[UNIX] Clam AntiVirus Heap Overflow (Win32-UPX)

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Clam AntiVirus Heap Overflow (Win32-UPX)
------------------------------------------------------------------------


SUMMARY

" <http://www.clamav.net> Clam AntiVirus is a GPL anti-virus toolkit for
UNIX. The main purpose of this software is the integration with mail
servers (attachment scanning)."

Remote exploitation of a heap overflow vulnerability allows execution of
arbitrary code.

DETAILS

Vulnerable Systems:
* Clam AntiVirus versions prior to 0.88.4

Immune Systems:
* Clam AntiVirus version 0.88.4

Vulnerability exists in pefromupx() function, that is used to buil Win32
PE file from UPX packed file.

Vulnerable code:
libclamav/upx.c:
------------
int pefromupx (char *src, char *dst, uint32_t *dsize, uint32_t ep,
uint32_t
upx0, uint32_t upx1, uint32_t magic)
{
char *imports, *sections, *pehdr, *newbuf;
int sectcnt, upd=1;
uint32_t realstuffsz;
uint32_t foffset=0xd0+0xf8;

imports = dst + cli_readint32(src + ep - upx1 + magic);
realstuffsz = imports-dst;

if (realstuffsz >= *dsize ) {
cli_dbgmsg("UPX: wrong realstuff size - giving up rebuild\n");
return 0;
}
...

OK first we check that realstuffsz is not larger than dsize.

...
foffset+=0x28*sectcnt;

if (!CLI_ISCONTAINED(dst, *dsize, sections, 0x28*sectcnt)) {
cli_dbgmsg("UPX: Not enough space for all sects - giving up
rebuild\n");
return 0;
}
...

Now we check that we have enough space for section headers.

...
for (upd = 0; upd <sectcnt ; upd++) {
uint32_t vsize=cli_readint32(sections+8)-1;
uint32_t rsize=cli_readint32(sections+16);
uint32_t urva=cli_readint32(sections+12);
....
cli_writeint32(sections+8, vsize);
cli_writeint32(sections+20, foffset);
foffset+=rsize;
sections+=0x28;
}
...

Now, we add to foffset rsize value of all sections and we DON`T check that
we have enough space in *dst.

...
/* CBA restoring the imports they'll look different from the originals
anyway... */
/* ...and yeap i miss the icon too :P */

memcpy(dst, newbuf, foffset);
*dsize = foffset;
free(newbuf);

cli_dbgmsg("UPX: PE structure rebuilt from compressed file\n");
return 1;
}
...

And there is our heap overflow. We copy from newbuf to dst pointer foffset
bytes, but we don`t check that foffset > *dsize.

Proof of concept:
The example of crafted upx file:
http://overflow.pl/poc/clamav_upx_heap.exe

[pucik@overflow UPX]$ clamscan clamav_upx_heap.exe
*** glibc detected *** double free or corruption (out): 0x08bcbbc0 ***
Przerwane (core dumped)

You can control value of foffset changing "SizeOfRawData" of section 1.


ADDITIONAL INFORMATION

The information has been provided by <mailto:pucik@xxxxxxxxxxx> Damian
Put.
The original article can be found at:
<http://www.overflow.pl/adv/clamav_upx_heap.txt>
http://www.overflow.pl/adv/clamav_upx_heap.txt



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