Disabled by default, NT can communicate passwords in plaintext. Can be done to support certain non-NT systems. Not recommended Following NT 4 Service Pack 3, the SMB file sharing enhancements by default do not allow connection to SMB servers such as Samba, HP LM/X, or LAN Manager for Unix unless plain text passwords are enabled. To enable:

Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Services\RDR\Parameters
Name: EnablePlainTextPassword
Type: REG_DWORD
Value: 1

See KB Q166730 - Unencrypted Passwords May Cause SP3 to Fail to Connect to SMB Servers.

A must have for NT administrators in corporate or governmental organizations or anyone being audited by a large outside audit firm.
It is not a secrets type guide but it has excellent sound advice and its used by PriceWaterhouse's auditors as a guide.

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