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The Power in Power Users

Placing Windows user accounts in the Power Users security group is a common approach IT organizations take to get users into a least-privilege environment while avoiding the many pains of truly running as a limited user. The Power Users group is able to install software, manage power and time-zone settings, and install ActiveX controls, actions that limited Users are denied.

What many administrators fail to realize, however, is that this power comes at the price of true limited-user security. Many articles, including this Microsoft Knowledge Base article and this blog post by Microsoft security specialist Jesper Johansen, point out that a user that belongs to the Power Users group can easily elevate themselves to fully-privileged administrators, but I was unable to find a detailed description of the elevation mechanisms they refer to. I therefore decided to investigate.

Before I could start the investigation, I had to define the problem. In the absence of a security flaw such as a buffer overflow privilege escalation is possible only if an account can configure arbitrary code to execute in the context of a more-privileged account. The default accounts that have more privilege than Power Users include Administrators and the Local System account, in which several Windows service processes run. Thus, if a Power Users member can modify a file executed by one of these accounts, configure one of their executables to load an arbitrary DLL, or add an executable auto-start to these accounts, they can obtain full administrative privileges.

My first step was to see what files and directories to which the Power Users group has write access, but that limited users do not. The systems I considered were stock Windows 2000 Professional SP4, Windows XP SP2, and Windows Vista. I'm not going to bother looking at server systems because the most common Power Users scenario is on a workstation.

The brute force method of seeing what file system objects Power Users can modify requires visiting each file and directory and examining its permissions, something that’s clearly not practical. The command-line Cacls utility that Windows includes dumps security descriptors, but I’ve never bothered learning Security Descriptor Description Language (SDDL) and parsing the output would require writing a script. The AccessEnum utility that Bryce wrote seemed promising and it can also look at Registry security, but it’s aimed at showing potential permissions weaknesses, not the accesses available to particular accounts. Further, I knew that I’d also need to examine the security applied to Windows services.

I concluded that I had to write a new utility for the job, so I created AccessChk. You pass AccessChk an account or group name and a file system path, Registry key, or Windows service name, and it reports the effective accesses the account or group has for the object, taking into consideration the account’s group memberships. For example, if the Mark account had access to a file, but Mark belongs to the Developers group that is explicitly denied access, then AccessChk would show Mark as having no access.

In order to make the output easy to read AccessChk prints ‘W’ next to the object name if an account has any permissions that would allow it to modify an object, and ‘R’ if an account can read the object’s data or status. Various switches cause AccessChk to recurse into subdirectories or Registry subkeys and the –v switch has it report the specific accesses available to the account. A switch I added specifically to seek out objects for which an account has write access is –w.

Armed with this new tool I was ready to start investigating. My first target was a Windows XP SP2 VMWare installation that has no installed applications other than the VMWare Tools. The first command I executed was:

accesschk –ws “power users” c:\windows

This shows all the files and directories under the \Windows directory that the Power Users group can modify. Of course, many of the files under \Windows are part of the operating system or Windows services and therefore execute in the Local System account. AccessChk reported that Power Users can modify most of the directories under \Windows, which allows member users to create files in those directories. Thus, a member of the Power Users group can create files in the \Windows and \Windows\System32 directory, which is a common requirement of poorly written legacy applications. In addition, Power Users needs to be able to create files in the \Windows\Downloaded Program Files directory so that they can install ActiveX controls, since Internet Explorer saves them to that directory. However, simply creating a file in these directories is not a path to privilege elevation.

Despite the fact that Power Users can create files underneath \Windows and most of its subdirectories, Windows configures default security permissions on most files contained in these directories so that only members of the Administrators group and the Local System account have write access. Exceptions include the font files (.fon), many system log files (.log), some help files (.chm), pictures and audio clips (.jpg, .gif, and .wmv) and installation files (.inf), but none of these files can be modified or replaced to gain administrative privilege. The device drivers in \Windows\System32\Drivers would allow easy escalation, but Power Users doesn’t have write access to any of them.

I did see a number of .exe’s and .dll’s in the list, though, so I examined them for possible exploits. Most of the executables for which Power Users has write access are interactive utilities or run with reduced privileges. Unless you can trick an administrator into logging into the system interactively, these can’t be used to elevate. But there’s one glaring exception: ntoskrnl.exe:

That’s right, Power Users can replace or modify Windows’ core operating system file. Five seconds after the file is modified, however, Windows File Protection (WFP) will replace it with a backup copy it retrieves, in most cases, from \Windows\System32\Dllcache. Power Users doesn’t have write access to files in Dllcache so it can’t subvert the backup copy. But members of the Power Users group can circumvent WFP by writing a simple program that replaces the file, flushes the modified data to disk, then reboots the system before WFP takes action.

I verified that this approach works, but the question remained of how this vulnerability can be used to elevate privilege. The answer is as easy as using a disassembler to find the function that Windows uses for privilege checks, SeSinglePrivilegeCheck, and patching its entry point in the on-disk image so that it always returns TRUE, which is the result code that indicates that a user has the privilege being checked for. Once a user is running on a kernel modified in this manner they will appear to have all privileges, including Load Driver, Take Ownership, and Create Token, to name just a few of the privileges that they can easily leverage to take full administrative control of a system. Although 64-bit Windows XP prevents kernel tampering with PatchGuard, few enterprises are running on 64-bit Windows.

Replacing Ntoksrnl.exe isn’t the only way to punch through to administrative privilege via the \Windows directory, however. At least one of the DLLs for which default permissions allow modification by Power User, Schedsvc.dll, runs as a Windows service in the Local System account. Schedsvc.dll is the DLL that implements the Windows Task Scheduler service. Windows can operate successfully without the service so Power Users can replace the DLL with an arbitrary DLL, such as one that simply adds their account to the Local Administrators group. Of course, WFP protects this file as well so replacing it requires the use of the WFP-bypass technique I’ve described.

I’d already identified several elevation vectors, but continued my investigation by looking at Power Users access to the \Program Files directory where I found default permissions similar to those in the \Windows directory. Power Users can create subdirectories under \Program Files, but can’t modify most of the preinstalled Windows components. Again, the exceptions, like Windows Messenger (\Program Files\Messenger\Msmgs.exe) and Windows Media Player (\Program Files\Windows Media Player\Wmplayer.exe) run interactively.

That doesn’t mean that \Program Files doesn’t have potential holes. When I examined the most recent output I saw that Power Users can modify any file or directory created in \Program Files subsequent to those created during the base Windows install. On my test system \Program Files\Vmware\Vmware Tools\Vmwareservice.exe, the image file for the Vmware Windows service that runs in the Local System account, was such a file. Another somewhat ironic example is Microsoft Windows Defender Beta 2, which installs its service executable in \Program Files\Windows Defender with default security settings. Replacing these service image files is a quick path to administrator privilege and is even easier than replacing files in the \Windows directory because WFP doesn’t meddle with replacements.

Next I turned my attention to the Registry by running this command:

accesschk –swk “power users” hklm

The output list was enormous because Power Users has write access to the vast majority of the HKLM\Software key. The first area I studied for possible elevations was the HKLM\System key, because write access to many settings beneath it, such as the Windows service and driver configuration keys in HKLM\System\CurrentControlSet\Services, would permit trivial subversion of the Local System account. The analysis revealed that Power Users doesn’t have write access to anything significant under that key.

Most of the Power Users-writeable areas under the other major branch of HKLM, Software, related to Internet Explorer, Explorer and its file associations, and power management configuration. Power Users also has write access to HKLM\Software\Microsoft\Windows\CurrentVersion\Run, allowing them to configure arbitrary executables to run whenever someone logs on interactively, but exploiting this requires a user with administrative privilege to log onto the system interactively (which, depending on the system, may never happen, or happen infrequently). And just as for the \Program Files directory, Power Users has default write access to non-Windows subkeys of HKLM\Software, meaning that third-party applications that configure executable code paths in their system-wide Registry keys could open security holes. VMWare, the only application installed on the system, did not.

The remaining area of exploration was Windows services. The only service permissions AccessChk considers to be write accesses are SERVICE_CHANGE_CONFIG and WRITE_DAC. A user with SERVICE_CHANGE_CONFIG can configure an arbitrary executable to launch when a service starts and given WRITE_DAC they can modify the permissions on a service to grant themselves SERVICE_CHANGE_CONFIG access. AccessChk revealed the following on my stock Windows XP SP2 system:

I next ran PsService to see the account in which the DcomLaunch service executes:

Thus, members of the Power Users group can simply change the image path of DComLauncher to point at their own image, reboot the system, and enjoy administrative privileges.

There can potentially be other services that introduce exploits in their security. The default permissions Windows sets on services created by third-party applications do not allow Power Users write access, but some third party applications might configure custom permissions to allow them to do so. In fact, on my production 64-bit Windows XP installation AccessChk reveals a hole that not only Power Users can use to elevate themselves, but that limited users can as well:

I’d now finished the major phase of my investigation and just confirmed what everyone has been saying: a determined member of the Power Users group can fairly easily make themselves full administrator using exploits in the operating system and ones created by third-party applications.

My final step was to see how Microsoft’s approach to the Power Users account has evolved over time. This 1999 Microsoft Knowledge Base article documents the famous screen-saver elevation vulnerability that existed on Windows NT 4, but Microsoft closed that hole before the release of Windows 2000. The KB article also shows that Microsoft was apparently unaware of other vulnerabilities that likely existed. Windows 2000 SP4 also includes holes, but is actually slightly more secure than the default Windows XP SP2 configuration: Power Users don’t have write access to Ntoskrnl.exe or the Task Scheduler image file, but instead of write-access to the DComLauncher service they can subvert the WMI service, which also runs in the Local System account.

Windows XP SP1 added more Power Users weaknesses, including write access to critical system files like Svchost.exe, the Windows service hosting process, and additional services, WMI and SSDPSRV, with exploitable permissions. Several services even allowed limited users to elevate as described in this Microsoft KB article from March of this year.

Microsoft’s newest operating system, Windows Vista, closes down all the vulnerabilities I’ve described by neutering Power Users so that it behaves identically to limited Users. Microsoft has thus closed the door on Power Users in order to force IT staffs into securing their systems by moving users into limited Users accounts or into administrative accounts where they must acknowledge end-user control over their systems.

The bottom line is that while Microsoft could fix the vulnerabilities I found in my investigation, they can’t prevent third-party applications from introducing new ones while at the same time preserving the ability of Power Users to install applications and ActiveX controls. The lesson is that as an IT administrator you shouldn’t fool yourself into thinking that the Power Users group is a secure compromise on the way to running as limited user.

posted by Mark Russinovich @ 11:01 AM 61 comments

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