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ERROR_CS_ENCRYPTION_UNSUPPORTED_SERVER
The server's advertised behavior and actual CSE support disagree
An ordinary SMB server can read and write normal files without implementing the Windows Client-Side Encryption workflow used for EFS-protected remote files. This error is especially diagnostic because the server claimed support but could not perform the required operation.
Do not downgrade to plaintext automatically. Verify the server product, version, underlying file system, and any gateway modifying SMB traffic. A supported migration may require a compatible Windows file server or a trusted local decrypt-and-reencrypt step.
What to inspect
- The capability or earlier response that caused the client to enter CSE mode.
- The server's documented EFS support for the negotiated SMB dialect and target share.
- Whether the backing file system can store the required encrypted-file metadata.
- Differences between local EFS support and remote share support.
References
- Microsoft: System error codes 6000-8199
- Microsoft Open Specifications: Win32 error codes
- Microsoft Open Specifications: EFSRPC relationship to SMB
- Microsoft Open Specifications: Existing encrypted-file open algorithm
- Linux ksmbd: SMB status definitions
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