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AD FS: Denial of service through uncontrolled resource allocation

CVE-2026-83645High 7.5

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Summary

Active Directory Federation Services (AD FS), a Windows Server role for identity federation and single sign-on, allocates resources without limits or throttling. An unauthenticated attacker with network access can exhaust these resources and make the service unavailable. Confidentiality and integrity are not affected, but an outage blocks SSO to dependent applications. Exposure is highest where AD FS has an internet-facing endpoint.

Affected

  • Windows Server 2012 and 2012 R2 (including Server Core), before 6.2.9200.26434 and 6.3.9600.23470
  • Windows Server 2016 and Windows 10 1607, before 10.0.14393.9612
  • Windows Server 2019 and Windows 10 1809, before 10.0.17763.9376
  • Windows Server 2022, before 10.0.20348.5761
  • Windows Server 2025, before 10.0.26100.33650
  • Note: the Windows 10 entries are unusual for a server role and should be verified on the MSRC advisory.

Fixed in

  • Update to the builds listed above or later, as published by Microsoft in the October 2026 Patch Tuesday.
  • Cumulative updates named by third parties: KB5126646 (Server 2025), KB5126651 (Windows 10 1809 / Server 2019), KB5126836 (Server 2012 R2). No KB confirmed for Windows 10 1607 / Server 2016.
  • Fixed build numbers and KB numbers come from third-party mirrors; verify them on MSRC before deployment.

Workaround

Unknown. No Microsoft-published workaround identified.

  • Monitor AD FS event logs and performance counters for resource exhaustion.
  • For internet-facing AD FS, limit exposure at the reverse proxy or firewall (general measure, not specific to this CVE).

Exploitation

  • No public proof-of-concept or in-the-wild exploitation reported as of the research date.
  • CISA SSVC: Exploitation none, Automatable yes, Technical Impact partial.
  • Not listed in CISA KEV.
  • EPSS about 1.1 percent.

Our recommendation

  • Check whether your AD FS farm has a public endpoint.
  • Patch in the next maintenance window, verifying the build numbers on MSRC first.
  • After patching, confirm that AD FS stays responsive and monitor event logs for resource exhaustion.
  • Priority: high for internet-facing AD FS, low to moderate for internal-only AD FS or organisations that use cloud identity.

Information from Microsoft (Patch Tuesday October 2026)

Severity (Microsoft)
Important
Impact
Denial of service – 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, KB5126835, 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 2 Nov 2026, 18:52.

Original description (NVD)

Allocation of resources without limits or throttling in Active Directory Federation Services (AD FS) allows an unauthorized attacker to deny service 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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