Modifying the DACL for a Service

A service control program can create or modify the DACL associated with a service to control access. To retrieve the DACL associated with a service object, use the QueryServiceObjectSecurity function. To set the DACL, use the SetServiceObjectSecurity function. Any changes made to the SECURITY_DESCRIPTOR associated with the service object are persistent until the service is removed from the system.

The following example creates and sets a new DACL for the service. The code merges one access control entry (ACE) to the existing DACL for the service. The new ACE grants the Guest account start, stop, delete, and READ_CONTROL access to the specified service. Access to the service can be modified by the AccessPermissions parameter passed to the BuildExplicitAccessWithName function.

The szSvcName variable is a global variable that contains the name of the service. For the complete example that sets this variable, see SvcControl.cpp.

//
// Purpose: 
//   Updates the service DACL to grant start, stop, delete, and read
//   control access to the Guest account.
//
// Parameters:
//   None
// 
// Return value:
//   None
//
VOID __stdcall DoUpdateSvcDacl()
{
    EXPLICIT_ACCESS      ea;
    SECURITY_DESCRIPTOR  sd;
    PSECURITY_DESCRIPTOR psd            = NULL;
    PACL                 pacl           = NULL;
    PACL                 pNewAcl        = NULL;
    BOOL                 bDaclPresent   = FALSE;
    BOOL                 bDaclDefaulted = FALSE;
    DWORD                dwError        = 0;
    DWORD                dwSize         = 0;
    DWORD                dwBytesNeeded  = 0;

    // Get a handle to the SCM database. 
 
    schSCManager = OpenSCManager( 
        NULL,                    // local computer
        NULL,                    // ServicesActive database 
        SC_MANAGER_ALL_ACCESS);  // full access rights 
 
    if (NULL == schSCManager) 
    {
        printf("OpenSCManager failed (%d)\n", GetLastError());
        return;
    }

    // Get a handle to the service

    schService = OpenService( 
        schSCManager,              // SCManager database 
        szSvcName,                 // name of service 
        READ_CONTROL | WRITE_DAC); // access
 
    if (schService == NULL)
    { 
        printf("OpenService failed (%d)\n", GetLastError()); 
        CloseServiceHandle(schSCManager);
        return;
    }    

    // Get the current security descriptor.

    if (!QueryServiceObjectSecurity(schService,
        DACL_SECURITY_INFORMATION, 
        &psd,           // using NULL does not work on all versions
        0, 
        &dwBytesNeeded))
    {
        if (GetLastError() == ERROR_INSUFFICIENT_BUFFER)
        {
            dwSize = dwBytesNeeded;
            psd = (PSECURITY_DESCRIPTOR)HeapAlloc(GetProcessHeap(),
                    HEAP_ZERO_MEMORY, dwSize);
            if (psd == NULL)
            {
                // Note: HeapAlloc does not support GetLastError.
                printf("HeapAlloc failed\n");
                goto dacl_cleanup;
            }
  
            if (!QueryServiceObjectSecurity(schService,
                DACL_SECURITY_INFORMATION, psd, dwSize, &dwBytesNeeded))
            {
                printf("QueryServiceObjectSecurity failed (%d)\n", GetLastError());
                goto dacl_cleanup;
            }
        }
        else 
        {
            printf("QueryServiceObjectSecurity failed (%d)\n", GetLastError());
            goto dacl_cleanup;
        }
    }

    // Get the DACL.

    if (!GetSecurityDescriptorDacl(psd, &bDaclPresent, &pacl,
                                   &bDaclDefaulted))
    {
        printf("GetSecurityDescriptorDacl failed(%d)\n", GetLastError());
        goto dacl_cleanup;
    }

    // Build the ACE.

    BuildExplicitAccessWithName(&ea, TEXT("GUEST"),
        SERVICE_START | SERVICE_STOP | READ_CONTROL | DELETE,
        SET_ACCESS, NO_INHERITANCE);

    dwError = SetEntriesInAcl(1, &ea, pacl, &pNewAcl);
    if (dwError != ERROR_SUCCESS)
    {
        printf("SetEntriesInAcl failed(%d)\n", dwError);
        goto dacl_cleanup;
    }

    // Initialize a new security descriptor.

    if (!InitializeSecurityDescriptor(&sd, 
        SECURITY_DESCRIPTOR_REVISION))
    {
        printf("InitializeSecurityDescriptor failed(%d)\n", GetLastError());
        goto dacl_cleanup;
    }

    // Set the new DACL in the security descriptor.

    if (!SetSecurityDescriptorDacl(&sd, TRUE, pNewAcl, FALSE))
    {
        printf("SetSecurityDescriptorDacl failed(%d)\n", GetLastError());
        goto dacl_cleanup;
    }

    // Set the new DACL for the service object.

    if (!SetServiceObjectSecurity(schService, 
        DACL_SECURITY_INFORMATION, &sd))
    {
        printf("SetServiceObjectSecurity failed(%d)\n", GetLastError());
        goto dacl_cleanup;
    }
    else printf("Service DACL updated successfully\n");

dacl_cleanup:
    CloseServiceHandle(schSCManager);
    CloseServiceHandle(schService);

    if(NULL != pNewAcl)
        LocalFree((HLOCAL)pNewAcl);
    if(NULL != psd)
        HeapFree(GetProcessHeap(), 0, (LPVOID)psd);
}

Related topics

The Complete Service Sample