Local System Account and Null Sessions in Windows NT

MORE INFORMATION

Background

The shell application, which is typically Progman.exe, runs under the Domain\User security context. Most processes that start from the shell process inherit the same security context.

When you set up Windows NT services, you choose a security context for the service to be started under (because these services are typically not started by a user). To check the security being used by a service:

1. Click Start, click Run, type control panel, and then double click Services.
2. Select a service, and then click Startup.

The security context of the service is set in the Log On As window pane.

Services Using The System Account

Services that use the system account start in the system context (without credentials). In Windows NT 3.5 and later, Windows NT services with no credentials (no domain name, user name, or password) that attempt to connect to network resources are denied access because they have no credentials and they are using a null session. For additional information, click the article number below to view the article in the Microsoft Knowledge Base: NOTE: System Account and This Account: Local System use the same account.



A null session is only established when there are no credentials for a process to start under (no user name or password). Typically, only the operating system itself runs as system.

On the local computer, the operating system is known as:

   Default Owner:   Administrators    local group
   User:            System            pseudo group - local group scope
   Groups:          Administrators    local group
                    Everyone          pseudo group - local group scope
				

When this context is used to access the network, a null session is used. This produces the following context on remote computers:

   Default Owner:   Everyone
   User:            Everyone
   Groups:          AnonymousLogon    pseudo group - local group scope
                    Network           pseudo group - local group scope
				

Only three identifiers can provide the null session access (Everyone, AnonymousLogon, and Network). The local system context and null session context have only the identifier Everyone in common. To configure Windows NT so that a service can access objects on its own computer directly, as well as over the network, use the Everyone identifier.

The default owners of these two contexts (as well as their default DACLs) are different. Any files you created in these contexts will be owned by Administrators. Any files you create through a null session will be owned by Everyone.