[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@overflow.pl> 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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