Product: Hancom Office Hwp 2014
Vendor: Hancom - www.hancom.com
Versions Affected (32 bits only):

HanWord Viewer 2007 (Korean)
HanWord Viewer 2010 ­ 8.5.6.1158 (English)
HwpViewer 2014 VP- 9.1.0.2186 (English)
Hwp 2014 VP - 9.0.0.1405 (English/Korean)
Version Not vulnerable:
Hwp 2014 VP - 9.1.0.2342 (English/Korean)
Credits:
              Daniel Regalado, FireEye
              Dan Caselden, FireEye

MITRE CVE: 2015-2810

Timeline:

     03/03/2015: FireEye contacted Hancom letting them know about the
vulnerability found.
     03/05/2015: Hancom replied asking for the technical details.
     03/06/2015: FireEye provides technical details and a PoC to Hancom to
replicate the crash.


Description:

Hancom is an office suite developer in South Korea. The HanWord processor
(also called Hangul a.k.a HWP) is vulnerable to an integer overflow when
assigning a long paragraph size value.

The Bug:

HWP accepts a maximum paragraph size of 0x7fffffff, which is used to
allocate memory for the content of a paragraph. Unchecked arithmetic on
this value can overflow the 32bit integer, resulting
 in an unexpectedly small allocation. Subsequent accesses to the buffer
disagree on the buffer¹s size, and may access memory outside of the
buffer. These accesses may corrupt the heap, allowing attackers to
influence the program¹s execution flow.

The integer overflow happens inside the HwpApp::CHncSDS_Manager function.
A sequence of arithmetic operations on the paragraph size value ends in a
multiplication by four. In the case of
 a paragraph size value of 0x7fffffff, the multiplication results in
0x4000001b*4 = 0x10000006c, which causes the 32-bit register to overflow
to 0x6c bytes:


eax=00000002 ebx=00000019 ecx=00000001
edx=0000006c esi=03d0c0c8 edi=4000001b
eip=048b163c esp=0d39ef88 ebp=40000000 iopl=0         nv up ei pl nz na pe
nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000
efl=00000206
HwpApp!CHncSDS_Manager::CHncSDS_Manager+0x4089ec:
048b163c 52
push    edx
048b1641 ff15e0059504    call    dword
ptr[HwpApp!CHncSDS_Manager::CHncSDS_Manager+0x4a7990
(049505e0)]={MSVCR90!malloc)}

The unexpected paragraph size may cause heap corruption when HWP writes
the contents of the paragraph in memory. In the crash below, the
instruction pointer has been overwritten:

(1b4.d1c): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00200020 ebx=01b10332 ecx=03cbefec
edx=31f21817 esi=03cbefec edi=00000000
eip=31f21817 esp=0012da74 ebp=0012dad4 iopl=0         nv up ei pl zr na pe
nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000
efl=00010246
31f21817 ??              ???


Impact:

Attackers may cause either a Denial of Service or in some circumstances
influence the program¹s execution flow.

Fix:

Hancom released a patch to fix this bug in the following version:
Hancom Hwp 2014 VP - 9.1.0.2342 (English/Korean)


--
Daniel Regalado

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