L0pht Security Advisory
Release date: February 18, 1999
Application: Microsoft Windows NT 4.0
Severity: any local user can gain administator privileges
and/or take full control over the system
Microsoft Windows NT 4.0 implements a system-wide cache of
file-mapping objects for the purpose of loading system dynamic link
libraries (DLLs) as quickly as possible. These cache objects, located in
the system's internal object namespace, are created with permissions such
that the 'Everyone' group has full control over them. Hence, it is
possible to delete these cache objects and replace them with others that
point to different DLLs.
When processes are created, the loader maps/loads the loading
executable's imported DLLs into the process space. If there is a DLL cache
object available, it is simply mapped into the process space, rather than
going to the disk. Hence, there is an exploitable condition, when a
low-privilege user replaces a DLL in the cache with a trojan DLL, followed
by a high-privelege account launching a process. The high priveleged
process will map in the trojan DLL and execute code on behalf of the low
privelege use r.
Windows NT 4.0 Server SP4
Windows NT 4.0 Workstation SP4
Other service packs are likely to be vulnerable, but the exploit has
not been tested on them, neither has the fix presented below.
The Windows NT object namespace is the place where the kernel
keeps the names of mutexes, semaphores, filemapping objects, and other
kernel objects. It is organized hierarchically, like a directory
structure. Amongst the directories are:
The NT object namespace is browsable with a tool called 'WinObj
2.0' from System Internals (their website is http://www.sysinternals.com).
You may wish to look around this namespace and browse the default
permissions of objects. It is quiet entertaining, really.
The "\Knowndlls" directory contains a list of DLLs in the
c:\winnt\system32 directory, like:
All of these objects are created at boot time, and are 'permanent
shared objects'. Normally, users can not create permanent shared objects
(it's an advanced user right, and it is normally not assigned to any
group, even Administrators). But the system pr eloads this cache for you.
Permanent shared objects differ from regular shared objects only in the
fact that they have a flag set, and an incremented reference count, such
that if you create one, and then terminate the creating process or close
all handle s to the object, it does not disappear from the object space.
To exploit the poor permissions on this cache, one first needs to
delete one of the shared objects by name, in order to later replace it. So
we make a call to the NTDLL.DLL native function "OpenSection()", getting a
handle to the object. Then we call the
NTOSKRNL.EXE native function "ZwMakeTemporaryObject()" which removes the
'permanent' flag and decrements the reference counter from the object. Now
we just call NTDLL.DLL:NtClose() on the handle and it is destroyed.
To create a section, one calls NTDLL.DLL:CreateSection(), which is
undocumented. There are other calls one needs to make in order to set up
the object and open the KnownDlls directory, but they are trivial and will
not be discussed here. Feel free to bro wse the source code presented at
the end of this advisory to see what you need to do though. Anyway, you
create a section (aka file-mapping) object that points to a trojan DLL. A
good candidate for DLL trojan is KERNEL32.DLL, since it is loaded by
pretty much every executable you're going to run.
Note that any DLL cache objects you create as a user can not be
'permanent', hence, when you log out, the cache object _will_ disappear.
So how can we get a higher privelege process to run while we're logged in?
There are many ways. We can wait for an 'A t' job to go off, or we can set
up the DLL hack as an 'At' job that goes off when someone else is logged
in. But more reliable is this:
When a new Windows NT subsystem is started, it creates a subsystem
process to handle various system details. Examples of these processes are
LSASS.EXE and PSXSS.EXE. The PSXSS.EXE is the POSIX subsystem. But since
no one ever really uses the POSIX subsys tem under NT. So, chances are, it
won't be loaded into memory yet. Once it is, though, it's loaded until the
machine reboots. If it loaded, reboot the machine, and it won't be :P.
So, we launch our DLL cache hack, and then run a POSIX subsystem
command, thus launching PSXSS.EXE (which runs as 'NT AUTHORITY\SYSTEM',
the system account), and running our DLL with local administrator
privileges. Incidentally, other subsystems have the
same effect, such as the OS/2 subsystem (the only other one that probably
isn't started yet).
I developed a patch for this security problem in the form of a
Win32 Service program that can be installed by the Administrator of the
system. It sets itself to run every time the system is started, and before
the user has the opportunity to start a program, it adjusts the
permissions of the DLL cache to something much safer. The source code for
t his service is also provided, along with a compiled version. Links to
the programs can be found at http://www.l0pht.com/advisories.html.
One can verify the validity of the patch by downloading the WinObj
v2.0 tool from System Internals (www.sysinternals.com) and inspecting the
permissions of the KnownDlls directory, and the section objects within it.
Microsoft has been sent a copy of this advisory, and I would
expect a hotfix from them at some point in the near future.
I wrote up a trojan to test exploitability, and it was a simple
'forwarder' DLL that had the same exported names as KERNEL32.DLL, but a
different 'DllMain()' function, to be called when the DLL is loaded. The
function calls in my trojan, simply forward o ff to the real KERNEL32.DLL
calls located in a copy of the kernel that you make in 'REALKERN.DLL' in
the c:\temp directory.
To try out this vulnerability, obtain an account as a
low-privilege guest user (referred to as 'Dick') and do the following:
1. Log in as Dick at the console.
2. Start up two "cmd.exe" shells. Do the following in one of them.
3. Copy c:\winnt\system32\kernel32.dll to c:\temp\realkern.dll
(The egg dll is hard coded to use the c:\temp directory to find this file.
If you can't put it in c:\temp, then modify the source '.def' file to
point to a different location and recompile eggdll.dll)
4. Copy the provided hackdll.exe and eggdll.dll to c:\temp
5. Ensure that there is no file named c:\lockout. If there is,
delete it. The exploit uses this file as a lockfile.
5. Delete the KERNEL32.DLL file-mapping object from the system cache:
c:\temp> hackdll -d kernel32.dll
6. Insert the new file-mapping object with:
c:\temp> hackdll -a kernel32.dll c:\temp\eggdll.dll
Don't hit a key in this window after hitting enter.
7. Now move to the other cmd.exe window that you started.
8. Run a POSIX subsystem command. A good way to start it is:
c:\temp> posix /c calc
(if you have calculator installed. If not, pick some other program)
9. Now the EGGDLL.DLL will prompt you with a few message boxes:
Say no to the "User is DOMAIN\DICK, Spawn Shell?" box.
Say no to the "User is \[garbage], Spawn Shell?" box.
Say YES to the "User is NT AUTHORITY\SYSTEM, Spawn Shell?" box.
Say YES to the "Winsta0" window station message box.
Say YES to the "Desktop" window desktop message box.
You will now see a "System Console" command.com shell open up.
(saying yes to the next 'winlogon' box will give you something
funny when you log out, btw :P)
10. Now go back to your first cmd.exe window and hit a key to
unpoison the DLL cache.
11. In the System Console window, run the User Manager program,
and modify Dick's account
(or anyone else's for that matter) to your hearts content.
(NT Server) c:\winnt\system32> usrmgr
(NT Workstation) c:\winnt\system32> musrmgr
Source and Compiled Code:
Exploit code can be downloaded from L0pht's website at
http://www.l0pht.com/advisories.html. It is available in compiled form,
and in pure source form as two zipfiles. The L0pht patch for this advisory
is also available in both source form and compiled f orm from the same
For more L0pht (that's L - zero - P - H - T) advisories check out:
Date: Fri, 19 Feb 1999 11:23:44 +0000
From: Paul Ashton
Subject: Re: L0pht Security Advisory: Windows NT
> L0pht Security Advisory
> I developed a patch for this security problem in the form of a
> Win32 Service program that can be installed by the Administrator of the
> system. It sets itself to run every time the system is started, and before
> the user has the opportunity to start a program, it adjusts the
> permissions of the DLL cache to something much safer.
Alternatively, you can set