Monday, March 12, 2012

How to disable USB sticks and limit access to USB storage devices on Windows systems

USB storage - a possible security risk?

Decent IT administrators secure their networks behind firewalls. They install mail filters on their SMTP servers and deploy anti-virus software on all client workstations. But securing the network is not sufficient -- what happens if the users bring their own USB memory sticks and connect them to the computers at their office? A 1 Gb USB stick can sometimes hold an entire company's vital data. Within minutes or even seconds an employee has all the files they need in order to start up their own business and take all the customers with them. Alternatively, what happens if a careless user accidentally compromises the network with an infected USB stick?

What does Microsoft have to say about it?

If you, the administrator, want to establish a minimum level of security, it is absolutely necessary to control which users can connect USB memory sticks to a computer. Unfortunately, a default Windows XP or Windows 2000 installation comes with no limitations on who is able to install and use USB storage media. Microsoft knowledge base article 823732 contains instructions on how to disable USB storage access for a certain group of users; however, the article only distinguishes between whether or not a USB storage device has been installed on a particular computer. Furthermore, the instructions are limited to a stand-alone computer. According to the general rule of thumb "If it's tedious, there is a better way", I try to avoid techniques that force me to repeat certain tasks for each computer that I manage. That's what group policy objects (GPO) are for.

Suggestions?

Mark Heitbrink describes how to disable USB storage devices entirely on all or some computers in the network. He employs an ADM template in a group policy object that disables the USB storage driver (USBSTOR). The ADM template simply sets the registry valueHKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\UsbStor\Start to 4 (Disable). But his technique has a serious drawback. It only works if the USB storage driver is already installed. If it has not yet been installed, Windows' plug & play subsystem automatically resets the Start value to 3 (Manual) when it installs USBSTOR after a USB storage device is plugged in for the first time. In that case, USBSTOR remains enabled until the GPO is re-applied, usually at the next reboot. If the storage device is plugged in during that reboot, it will still be available because the USBSTOR driver is started before any GPOs are processed.

The Howto!

If we combine Mark Heitbrink's approach with the one outlined in knowledge base article 823732, we get a more reliable solution. Firstly, we need to prevent USBSTOR from being installed unless the currently logged on user is allowed to use USB storage. We do that by restricting access toUSBSTOR.INF and USBSTORE.PNF in a GPO such that PNP can't automatically install the driver. This is possible because when PNP installs a driver, the installation is performed using the priviledges of the currently logged on user. Secondly, we need to make sure that USBSTOR is not started when a USB storage device is plugged in. For that we use Mark's ADM template. The only minor drawback of my solution is that users with access to USB storage need to manually start USBSTOR before connecting USB storage devices.

1. In Active Directory Users and Computers, open an existing GPO or create a new one and open it. Use the security settings of that GPO to specify which computers it affects.

2. In that GPO, go to Computer Configuration – Windows Settings – Security Settings – File System and create a new entry (right-click File System and select Add File). Specify the location of USBSTOR.INF (usually SystemRoot%\Inf\USBSTOR.INF)

3. Change the security settings of the new entry. The security settings that you specify here will be enforced on the USBSTOR.INF of every computer to which the GPO is applied. This process is not additive, which means that the previous security settings ofUSBSTOR.INF will be overwritten by the ones given in the GPO. It is therefore recommended to grant full control to SYSTEM and local administrators. But unlike in the default security settings of USBSTOR.INF, you should not grant any priviledges to Everybody. You do not need to explicitly deny access – just omit an entry for Everybody. Optionally, you can grant read access to a certain group. Members of this group will be able to use USB storage.

4. Repeat the above two steps for USBSTOR.PNF.

5. Download USBSTOR.ADM.

6. Back in the GPO, right-click Administrative Templates under Computer Configuration and select Add/Remove Templates. Click Add and browse to the location of USBSTOR.ADM. Close the dialog.

7. You should now have an additional entry called Services and Drivers in Administrative Templates. Click on it. If it is empty, select View from the menu and uncheck Show Policies Only. Click back on Services and Drivers in Administrative Templates. It should now show the USB Storage policy. Double click it, select Enabled and pick Disabled from the Startup Type drop down. Again, the policy must be enabled wheras Startup Type must be Disabled.

8. Close the dialog as well as the GPO and boot/reboot one of your workstations. Make sure no USB strorage device is connected to that computer. Log on with administrative privileges and check the permissions of USBSTOR.INF and USBSTOR.PNF. Check the value of the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\UsbStor\Start. It should be 4. It is also ok if the UsbStor key doesn't exist at all.

9. On the same workstation, log off and back on as a user that should not have access to USB storage. Connect a USB memory stick or a similar device. Nothing should happen. Remove the memory stick.

10. Log on as a user that should have access to USB storage and execute net start usbstorin a command shell or at Start – Run before connecting the memory stick. The memory stick should initialized and mapped to a drive letter. If USBSTOR fails to start, it's probably because this is the first time a memory stick is plugged into the workstation in which case USBSTOR is not yet installed. Nevertheless, the memory stick should be initialized and mapped correctly but you need to reboot in order to reapply the administrative template such that USBSTOR is disabled again. Alternatively, you can disable it manually by downloading and double clicking USBSTOR.REG as well as executing net stop usbstor.

11. Instruct the users with access to USB storage that they need to execute net start usbstor before they can connect a USB storage device.