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WININET_E_CLIENT_AUTH_CERT_NEEDED
WININET_E_CLIENT_AUTH_CERT_NEEDED means that the server requested a client certificate during TLS authentication. It does not indicate that the server certificate is invalid; the server is asking the client to prove its own identity.
Selection criteria
- Choose a certificate with an available private key and a client-authentication purpose appropriate for the server.
- Use the issuer constraints supplied by the server when selecting from multiple certificates.
- Make the selection policy explicit for non-interactive applications; a UI prompt is not a safe fallback for a service.
Do not assume anonymous fallback succeeds
Microsoft notes that a UI path may continue anonymously if no certificate is selected, but the server can still reject the next request. Treat this status as a TLS credential requirement and log the requested endpoint and selected certificate identity without exposing private material.
Microsoft: InternetErrorDlg client certificate handling · Microsoft: WinINet error messages
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