Message-ID: <7a282fc30512131028g40d65517k2254283bfecec6db@mail.gmail.com>
Date: Tue Dec 13 18:28:44 2005
From: joshua.russel at gmail.com (Joshua Russel)
Subject: [EEYEB-20050523] Windows Kernel APC Data-Free
	Local Privilege Escalation Vulnerability

It is a local vulnerability, then how does Retina claims to scan it remotely?


On 12/13/05, Advisories <Advisories@...e.com> wrote:
> Windows Kernel APC Data-Free Local Privilege Escalation Vulnerability
>
> Release Date:
> December 13, 2005
>
> Date Reported:
> May 23, 2005
>
> External Refferences:
> eEye ID# EEYEB-20050523
> OSVDB ID# 18823
> CVE # CAN-2005-2827
> Microsoft #  MS05-055
>
> Severity:
> Medium (Local Privilege Escalation to Kernel)
>
> Systems Affected:
> Windows NT 4.0
> Windows 2000
>
> Overview:
> eEye Digital Security has discovered a local privilege escalation
> vulnerability in the Windows kernel that could allow any code executing
> on a Windows NT 4.0 or Windows 2000 system to elevate itself to the
> highest possible local privilege level (kernel).  For example, a
> malicious user, network worm, or e-mail virus could take advantage of
> this vulnerability in order to completely compromise the vulnerable
> system on which the exploit code is executing, regardless of that code's
> original privilege level.
>
> The vulnerability exists in the thread termination routine contained
> within NTOSKRNL.EXE.  Through a specific series of steps, a local
> attacker can cause the code responsible for discarding queued
> Asynchronous Procedure Call (APC) entries to erroneously attempt to free
> a region of kernel data, producing a "data free" vulnerability that may
> be exploited in order to alter arbitrary kernel memory, or even divert
> the flow of execution directly.
>
> Technical Details:
> The basis of this vulnerability is in PspExitThread's APC freeing loop
> and in the behavior of KiMoveApcState, invoked from KiAttachProcess and
> KeUnstackDetachProcess.  We'll give a description of the problem below,
> followed by a "call flow" illustration to outline the specific sequence
> of events.
>
> When a thread is exiting, PspExitThread will detach the thread's APC
> queues from ETHREAD.ApcState.ApcListHead[0] and ApcListHead[1], so that
> each queue is now a circular, doubly-linked list in which the first and
> last nodes do not point back to the list head (LIST_ENTRY structure).
> However, since the list heads' pointers are not modified, the purpose is
> presumably just to allow the APC freeing loop within PspExitThread to
> walk each list and free its nodes, without navigating back to the list
> head and erroneously attempting to free memory within the ETHREAD
> structure.  Of course, the vulnerability is that this can be made to
> happen, and the result is a "data free" condition that eventually causes
> ExFreePoolWithTag to operate on user memory.
>
> APCs queued by an external process count against that process's pool
> quota, and therefore the quota block of the pool block containing the
> APC structure has a reference to the queuing process.  If the exiting
> thread contains an APC queued by a now-terminated external process in
> its lists, and if that APC node represents the last reference to the
> process's Process object, then freeing that node will cause the Process
> object to be destroyed from within ExFreePoolWithTag.  Part of this
> sequence involves executing PspProcessDelete, which switches to the
> ending process's address space using KeStackAttachProcess, calls
> PspExitProcess, and then reverses the switch with
> KeUnstackDetachProcess.
>
> Both the "attach" and "detach" functions call KiMoveApcState, which is
> intended to temporarily strip the thread of its APCs so that none are
> dispatched in an address space for which they were not intended, then
> re-link the list of APCs after the thread's native address space is
> reinstated.  During attach, the ETHREAD.ApcState structure is
> duplicated, and the pointers of the lists' first and last nodes are
> adjusted to refer to the copy.  Upon detach, the first and last nodes'
> pointers are adjusted to re-link the lists to the original
> ETHREAD.ApcState -- even though they were supposed to remain
> disconnected, since the APC free loop is still in progress.  The end
> result is that the free loop will continue and attempt to free a portion
> of the ETHREAD structure as though it were a pool block header,
> culminating in the kernel operating on attacker-supplied pointers from
> user-land memory, because the accessed portion of ETHREAD contains
> predictable and mostly zeroed values.
>
> The following depicts the sequence of function calls and parameters
> involved in producing the vulnerable condition:
>
> . PspExitThread
> . . KeFlushQueueApc
> . . (detaches APC queues from ETHREAD.ApcState.ApcListHead)
> . . (APC free loop begins)
> . . ExFreePool(1st_APC -- queued by exited_process)
> . . . ExFreePoolWithTag(1st_APC)
> . . . . ObfDereferenceObject(exited_process)
> . . . . . ObpRemoveObjectRoutine
> . . . . . . PspProcessDelete
> . . . . . . . KeStackAttachProcess(exited_process)
> . . . . . . . . KiAttachProcess
> . . . . . . . . . KiMoveApcState(ETHREAD.ApcState --> duplicate)
> . . . . . . . . . KiSwapProcess
> . . . . . . . PspExitProcess(0)
> . . . . . . . KeUnstackDetachProcess
> . . . . . . . . KiMoveApcState(duplicate --> ETHREAD.ApcState)
> . . . . . . . . KiSwapProcess
> . . ExFreePool(2nd_APC)
> . . ExFreePool(ETHREAD + 30h)
> . . (APC free loop ends)
>
> The ETHREAD data upon which ExFreePool is called is mostly predictable,
> KernelStack at offset +28h being the single true variable; however,
> methods for leaking a thread's kernel ESP permit complete control over
> the path execution will take through ExFreePoolWithTag.  With enough
> crafting, an arbitrary function pointer can be supplied as an object
> type method, allowing execution to be hijacked directly.
>
> Beginning with Windows XP, KeFlushQueueApc contains a code fix that
> resolves this vulnerability.
>
> Protection:
> Retina Network Security Scanner has been updated to identify this
> vulnerability.
>
> Vendor Status:
> Microsoft has released a patch for this vulnerability.  The patch is
> available at:
> http://www.microsoft.com/technet/security/bulletin/MS05-055.mspx
>
> Credit:
> Derek Soeder
>
> Greetings:
> Dedicated to
>
> R. W. S., Sr.
> 1928 - 2005
>
> >From my father to his:
>
> "He was a good man; liked by all, loved by many.  He was always upbeat,
> outgoing and loved to kid around.  He was always willing to help others
> in their time of need and gave a lot of himself.  He was very creative,
> handy with tools, and could fix about anything.  He was the one everyone
> turned to for advice and direction.  He was my father, and I miss him
> dearly."
>
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