This is one of those topics that are probably on the top 10 to do’s of anyone’s list when it comes to securing their Windows desktops. Whether it is plain dictatorship, security/confidentiality concerns/requirements, unpatched OS’s, weak/no AV solutions, the golden POLP (Principle Of Least Privileges) may force you to come up with a solution to this problem. If you are using anything else (XP, 2000, 2003 Server) except the newer versions of Windows (Vista, 7) which allow you to do this via a GPO setting, you are out of luck, there is no GPO setting or quick-fix that works.
As a short history, I went through CIA documents that were published (can’t find them anymore), Forums, Microsoft KB’s, Whitepapers, and finally came up at the other end of the tunnel with a working process.
The goal is to devise a process of denying access to USB Storage that meets following criteria:
- Must be implemented at OS level
- Must be deployed scripted/automatically and/or via GPO
- Must not cripple other OS functionality (e.g. installing printer/scanner drivers)
- Must be fully reversible by Administrators only
- Must be working regardless if USB Storage was used before the process is put in place
The solution – explained
For disabling USB Storage there are 2 situations to cover:
- No USB storage ever installed, user must not be able to install device
- USB storage was previously installed by user or admin, user must not be able to use USB Storage again
Both scenarios are covered in these 6 steps:
- Copy usbstor.inf, usbstor.pnf, usbstor.sys to their default locations, as if a USB storage device would be installed.
- Restrict access to the 3 files mentioned above. We will use an implicit DENY for the local “SYSTEM” Account for these files.
- Remove Registry Keys that handle USB Storage device startup: HKLM\SYSTEM\CurrentControlSet\Services\USBSTOR and HKLM\SYSTEM\ControlSet001\Services\USBSTOR and HKLM\SYSTEM\ControlSet002\Services\USBSTOR
- Replace USB Storage related registry keys with specially crafted keys that disable startup of the USB Storage driver
- Apply an implicit DENY for the local SYSTEM Account on the Registry Keys mentioned above
- Insert USB Storage device, wait for it to be detected by OS and marvel at the fact it won’t let you install the device 🙁 🙂
For enabling USB Storage these steps must be taken from an Account that is member of the Administrators Group
- Remove restrictions placed on the ubstor.* files.
- Remove following specially crafted Registry Keys: HKLM\SYSTEM\CurrentControlSet\Services\USBSTOR and HKLM\SYSTEM\ControlSet001\Services\USBSTOR and HKLM\SYSTEM\ControlSet002\Services\USBSTOR
- Remove restrictions placed on the registry keys from above
- Delete incompletely installed USB storage devices fron Device Manager and Reboot Computer
- Insert USB Storage device, wait for it to be detected by OS/go to device manager and refresh device list and marvel at the fact it works 🙂
Implementation – explained
For implementing this in a scripted manner we will use batch scripting, I’m going for a low level approach, assuming you don’t have vbs / powershell on hand, vbs would be rather complicated anyway and Powershell is not installed by default on the OS. You do have some prerequisites:
- reg.exe (available by default on XP)
- A network share
- set-acl (open source utility – get it, copy to a network share of choice and be happy it exists)
- The 3 usbstor files mentioned earlier, 2 are available by default (usbstor.inf and usbstor.pnf) under %WINDIR%\inf. The 3rd, usbstor.sys, unless a usb storage device was previously installed is not present. Find it under %WINDIR%\Driver Cache\i386\Sp3.cab or the other cab files there. Extract it from the cab file to the network share.
- The piece of code that disables USB is written below, but requires that set-acl, the specified .txt, .reg, usbstor.sys files be present in the same directory from which it is executed
::Copy ubstor.sys xcopy /R /H /Y %CD%\usbstor.sys %windir%\system32\drivers ::Secure USBSTOR.* files with ACE (only Local Administrators Full Control, local "SYSTEM" denied Full Control) SetACL.exe -on "c:\windows" -ot file -actn restore -bckp "%CD%\usbstor_ACL.txt" ::Delete settings related to USBSTOR Service REG DELETE HKLM\SYSTEM\CurrentControlSet\Services\USBSTOR /f REG DELETE HKLM\SYSTEM\ControlSet001\Services\USBSTOR /f REG DELETE HKLM\SYSTEM\ControlSet002\Services\USBSTOR /f ::Add special crafted registry keys regedit /s "%CD%\disable_usb.reg" ::Secure keys from above with ACE (only Local Administrators Full Control, local "SYSTEM" denied Full Control) SetACL.exe -on "hklm\SYSTEM" -ot reg -actn restore -bckp "%CD%\HKLM_ControlSet.txt"
- Line 5 of the code uses a file that contains a specially formatted ACL applicable to the 3 usbstor files. To generate a different ACL, use the syntax below for each file you are interested in. When you are finished you can merge all text files in a single text file and add it to the script.
SetACL.exe -on "c:\windows\inf\usbstor.inf" -ot file -actn list -lst "f:sddl;w:d,s,o,g;s:b" -bckp "%CD%\usbstor_inf_ACL.txt"
- REG command is used to delete any data that may exist in the specified registry keys (think previous installed USB Storage)
- Once the Registry is clean of the keys, we then push a customized reg file (find it at the end of the post), that essentially changes this:
USBSTOR driver points to the file you defined (usbstor.sys, that you just set a restrictive ACL on)
DeviceCount equals zero 🙂
DeviceStartUp Type is set to Disabled (more details here)
Other standard settings for that key
- Line 16 of code, similar to the ACL for USBSTOR Files, configures the security for the registry keys we added. To customize the ACL, change it to your liking then export the ACL using the command below and update the batch code to include it.
SetACL.exe -on "hklm\SYSTEM\CurrentControlSet\Services\usbstor" -ot reg -actn list -lst "f:sddl;w:d,s,o,g;s:b" -bckp "%CD%\HKLM_CurrentControlSet.txt"
This is just a question of reversing the changes made by the Disabling process. The following piece of code does just that:
::enable inheritance of permissions SetACL.exe -on "c:\windows\inf\usbstor.inf" -ot file -actn setprot -op "DACL:np;SACL:np" SetACL.exe -on "c:\windows\inf\usbstor.pnf" -ot file -actn setprot -op "DACL:np;SACL:np" SetACL.exe -on "c:\windows\system32\drivers\usbstor.sys" -ot file -actn setprot -op "DACL:np;SACL:np" ::clear any non-inherited ACE SetACL.exe -on "c:\windows\inf\usbstor.inf" -ot file -actn clear -clr "dacl,sacl" SetACL.exe -on "c:\windows\inf\usbstor.pnf" -ot file -actn clear -clr "dacl,sacl" SetACL.exe -on "c:\windows\system32\drivers\usbstor.sys" -ot file -actn clear -clr "dacl,sacl" ::deleting custom Registry Keys REG DELETE HKLM\SYSTEM\CurrentControlSet\Services\USBSTOR /f REG DELETE HKLM\SYSTEM\ControlSet001\Services\USBSTOR /f REG DELETE HKLM\SYSTEM\ControlSet002\Services\USBSTOR /f
- As you can see we are enabling inheritance of permissions, clearing any ACE defined explicitly on that object (the ones we pushed actually) and removing the Registry keys we also pushed. Make sure the user running this enabling process has rights to change these objects (in our case he is member of the Local Administrators Group)
- After this is done manually clean it of any hidden installed USB Storage devices and reboot the computer. After the reboot replugging the device should allow you to install and use it again.
Phew, this was also a long post, but believe me, reaching this compressed format was a lot of work :).
Now I’ve attached this zip file containing the contents of what I’ve been talking about, it should be usable out of the box.
There is also there question I guess of securing these files so that they apply to users but users can’t get to them to “help themselves”, but that is another topics for another post perhaps.
As always any feedback is welcomed.