dot1x failed

Handian Sudianto 7,511 Reputation points
2026-10-08T04:01:39.2+00:00

Sometime we facing issue the client suddently disconnect from the network and i got below error.

Is the client send different certificate to the nac? This is not permanent issue because if i re-plug the cable then the client can connect again

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Windows for business | Windows Client for IT Pros | Networking | Network connectivity and file sharing
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  1. AI answer

    2026-10-08T04:02:00.94+00:00

    Yes. The error indicates that certificate-based 802.1X authentication was attempted, and the certificate presented for user authentication was rejected by the server.

    From the message shown:

    • Reason Text: "The authentication failed because the user certificate required for this network was rejected by the server"
    • Identity: host/...

    That points to a certificate validation problem during EAP authentication, not a random disconnect by itself. Common certificate-related causes in this scenario include:

    • expired certificate
    • invalid certificate
    • certificate chain verification failure
    • certificate revocation check failure
    • wrong or missing extended key usage (EKU)
    • multiple matching certificates on the device, causing the wrong certificate to be selected

    For Windows 802.1X, these are the most useful checks:

    1. Check the wired 802.1X client log
    • Open Event Viewer
    • Go to Applications and Services Logs > Microsoft > Windows > Wired-AutoConfig > Operational
    • Review the failure event at the disconnect time
    1. Check certificate validation details
    • In Event Viewer, go to Applications and Services Logs > Microsoft > Windows > CAPI2
    • Right-click Operational and select Enable Log if it is not already enabled
    • Reproduce the issue and review certificate chain, revocation, and trust errors
    1. Verify the certificate on the client
    • Confirm the certificate used for 802.1X is still valid and not expired
    • Confirm the certificate chain is trusted
    • Confirm it has the proper EKU for authentication
    1. Check whether the wrong certificate is being selected
    • If the device has multiple certificates and the wired or Wi-Fi EAP profile does not use strict filtering, Windows can auto-select the wrong certificate and authentication can fail
    • The profile should be configured so it matches only one intended certificate
    1. Verify the server-side EAP configuration
    • On the NPS side, check which certificate is configured for the EAP method
    • In NPS snap-in, go to Policies > Network Policies > [policy] > Properties > Constraints > Authentication Methods
    • Confirm the expected certificate and EAP method are configured

    Relevant 802.1X certificate failure categories documented by Windows include:

    • server certificate expired, invalid, not found, or revoked
    • user certificate expired, invalid, or revoked
    • trusted root certificate expired or invalid

    So the most likely answer is: the client is sending a certificate, but the NAC/NPS is rejecting it. If multiple certificates exist on the endpoint, it can also be sending a different certificate than intended.

    AI-generated content may be incorrect. Read our transparency notes for more information.

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  2. Harry Phan 34,045 Reputation points Independent Advisor
    2026-10-08T04:11:34.3+00:00

    Hello Handian,

    No, this does not necessarily mean the client is sending a different certificate. Error 0x80420105 indicates that the certificate presented during 802.1X authentication was rejected by the NAC/RADIUS server.

    Since re-plugging the cable restores connectivity, this is more likely an intermittent reauthentication or certificate-selection issue than a permanently invalid certificate. Check the NAC/RADIUS logs at the failure timestamp and compare the certificate thumbprint/serial number with the successful authentication after re-plugging. Also check Event Viewer > Microsoft > Windows > Wired-AutoConfig > Operational on the client; if the same certificate fails first and succeeds after re-plugging, focus on the 802.1X reauthentication/session handling rather than certificate replacement.

    Harry P.

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