Microsoft Windows winhlp32.exe Heap Overflow Vulnerability Dec 23 2004 03:00PM
flashsky fangxing (flashsky xfocus org)

Venustech AD-Lab


[Security Advisory]

Advisory: [AD_LAB-04006]Microsoft Windows winhlp32.exe Heap Overflow Vulnerability

Class: Design Error


Remote: Yes


Windows NT

Windows 2000 SP0

Windows 2000 SP1

Windows 2000 SP2

Windows 2000 SP3

Windows 2000 SP4

Windows XP SP0

Windows XP SP1

Windows 2003

Windows XP SP2







There is a vulnerability in Microsoft Windows .hlp file parsing program winhlp32.exe.

The vulnerability is caused due to a decoding error within the windows .hlp header

processing.This can be exploited to cause a heap-based buffer overflow.



If the help file is phrase compressed, it contains an internal file named phrases.

The table header of phrases table is located at offset 0x19 in the .hlp file

and its file structure includes:

unsigned short wNumberOfPhrases;

unsigned short wOneHundred; 0x0100;

long decompressedsize;

The phrases table header is right followed by phrases talbe, and each phrase

occupies 2 bytes, which is unsigned short type.

The function of 0100A1EF has 3 parameters. The 3rd parameter is pointed to

the phrases table header. The second one is pointed to a heap memory, which

is used for saving phrases data. But, during calculating data length, there is

not sufficient check of the data length. This can be exploited by using a

malformed .hlp file to cover the heap memory which is pointed by the second


The analysis for the function of 0100A1EF is as follows:

0100A1EF sub_100A1EF proc near ; CODE XREF: sub_100A14C+6Fp


.text:0100A1EF arg_0 = dword ptr 4

.text:0100A1EF arg_4 = dword ptr 8

.text:0100A1EF arg_8 = dword ptr 0Ch


.text:0100A1EF mov eax, [esp+arg_8] ;arg_8 pointed to phrase table header

.text:0100A1F3 push ebx

.text:0100A1F4 push esi

.text:0100A1F5 push edi

.text:0100A1F6 movzx edx, word ptr [eax+2] ;[eax+2] -> wOneHundred

.text:0100A1FA mov ecx, [eax+0Ch] ;[eax+0Ch] -> phrase table

.text:0100A1FD mov eax, [esp+0Ch+arg_0] ;the following calculates the offset of phrase table

.text:0100A201 sub eax, edx

.text:0100A203 mov ebx, [esp+0Ch+arg_4]

.text:0100A207 mov edi, eax

.text:0100A209 shr eax, 1

.text:0100A20B and edi, 1

.text:0100A20E movzx edx, word ptr [ecx+eax*2] ;phrase_offset1

.text:0100A212 movzx esi, word ptr [ecx+eax*2+2] ;phrase_offset2

.text:0100A217 sub esi, edx

.text:0100A219 add ecx, edx

.text:0100A21B push esi ; size_t ;size = phrase_offset2 - phrase_offset1

.text:0100A21C push ecx ; void *

.text:0100A21D push ebx ; void * ;ebx -> No.2 pointer, to heap memory

.text:0100A21E call ds:memmove

There are 2 vulns here:

1. A Integer bufferoverflow, size = phrase_offset2 - phrase_offset1if phrase_offset2 less than phrase_offset1 the

size will be negative number and then memmove use this negative number size for memory copye cause of

the heap overflow.

2. The allocated heap size depends on a item of phrase table not the phrasesEndOffset-phrasesHeadOffset size

so if we changed the phrasesEndOffset size and will cause another heap overflow here.

More details and POC at http://www.xfocus.net/flashsky/icoExp/index.html .



Keji(yu_keji (at) venustech.com (dot) cn [email concealed]) discovery this vuln:)

Vulnerability analysis and advisory by Keji,Flashsky and icbm.

Special thanks to "Fengshou" project members and all Venustech AD-Lab guys:P



The information in this bulletin is provided "AS IS" without warranty of any

kind. In no event shall we be liable for any damages whatsoever including direct,

indirect, incidental, consequential, loss of business profits or special damages.

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