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WPS Office Wpsio.dll Stack Buffer Overflow Vulnerability
From: zhangjiantao <zhangjiantao () dptechnology net>
Date: Sat, 27 Apr 2013 10:46:23 +0800

WPS Office Wpsio.dll Stack Buffer Overflow Vulnerability

1 Summary
CVE number: CVE-2012-4886
Impact: High
Vendor homepage: http://www.wps.cn
Credit: Zhangjiantao of Hangzhou DPtech Technologies
2 Affected Prodects
Affected Version: http://wdl.cache.ijinshan.com/wps/download/special/WPS2012.12012.exe
The WPS office is a free desktop office suite (compatible with Microsoft office),popular in China.
 
3 Vulnerability Details
In module wpsio.dll, a BSTR string stored in the file is copied to the stack buffer, without strict length inspection, 
leading to a stack buffer overflow.  This sample exploit this issue to cover an object stored in the stack, leading to 
crash during the virtual function call. Successfully exploited this vulnerability will lead to arbitrary code execution.
 
4 Crash info
crash info:
(b70.eb8): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=0012c0a4 ebx=770f4b39 ecx=90909090 edx=0012be00 esi=0012c0a4 edi=0018bd54
eip=45e25208 esp=0012bdec ebp=0012bdf8 iopl=0  nv up ei pl zr na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00210246
wpsio!TxExport+0x37b1:
45e25208 ff5114  call    dword ptr [ecx+14h] ds:0023:909090a4=????????
 
module info:
start    end        module name
45e00000 4606f000   wpsio      (export symbols)       C:\Program Files\Kingsoft\WPS Office Personal\office6\wpsio.dll
    Loaded symbol image file: C:\Program Files\Kingsoft\WPS Office Personal\office6\wpsio.dll
    Image path: C:\Program Files\Kingsoft\WPS Office Personal\office6\wpsio.dll
    Image name: wpsio.dll
    Timestamp:        Mon May 28 04:10:12 2012 (4FC28A24)
    CheckSum:         0026D933
    ImageSize:        0026F000
    File version:     8.1.0.3238
    Product version:  8.1.0.3238
    File flags:       0 (Mask 3F)
    File OS:          40004 NT Win32
    File type:        0.0 Unknown
    File date:        00000000.00000000
    Translations:     0000.04b0
    CompanyName:      Zhuhai Kingsoft Office-software Co.,Ltd
    ProductName:      Kingsoft Office
    InternalName:     wpsio
    OriginalFilename: wpsio.dll
    ProductVersion:   8,1,0,3238
    FileVersion:      8,1,0,3238
    FileDescription:  wpsio
    LegalCopyright:   Copyright©1988-2011 Kingsoft Corporation.  All rights reserved.
 
5 Analysis
In sub_45E2CC84:
 
.text:45E2CC84 var_210         = byte ptr -210h ;buffer size 0x200
.text:45E2CC84 var_4           = dword ptr -4
 
 
.text:45E2CDB3                 push    [ebp+Src]       ; BSTR
.text:45E2CDB9                 call    esi ; SysStringLen
.text:45E2CDBB                 mov     [ebp+var_244], eax
.text:45E2CDC1                 add     eax, eax        ;size is 0x170
.text:45E2CDC3                 push    eax             ; Size
.text:45E2CDC4                 push    [ebp+Src]       ; Src
.text:45E2CDCA                 lea     eax, [ebp+var_210]
.text:45E2CDD0                 push    eax             ; Dst
.text:45E2CDD1                 call    memcpy
 
First time,copy 0x170 bytes to buffer var_210.
 
.text:45E2CE16                 push    edi             ; BSTR
.text:45E2CE17                 mov     [ebp+var_234], ax
.text:45E2CE1E                 call    esi ; SysStringLen
.text:45E2CE20                 add     eax, eax
.text:45E2CE22                 push    eax             ; Size
.text:45E2CE23                 movzx   eax, [ebp+var_234] ;length
.text:45E2CE2A                 lea     eax, [ebp+eax*2+var_210]
.text:45E2CE31                 push    edi             ; Src
.text:45E2CE32                 push    eax             ; Dst
.text:45E2CE33                 call    memcpy
 
Second time,copy the same string, placed after the first string. var_234 is the length of the string. Total copy 0x2e0 
bytes.
After copy,return address and SEH record has been overwritten.
 
0:000> k
ChildEBP RetAddr  
WARNING: Stack unwind information not available. Following frames may be wrong.
0012c070 90909090 wpsio!TxExport+0xb3e1
0012c148 45e2a113 0x90909090
 
0:000> !exchain
0012c064: 90909090
Invalid exception stack at 90909090
 
The source data of memcpy is from the file poc.wps,offset 0x41d7.

 
6 Exploit
As described above, using a suitable data overwrite SEH record or return address, eip is controllable.
So,We think this is a security vulnerability.





zhangjiantao
Hangzhou DPtech Technologies Co., Ltd.
http://www.dptechnology.net

Attachment: poc.zip
Description:

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