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Anthony Mastrean
Asked: 2012-06-14 12:31:49 +0800 CST

What are the minimum permissions to read the WMI class 'MSSerial_PortName'?

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I am configuring a local application user (part of BUILTIN\Users) on a Windows 7 kiosk. The kiosk has a special USB device running on a virtual COM port. The user needs permission to read the WMI class, MSSerial_PortName, in the root\WMI namespace, to find the COM port. In PowerShell (which I'm using to verify the configuration)

PS> Get-WmiObject -namespace 'root\WMI' -class 'MSSerial_PortName'

and by regular .NET code (which is how the application is written)

ManagementObjectSearcher searcher = new ManagementObjectSearcher("root\\WMI", "SELECT * FROM MSSerial_PortName");

I get "access denied" errors unless running as an Administrator or elevated session. I have read plenty of Q&A on similar access problems, but most seem to recommend running as Administrator. That's not an option for this user/kiosk/configuration. And, at the moment, I am unable to use the managed wrapper from the device vendor.

I played around in the Microsoft Management Console, loading the WMI Control, and modified the Properties | Security | namespace: Root\WMI. I set the Users group to have the same permissions as the Administrators group. But that didn't work (I was just guessing anyway).

I can't find any MSDN documentation on MSSerial_PortName the MSSerial "base" class or other related classes in the namespace (as suggested by this article). And I don't know anything else about WMI security.

windows wmi serial

2 Answers

  1. I hit the same wall today. This works for me temporarily...

    catch (ManagementException ex)
{
    Debug.WriteLine( string.Format( "##DBG An error occurred while querying for WMI data to find available COM ports:\n Message: {0}\n Stacktrace: {1}",  ex.Message, ex.StackTrace) );

    bool bSucceed = true;

    // TODO Q&D solutions. As it does not work as expected (on windows 7 ) we create our ow default list here and check if we can open the ports
    for (int x = 1; x <= 9; x++)
    {
        bSucceed = true;
        cComportName = string.Format("COM{0}", x);

        /////////////////////
        // Check if we can open it here

        // Set the port's settings
        m_comport.BaudRate = 9600; 
        m_comport.DataBits = 8; // int.Parse(cmbDataBits.Text);
        m_comport.StopBits = (StopBits)Enum.Parse(typeof(StopBits), "1" );
        m_comport.Parity = (Parity)Enum.Parse(typeof(Parity), "None" );
        m_comport.PortName = cComportName;
        try
        {
            // Open the port
            m_comport.Open();
        }
        catch (UnauthorizedAccessException) { bSucceed = false; }
        catch (IOException) { bSucceed = false; }
        catch (ArgumentException) { bSucceed = false; }

        if (bSucceed) 
        {
            m_comport.Close();
            m_listComPorts.Add(new string[ConstComPortAttr.COMPORT_MAX_COLUMNS] { cComportName, cInstanceName });
        }

    }
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  2. I've found another way in PowerShell and it does not require admin privileges:

    PS> Get-WmiObject -Namespace root/cimv2 -Class Win32_PnPEntity -Filter "ClassGuid='{4d36e978-e325-11ce-bfc1-08002be10318}'"

    Unfortunately it does not give you portname in COMx form, but using a simple text parsing you should be able to extract the port name from device name.

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