A systems administrator establishes a CIFS share on a Unix device to share data to windows systems. The security authentication on the windows domain is set to the highest level. Windows users are…

When Windows domain security is at its highest level, clients send only NTLMv2 and refuse to fall back to LM or NTLM. However, configuring the Unix Samba server to refuse LM and accept only NTLMv2 exposes a known compatibility gap - older Samba implementations do not fully support the extended security negotiation and signing requirements that Windows enforces at this level. This NTLMv2 implementation mismatch on the Samba side is the direct cause of the authentication failure.

Accepting only LM on the Unix server would also block Windows clients at high security (since they send NTLMv2, not LM), but this is not the configured setting and does not reflect the NTLMv2 compatibility issue specific to this scenario.

Refusing NTLMv2 and accepting LM would create an obvious protocol mismatch since Windows sends NTLMv2, but this setting is not what is configured on the Unix server in question.

Accepting only NTLM (v1) would fail because Windows at the highest security level sends NTLMv2 exclusively, but this setting is not the one causing the described problem.