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SEC_E_ILLEGAL_MESSAGE
SEC_E_ILLEGAL_MESSAGE signals this condition: The security package received an unexpected or malformed authentication message. This often means the SSPI token was corrupted, truncated, sent to the wrong package, or exchanged out of protocol order. It is not normally solved by changing the target user account.
What to check
- Confirm that both endpoints negotiated the same security package and protocol version.
- Capture the stage of the handshake and any preceding transport or proxy errors.
- Check application code for token reuse, incorrect buffer lengths, or a message sent to the wrong SSPI context.
Microsoft: InitializeSecurityContext
Microsoft: CredSSP token errors
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