Service Running as System Account Fails Accessing Network


When an application is started using the system account, it logs on with null credentials. If it attempts to access a remote Windows NT 3.5 server resource (using a null session), it fails. Scheduler service scripts (AT command) and other service or application operations attempting to access server resources over the network receive errors such as the following:

Error 5: Access is denied.
When an application using the system account uses a UNC name to access a shared resource, the following error message is returned:

File Not Found
NOTE: Scheduler service script error messages can be caught by redirecting the AT command to a file.


Services, including the Scheduler, use null session support when logged on as system account to interact with desktop.

Windows NT version 3.1 supports null sessions by default. However, Windows NT version 3.5 does not provide this support unless specifically configured to do so. This change was made in order to provide a higher default level of security.

When a process is running under the context of the system account, it attempts to use a null session for credentials to acquire a resource access. AT scripts performing commands, such as NET USE, will default to these credentials. A protocol analyzer trace reveals the null session request associated with the Server Message Block (SMB) Session Setup & X in which no explicit credentials are passed.


Do not use the system account. Services, such as Scheduler, and custom applications can be configured using user-specific accounts. User-specific accounts provide user level security based on a specific account and associated password.

NOTE: For more information, see Chapter 15: Server Manager, Assigning a Logon Account to the Schedule Service in the "Windows NT Server 3.5 System Guide."

If you desire null session support, you can modify the registry of each remote resource computer as follows.


  • A shared resource configured in this manner is not secure. Microsoft does not recommend this configuration if null session security is a consideration.
  • Using the Registry Editor incorrectly can cause serious, system- wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of the Registry Editor can be solved. Use this tool at your own risk.
  1. Start the registry editor, REGEDT32.EXE
  2. From the HKEY_LOCAL_MACHINE subtree, go to the following key:


    NOTE: NullSessionShares is type REG_MULTI_SZ
  3. On a new line within the NullSessionShares key, type in the share you want to access with a null session (for example: "PUBLIC")
  4. If your application uses Named Pipes and requires null session support.

    1. From the HKEY_LOCAL_MACHINE subtree, go to the following key:


      NOTE: NullSessionPipes is type REG_MULTI_SZ
    2. On a new line within the NullSessionShares key, type in the pipe you want to access with a null session.
  5. Stop and restart the server.
For additional information, please see the following articles in the Microsoft Knowledge Base:

ARTICLE-ID: 132679
TITLE : Local System Account and Null Sessions in Windows NT

ARTICLE-ID: 122702
TITLE : Using the System Account as a Service in Windows NT 3.5

ARTICLE-ID: 121281
TITLE : LMHOSTS #Include Directives Requires Null Session Support

ID članka: 124184 – Zadnji pregled: 31. okt. 2006 – Revizija: 1

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