Practical recommendations for securing Internet-connected Windows NT Systems

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Do not download or run programs from untrusted sources. Programs can contain instructions to violate security in several ways including data theft, denial of service, and data destruction.

Review the following list to learn how to reduce security risks:

• If your private network runs TCP/IP, the servers Enable IP Forwarding check box in the Network application should not be selected.
  
  Clearing the Enable IP Forwarding check box prevents unauthorized IP packets from infiltrating your network. The Enable IP Forwarding check box is located in the Microsoft TCP/IP Properties dialog box. To open this, use the Network application in Control Panel.

To disable IP forwarding on Microsoft Windows NT Server version 4.0:

1. From the Start menu, select Settings, and then click Control Panel.
2. In Control Panel, double-click the Network icon.
3. In the Network dialog box, click the Protocols tab, select TCP/IP Protocol, and then click Properties.
4. In the Microsoft TCP/IP Properties dialog box, click Routing.
5. Make sure the check box for Enable IP Forwarding is cleared.
6. Click OK, then click OK again.

WARNING: If the Windows NT Remote Access Service (RAS) is installed on your gateway after Microsoft Proxy Server is installed, IP forwarding will be enabled. You must disable IP forwarding after installing RAS.

• Block Nonessential Inbound TCP/IP Ports.
  
  If your Windows NT Server is highly exposed, with the mission of offering services like Web and FTP, then only two inbound paths need to exist from the router to the server: HTTP on port 80 and FTP on port 21. The router should block all other inbound traffic.
  
  If you are using the Proxy Server and have 2 netcards on your Computer, you can bind ONLY IPX on the Internal netcard and ONLY IP on the external netcard.
• Disable NetBios over TCP/IP.
  
  By default an Internet-connected Windows NT computer will support two transport protocols: NetBeui and TCP/IP. Windows networking operations require syntax of the form of \\Name. These operations include directory and printer sharing, NetDDE, and remote administration. Connecting to a drive or editing a registry across the Internet requires only a mapping, in the local LMHOSTS file, between the remote computer's NetBIOS name and its IP address.
  
  You can control the use of NetBIOS over TCP/IP (NBT) by going into Control Panel, Network and the Bindings tab and disabling any of the bindings between NetBIOS-based services and TCP/IP. This way no one can try to remote-mount drives or remote-edit registries. Windows NT networking services run promiscuously over multiple transports; therefore, internally your computers can still talk to each other over the NetBEUI protocol, which does not go over the Internet.
• Use NTFS volumes.
  
  The Windows NT File System (NTFS) provides security and access control for your data files. By using NTFS, you can limit access to portions of your file system for specific users and services. A File Allocation Table (FAT) only supports share level security.
  
  For safety's sake it is best to layer multiple defenses, so use NTFS on Internet-connected Windows NT computers. Windows NT takes the intersection of NTFS ACLs and share permissions, for example if NTFS ACLs gives a network user full access to a partition but the share-level permissions grant only read access, then the effective access is read only. If you create new shares, be sure to alter the default permissions assigned by Windows NT. Otherwise, by default, the group Everyone will have Full Control of all that is visible through the share.
• Run only the services that you need.
  
  The fewer services you are running on your computer, the less likely a mistake will be made in administration that could be exploited. Use the Services application in the Control Panel to disable any services not absolutely necessary. Also, if FTP or Gopher services are not needed or used, turn off these services using Internet Service Manager to stop each service.
• Unbind unnecessary services from your Internet adapter cards.
  
  Use the Bindings feature in the Network application in the Control Panel to unbind any unnecessary services from any network adapter cards connected to the Internet. For example, you might use the Server service to upload new images and documents from computers in your internal network, but you might not want users to have direct access to the Server service from the Internet. If you need to use the Server service on your private network, the Server service binding to any network adapter cards connected to the Internet should be disabled.
  
  You can use the Windows NT Server service over the Internet; however, you should fully understand the security implications and comply with Windows NT Server licensing requirements issues. When you are using the Windows NT Server service you are using Microsoft networking or the Server Message Block (SMB) protocol and all Windows NT Server licensing requirements still apply.
• Check permissions set on network shares.
  
  If you are running the Server service on your Internet adapter cards, be sure to double check the permissions set on the shares you have created on the computer. It is also wise to double check the permissions set on the files contained in the shares directories to ensure that you have set them appropriately.
• Access from Network privilege can be revoked.
  
  By default, Windows NT grants the group Everyone the right to Access from the network. By revoking this right you can block all networking services, but maintain support for the Web service because the Web server runs either as a System or Local user.
• Rename and limit the membership of the Administrator group.
  
  Rename the Administrator account by selecting User, Rename from User Manager menu. By limiting the members of the Administrator group, you limit the number of users who might choose bad passwords.
• Enforce strict account policies.
  
  User Manager for Domains provides configuration options called security policies, such as one that allows a system administrator to specify how quickly account passwords expire (forcing users to regularly change passwords), and another that determines how many bad logon attempts will be tolerated before a user is locked out. Use the User Manager for Domains security policies to configure the server against exhaustive or random password attacks.
• Choose good passwords.
  
  Although this may seem obvious, a stolen or easily guessed password is the best opportunity for someone to gain access to your computer. Make sure that all passwords used, especially those with administrative rights, have difficult-to-guess passwords. In particular make sure to select a good administrator password (long, mixed-case, alphanumeric password) and set the appropriate account policies. Passwords can be set by using Windows NT User Manager for Domains.
  
  For additional information, please see Chapter 2 of the Microsoft Proxy Server documentation has information on the above topic.

More information on securing an Internet connected Web server can be found in Chapter 8 of the Microsoft Internet Information Server Resource Kit. ISBN:1-57231-638-1