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Microsoft AD FS: Spoofing vulnerability due to weak authentication

CVE-2026-83511High 7.4

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Summary

Microsoft Active Directory Federation Services (AD FS) on several Windows Server and Windows 10 builds contains a spoofing vulnerability caused by weak authentication (CWE-1390). An unauthenticated attacker on the network can exploit it without privileges or user interaction, but attack complexity is high. Successful exploitation could impact confidentiality and integrity; no exploitation or public exploit code is known.

Affected

  • Windows 10 Version 1607 and Windows Server 2016 (incl. Server Core): 10.0.14393.0 to below 10.0.14393.9612
  • Windows 10 Version 1809 and Windows Server 2019 (incl. Server Core): 10.0.17763.0 to below 10.0.17763.9376
  • Windows Server 2012 R2 (incl. Server Core): 6.3.9600.0 to below 6.3.9600.23470
  • Windows Server 2022: 10.0.20348.0 to below 10.0.20348.5761
  • Windows Server 2025 (incl. Server Core): 10.0.26100.0 to below 10.0.26100.33650
  • Note: the CPE data lists only Windows 10 1607; AD FS runs on the Server editions above.

Fixed in

  • Windows 10 1607 / Server 2016: 10.0.14393.9612
  • Windows 10 1809 / Server 2019: 10.0.17763.9376 (KB5126651 per third-party index)
  • Windows Server 2012 R2: 6.3.9600.23470
  • Windows Server 2022: 10.0.20348.5761
  • Windows Server 2025: 10.0.26100.33650 (KB5126646 per third-party index)
  • Fixed versions are from secondary sources; verify them on the MSRC page before deploying.

Workaround

Unknown. No official Microsoft workaround was found.

  • General hardening suggested by a third party: configure AD FS to use only secure authentication methods such as MFA or certificate-based authentication.

Exploitation

  • Microsoft: not publicly disclosed, not exploited.
  • CISA SSVC: exploitation none, not automatable.
  • Not listed in the CISA KEV catalog.
  • No public proof of concept found.

Our recommendation

  • Apply the October 2026 security updates to all affected AD FS servers, verifying the build numbers on the MSRC page first.
  • Prioritise AD FS servers that are reachable from the internet and patch them first.
  • Patch internal-only AD FS servers in the next regular maintenance window.
  • Enforce MFA or certificate-based authentication for federated sign-on as a hardening measure.
  • Check your AD FS exposure and monitor sign-in logs for unusual spoofing or authentication activity.

Information from Microsoft (Patch Tuesday October 2026)

Severity (Microsoft)
Important
Impact
Spoofing – Active Directory Federation Services (AD FS)
Status on Patch Tuesday
Neither exploited nor publicly disclosed
Exploitability assessment
Exploitation unlikely
Product family
Windows, Windows ESU (Extended Security Updates)
Updates (KB)
KB5126651, KB5126655, KB5126646, KB5126868, KB5126836

Entry in the Microsoft Security Update Guide

Public exploits

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 3 Nov 2026, 01:08.

Original description (NVD)

Weak authentication in Active Directory Federation Services (AD FS) allows an unauthorized attacker to perform spoofing over a network.

NVD references (1)

Sources

The tracker combines official, freely usable data sources – updated continuously:

This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.

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