Compuware Softice (DbgMsg driver) Local Denial Of Service
by Piotr Bania <bania.piotr@gmail.com>
http://pb.specialised.info
Original location: http://pb.specialised.info/all/adv/sice-adv.txt
Severity: Low / Medium - BSOD (Blue Screen Of Death) DOS
Software affected: Tested on Softice from DriverStudio 3.2 and 3.1.
However all version with DbgMsg.sys driver
installed should be marked as vulnerable.
I. BACKGROUND
SoftICE is a powerful system-wide debugger that supports source level
debugging of any software, driver, service, and most bios code on either
a single or dual machine configuration.
II. DESCRIPTION
On NT based operating systems Debug Messages are being sent by executing
the OutputDebugString API (ring3 applications) or by executing
DbgPrint() function (also known as KdPrint(), mostly used by ring0
"applications").
However not only kernel mode stuff can execute DbgPrint() since it is
exported by NTDLL.DLL - so called native calls. If we go forward we will
see that actually the final pass of the Debug Message is done by
executing INT 2Dh, like following example shows:
----// SNIP SNIP //-------------------------------------------------
mov ecx,offset some_string ; Debug Message offset
xor edx,edx ; second Param (NULL in this case)
mov eax,1 ; Service Type here defined as
; BREAKPOINT_PRINT (=1)
int 2dh
int 3
nop <--- int 2dh returns here not to int 3
----// SNIP SNIP //-------------------------------------------------
When Softice is installed - DbgMsg.sys driver is loaded (user should
also notice that even when Softice is not active, the system is still
vulnerable), executed 2d interrupt will be firstly filtered by
DbgMsg.sys routine. This is probably provided to capture are flying
Debug Messages and display them into the debugger window.
However the driver doesn't validate the Debug Message pointer properly,
this can cause an machine crash when the pointer is invalid.
III. IMPACT
Executing special crafted "int 2dh" request will cause local machine
crash on system with DbgMsg.sys driver installed, *even* when Softice is
not active. This attack can be also used as a very nasty anti-debugging
trick against Softice users.
IV. POC CODE
Sample POC code was developed.
best regards,
Piotr Bania
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Piotr Bania - <bania.piotr@gmail.com> - 0xCD, 0x19
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