CVE-2021-40456Network attack vector

Authentication restriction bypass via WS-Trust IP filtering

Published Oct 13, 2021 · Updated Aug 4, 2024

Incorrect authorization in Microsoft AD FS on affected Windows Server releases allows remote attackers to bypass banned IP restrictions. The AD FS WS-Trust workflow fails to enforce BannedIPList entries against requests originating from blocked addresses. Exposure requires a reachable WS-Trust endpoint; the bypass permits authentication requests from a blocked source but does not itself grant credentials.

CVSS severity5.3
Medium
EPSS probability2.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect authorization in Microsoft AD FS on affected Windows Server releases allows remote attackers to bypass banned IP restrictions. The AD FS WS-Trust workflow fails to enforce BannedIPList entries against requests originating from blocked addresses. Exposure requires a reachable WS-Trust endpoint; the bypass permits authentication requests from a blocked source but does not itself grant credentials.

The record

CVE
CVE-2021-40456
Published
Oct 13, 2021
Updated
Aug 4, 2024
Vendor
Microsoft
Product
Windows Server 2019
Classifications
CWE-863, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record 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=10.0.0 <10.0.17763.2237

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Windows Server 2019version=10.0.0 <10.0.17763.2237
Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19041.1288
Microsoft · Windows Server 2019 (Server Core installation)version=10.0.0 <10.0.17763.2237
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19042.1288
Microsoft · Windows Server 2022version=10.0.0 <10.0.20348.288

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:P/I:N/A:N
5.3Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

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

Weakness, pattern, technique

CWE-863Incorrect Authorization
T1190Exploit Public-Facing Application

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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