Type packetstorm
Modified 2000-01-14T00:00:00


                                            `BindView Security Advisory  
Local Promotion Vulnerability in Windows NT 4  
Issue Date: January 13, 2000  
Contact: Todd Sabin <>  
Problem in NtImpersonateClientOfPort system call on NT 4  
Due to a flaw in the NtImpersonateClientOfPort Windows NT 4 system  
call, any local user on a machine is able to impersonate any other  
user on the machine, including LocalSystem. We have written a  
demonstration exploit which allows any user to spawn a cmd.exe window  
as LocalSystem.  
Affected Systems:  
All Windows NT 4.0 systems up to and including SP6a. We tested our  
exploit on W2K RC2, and it was not vulnerable.  
All Windows NT 4.0 machines are subject to compromise by any user who  
can log in locally and run arbitrary programs. This may lead to  
Domain Admin access, if Domain Admin credentials are on the machine.  
In the case of Terminal Server, it should also be possible to use the  
credentials of other users on the compromised machine to take actions  
across the network as those other users. This has not been tested,  
Windows NT includes a mostly undocumented feature called Lpc ports,  
which are used for making Local Procedure Calls on a machine. One of  
the system apis used with Lpc ports is NtImpersonateClientOfPort,  
which allows a server to act in the security context of the client who  
is calling it. However, the interface to the call lets the server  
specify which client to impersonate based on process and thread IDs.  
The kernel does do some sanity checking of the parameters to verify  
that the call is legitimate, but it's possible to fake it. First it  
verifies that the port you're trying to impersonate on actually has an  
outstanding request. This is easy to satify by making a request to it  
ourselves. Next, it checks that the message ID in the request matches  
the outstanding message ID in the thread you're asking to impersonate.  
This is also easy to satisfy, because if a thread is _not_ making a  
request, it's outstanding message ID will be zero. So, as the server,  
when the request comes in, we just change the pid and tid to the ones  
we want, and change the Message ID to 0. Once we're impersonating we  
can do whatever we want as that user.  
The pseudo-code for our exploit works like this. There are two  
Server thread Client thread  
modify the LpcMessage received in the request  
so that the process and thread ids point to the  
thread we want, and change the message id to 0.  
At this point, we're running under the token of the thread we  
specified above. For our exploit, we choose to impersonate a thread  
of lsass. The reason has to do with the privileges that lsass has  
When impersonating, it seems that the impersonation token only gets  
those privileges that are _enabled_ in the client at the time of  
impersonation. Privileges that are disabled in the client, are not  
put into the impersonation token, even in a disabled state. Now,  
lsass happens to have the CREATE_TOKEN privilege enabled, so we can  
impersonate lsass, and use that privilege to create a new token for  
ourselves based on the lsass impersonation token, but with _all_  
privileges enabled. We can then launch another process under that  
token. So to continue:  
// get the information about the current token.  
GetTokenInformation // (several times)  
// Add _all_ privileges to our TOKEN_PRIVILEGES struct  
// all user space, preparing for NtCreateToken  
NtCreateToken // with info from the lsass token, except that  
// all privileges are enabled  
Finally. We have our token. Now, we can CreateProcessAsUser with  
that, except that there are a couple more hoops to jump through.  
CreateProcessAsUser requires more privileges than lsass had enabled,  
so we don't currently have them, however, our new token does! So, we  
can just impersonate the new token. But again, that's not enough.  
CreateProcessAsUser checks for the privileges in the _process_ token,  
ignoring any impersonation token. So as a final step, we change the  
new token to be our primary process token.  
NtSetInformationProcess (... ProcessAccessToken ...)  
and we're finally there.  
Install the hotfix from Microsoft.  
Limit local logon privileges, if possible.  
Microsoft's security bulletin:  
Microsoft's Hotfix:  
Microsoft's Knowledge Base article:  
(may take a couple days to appear)  
For more information on the LPC ports APIs, see Undocumented Windows  
NT, ISBN# 0-7645-4569-8, Chapter 8. The rest of the book isn't bad