The NT environment allows the security to be very flexible. For an administrator, they should be aware of the issues for having a secure NT machine. Here are some of the major security issues.
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To really lock NT down hard, set the root directory to full access for administrators and system, list access to users (not Everyone). Let that work all the way down the tree. Loosen things up as need be, but what has been done is ensure that any new directory that gets created will have those permissions.
Make sure the print spool directory has full access to creator\owner (see the NT Resource Kit, 3.51 Update 1 (also known as vol 5)).
Go through (using cacls, or use the search facility of either file manager or explorer) and set the permissions on all of the executables and DLLs to full access to admins (or if people normally work on that machine under admin status, remove write permission for admins), and list only (read-execute) permissions to users.
Note that it is now difficult for users to install any software. This could be good or bad, depending on what you want to do. Make a list of common DLLs that are updated often and give users delete permission.
Now apply the "smoke test" - log in as a user, and see what is broken. Some programs insist on being able to write to an .ini file in the system tree - if users can't write to (or create) these files, these programs will fail. Change the permissions as need be.
Be careful, it is possible where non-admins either can't successfully log in, or get a desktop that is completely blank.
If users are allowed to store files locally, make sure that they have full rights to their own directories. Note that under NT 4.0, a user's desktop profile, and numerous other things are stored under the system tree - look in %systemroot%\profiles, and make sure each user has full rights to their subdirectory - it should be admin, system, and user have full access.
It is a good idea to loosen up the temp directory - a good thing is to give users list access, but creator\owner full access. There may be other directories that need work, depending on what apps are installed, and whether they have any notion of multiple users - one example would be the cache directory for a web browser.
Since people have a lot of different needs, there is no single answer - it depends on the environment.
As to user rights, go through and make sure Guest is not only disabled, but that it has no rights to anything.
Give careful attention to who is allowed to log on from the network and locally.
One thing to consider is that the administrator account is on every machine, and can't be locked out from too many bad passwords. A good way around this is to remove the administrator's group from the permissions to log on from the network, and add back in the individual users who are the admins.
Now go set it up to audit failed login attempts, lock out users for a few minutes if there are too many login failures, and require a password of decent length - 6 characters is acceptable. This makes brute force attacks very difficult. If you want to prevent other users from accessing the machine remotely, you can also remove the users from the right to log on from the network - that confines the users to having to use the shares on the server. This also prevents anyone not given that right from accessing the event log, the registry, and the shares on the machine. Pay attention to who can and cannot shut the machine down, and make it require you to log in to shut it down.
Point to Point Tunneling Protocal
This is a feature in NT 4.0 that allows encryption between an NT 4.0 server and possible dialins. There is source code available on http://www.microsoft.com. There are several companies that provide dialin access such as US Robotics that is adding in support for PPTP.
In the registry, Remove write permission to Everyone from HKEY_CLASSES_ROOT, and give full access to creator\owner, which is what Microsoft did with NT 4.0 - much more secure.
Go through all the shares that are available and make sure that the permissions are set correctly - don't accept the default of full access to everyone.
The file sharing service if available and accessible by anyone can crash the NT 3.51 machine by using the dot..dot bug and require it to be rebooted. This technique on a Windows 95 machine potentially allows anyone to gain access to the whole hard drive. This vulnerability is documented in Microsoft Knowledge Base article number Q140818 last revision dated March 15, 1996. Resolution is to install the latest service pack for Windows NT version 3.51. The latest service pack to have the patch is in service pack 4.
Versions prior to 1.0c were vulnerable to allowing users to execute commands remotely and allow access to all the files on the same hard drive partition as the IIS Server. Make sure that the web server is version 1.0c or higher. NT 4.0 comes with IIS Version 2.0 that fixes these known problems.
Additonal Information on the IIS Web Server bugs is available at http://www.omna.com/msiis .
Many times FTP is configured to allow anyone to log in and have access to the whole hard drive. Attempt to log in and check to see what files are accessible. By doing a "cd ..", it may allow people to go higher in the file system that what is intended.
Network File System can easily be configured to allow anyone to have access to files being exported. Check to see if they are correctly configured for the proper exports.
There is an rsh server that comes with NT. Rsh is a service that allows people to configure their login to not require a password if coming from certain machines. Intruders have figured out ways to by-pass this security and it is recommended to not allow this server to run.
Here are two other great resources of NT Security Information: