One of the things you might expect an Antivirus engine to do reliably is parse PE files. However, after some simple testing with Avira, I found a heap underflow (that is, writing *before* a heap allocation) parsing section headers. If a section header has a very large relative virtual address, Avira will wrap calculating the offset into a heap buffer, and write attacker controlled data to it (the data from section->PointerToRawData in the input file).

The code is doing something like:

if (Section->SizeOfRawData + Section->VirtualAddress < 8192) {
    buf = malloc(8192);

    memcpy(buf + Section->VirtualAddress, input + Section->PointerToRawData, Section->SizeOfRawData);
}


The bug is that you need to check if Section->VirtualAddress + Section->SizeOfRawData wraps. This vulnerability is obviously exploitable for remote code execution as NT AUTHORITY\SYSTEM.

To reproduce this bug, create an executable with a section like this:

  NAME          RVA      VSZ   RAW_SZ  RAW_PTR  nREL  REL_PTR nLINE LINE_PTR     FLAGS
  .text    ff003fff     1fff     1fff      200     0        0     0        0         0  ---

With Page heap enabled, this should crash reliably trying to memcpy the data from section.PointerToRawData

(e58.2b8): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00000041 ebx=00000000 ecx=000007f7 edx=00000002 esi=35785219 edi=41294000
eip=7291545c esp=41bedaf0 ebp=41bedaf8 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
aecore!ave_proc+0x1fc2c:
7291545c f3a5            rep movs dword ptr es:[edi],dword ptr [esi]
0:011> db esi
35785219  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785229  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785239  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785249  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785259  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785269  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785279  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
35785289  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA

I think it started writing to ptr - 8192, lets see what's there:

0:011> db edi - (0n8192 - (ecx * 4)) 
41293fdc  00 00 00 41 41 41 41 41-41 41 41 41 41 41 41 41  ...AAAAAAAAAAAAA
41293fec  41 41 41 41 41 41 41 41-41 41 41 41 41 41 41 41  AAAAAAAAAAAAAAAA
41293ffc  41 41 41 41 ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  AAAA????????????
4129400c  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
4129401c  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
4129402c  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
4129403c  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
4129404c  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????

Yes! 

Without page heap, you should get heap corruption, probably writing to 0x41414141.

I've attached a simple testcase.

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

 

testcase.txt 52.5 KB View Download