[NT] Windows VDM #UD Local Privilege Escalation
From: SecuriTeam (support_at_securiteam.com)
To: firstname.lastname@example.org Date: 13 Oct 2004 18:06:07 +0200
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Windows VDM #UD Local Privilege Escalation
eEye Digital Security has discovered a third local privilege escalation
vulnerability in the Windows kernel that would allow any code running on
an affected system to elevate itself to the highest possible local
privilege level (kernel), regardless of the privileges with which the code
executes initially. For instance, a malicious user with legitimate access
to a machine, or a remote attacker or worm payload able to gain
unprivileged access through an unrelated exploit, could leverage this
vulnerability to fully compromise a Windows NT 4.0, Windows 2000, Windows
XP, or Windows Server 2003 system.
This vulnerability is located in a portion of the Windows kernel that
handles some low-level aspects of executing 16-bit code inside a Virtual
DOS Machine (VDM).A certain invalid opcode byte sequence is used in the
16-bit DOS emulation code to pass requests (referred to as "bops") to the
32-bit VDM "host" code, and the invalid opcode fault handler within the
Windows kernel gives these sequences special treatment when relaying them
to the 32-bit host code executing in user space (normally an NTVDM.EXE
process).The kernel does not validate the address to which execution is
transferred after one of these invalid instructions is encountered, and
because the memory containing the address is fully accessible to user-mode
code, it is possible to redirect execution to an arbitrary location with
kernel privileges still in effect.
The interrupt 06h (#UD) handler in NTOSKRNL.EXE contains a branch of code
for quickly handling C4h/C4h machine code byte sequences according to the
control word specified in the two bytes that follow, when the sequence
occurs in Virtual-8086 mode (bit 17 of EFLAGS is set). If a control word
value other than 4250h or 4350h (both used for fast file I/O) is given,
the "bop" is passed off to another section of code in the process hosting
the VDM. In NTVDM.EXE, this transition normally corresponds to returning
from a call to NtVdmControl(0) (VdmpStartExecution), but in actuality,
execution can be redirected anywhere, since the switch is just
accomplished by swapping out context structures. The VDM TIB (arrived at
by way of [[[[FFDFF124h]+44h]+1DCh]+98h] on Windows 2000, FS:[F18h] on
Windows NT 4.0, Windows XP, and Windows Server 2003) is used to hold a
copy of the V86-mode context in effect at the time the fault occurred (at
offset +CD0h on NT4 and 2000, +2D8h for XP and 2003), then the context for
resuming execution of the host code is retrieved (from offset +A04h on NT4
and 2000, +0Ch on XP and 2003) and loaded into the appropriate registers.
As mentioned above, this context is contained in user memory but is not
sanitized in any way by the #UD handler, so any process with or without a
formally-initialized VDM can place arbitrary values in the host execution
context and get the handler to IRETD to any CS:EIP, allowing kernel
privileges to be retained while user-supplied code is executed. On any
version of Windows, it is sufficient to modify the VDM TIB in a process
with a properly initialized VDM (most easily done by code executing in a
COM file). For Windows NT 4.0, XP, and 2003, it is only necessary to set
the pointer at offset F18h in the user-land TIB to reference a fake VDM
TIB, then execute V86-mode code using NtContinue().
Microsoft has released a patch for this vulnerability. The patch is
The information has been provided by <mailto:email@example.com> Derek
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