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AD FS: Spoofing through improper SAML signature verification

CVE-2026-96252Critical 9.1

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Summary

Active Directory Federation Services (AD FS) does not properly verify the cryptographic signature of SAML responses in affected Windows Server and Windows 10 builds. An unauthenticated attacker on the network can replay a validly signed SAML response intended for a different endpoint against an affected AD FS endpoint, which enables spoofing. Microsoft rates the severity as Important, while NVD scores it 9.1 (Critical). Microsoft reported no exploitation at disclosure.

Affected

  • Windows Server 2012 and 2012 R2 (including Server Core): builds before 6.2.9200.26434 and 6.3.9600.23470
  • Windows Server 2016 and Windows 10 Version 1607: builds before 10.0.14393.9612
  • Windows Server 2019 and Windows 10 Version 1809: builds before 10.0.17763.9376
  • Windows Server 2022: builds before 10.0.20348.5761
  • Windows Server 2025: builds before 10.0.26100.33650
  • Only systems running the AD FS role are relevant. NVD lists only Windows 10 1607 in its CPE data.

Fixed in

  • Install the October 2026 security update for each affected version (released 13 October 2026).
  • Exact KB numbers are not confirmed. Verify them on the MSRC page or in the Microsoft Update Catalog before deployment.
  • Known issue: KB5126651 (Server 2019) has a reported Exchange/OWA issue. Check Microsoft's known-issues notes first.

Workaround

Unknown. No vendor workaround was found, and the patch is available.

  • Interim measures (general good practice, not from a vendor source): restrict which networks can reach AD FS endpoints, review relying-party and SAML endpoint configuration, and monitor AD FS logs for unexpected token requests.

Exploitation

  • Microsoft assessed no exploitation at the time of disclosure (secondary source: Forkast).
  • CISA SSVC: Exploitation none, Automatable yes, Technical Impact total.
  • Not listed in CISA KEV.
  • No public proof of concept found. Beyond that, unknown.

Our recommendation

  • If you run AD FS, patch all AD FS servers and proxies in the October 2026 update cycle, after confirming the KB number on MSRC.
  • Check the Exchange/OWA known issue before patching if the server also hosts Exchange.
  • Check whether AD FS endpoints or proxies are published to the internet and restrict access where possible.
  • Monitor AD FS logs for unexpected token requests and review relying-party configuration.
  • Treat the issue as critical while AD FS is exposed, even though Microsoft rates it Important.

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 less likely
Product family
Windows, Windows ESU (Extended Security Updates)
Updates (KB)
KB5126835, KB5126836, KB5126868, KB5126651, KB5126655, KB5126646

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 31 Oct 2026, 06:43.

Original description (NVD)

Improper verification of cryptographic signature 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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