h2 · Microsoft
before 10.0.26200.7623 · source: NVD
WS
Windows Server 2012 · Microsoft
r2 · source: NVD
WS
Windows Server 2016 · Microsoft
before 10.0.14393.8783 · source: NVD
WS
Windows Server 2019 · Microsoft
before 10.0.17763.8276 · source: NVD
WS
Windows Server 2022 · Microsoft
before 10.0.20348.4648 · source: NVD
WS
Windows Server 2022 23h2 · Microsoft
before 10.0.25398.2092 · source: NVD
Affected product
W1
Windows 10 1607
Microsoft
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-20856
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-20856
Vendor Advisory
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20856
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