Source: https://code.google.com/p/google-security-research/issues/detail?id=522
Fuzzing VB6 executables produced the attached crash testcase:
(5a8.dc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=ffffffff ebx=0a07e8ec ecx=0a07eb04 edx=00000000 esi=0907e924 edi=00000010
eip=13d64b78 esp=0ea6ee30 ebp=0ea6ee38 iopl=0 nv up ei pl nz na po nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202
13d64b78 0fb60e movzx ecx,byte ptr [esi] ds:002b:0907e924=??
# where does esi come from?
0:121> ub @eip La
13d64b60 55 push ebp
13d64b61 8bec mov ebp,esp
13d64b63 8b4514 mov eax,dword ptr [ebp+14h]
13d64b66 57 push edi
13d64b67 8b7d0c mov edi,dword ptr [ebp+0Ch]
13d64b6a f7d0 not eax
13d64b6c 85ff test edi,edi
13d64b6e 0f849e000000 je 13d64c12
13d64b74 56 push esi
13d64b75 8b7510 mov esi,dword ptr [ebp+10h]
# Okay, it's a parameter
0:121> kvn1
# ChildEBP RetAddr Args to Child
00 0ea6ee38 14424d8f 1656cae4 00000010 0907e924 0x13d64b78
0:121> ub 14424d8f La
14424d77 8b4304 mov eax,dword ptr [ebx+4] <-- load index
14424d7a 03c3 add eax,ebx <-- add to pointer
14424d7c 8d4c3bf0 lea ecx,[ebx+edi-10h] <-- probably load bounds of buffer
14424d80 3bc1 cmp eax,ecx <-- check if index is in bounds
14424d82 771f ja 14424da3 <-- too late, overflow has already happened
14424d84 6a00 push 0
14424d86 50 push eax < +0x10
14424d87 6a10 push 10h
14424d89 56 push esi
14424d8a e8d1fd93ff call 13d64b60
Looks like the code is doing
ptr += offset;
if (ptr > ptr+SizeOfBuffer)
goto error;
This is obviously incorrect, because the offset can wrap. Where does that value come from?
0:121> dd ebx
0a07e8ec 00000228 ff000038 000000d0 000000f8
0a07e8fc 0000014f 00000120 00000158 000001bc
0a07e90c 00000048 00000000 00000204 00000211
0a07e91c 38000208 00000000 02a69b00 101b081b
0a07e92c 00083389 5a4f2f2b 02a69b02 101b081b
0a07e93c 00083389 5a4f2f2b 09194000 11cfdf6e
0a07e94c a000748e f8260fc9 bac300ac 4551fc30
0a07e95c 204f1db8 383f2a55 77696e7e 4df2a25e
That is from the input file:
*0001e10: 2802 0000 3800 00ff d000 0000 f800 0000 (...8...........
0001e20: 4f01 0000 2001 0000 5801 0000 bc01 0000 O... ...X.......
0001e30: 4800 0000 0000 0000 0402 0000 1102 0000 H...............
0001e40: 0802 0038 0000 0000 009b a602 1b08 1b10 ...8............
0001e50: 8933 0800 2b2f 4f5a 029b a602 1b08 1b10 .3..+/OZ........
0001e60: 8933 0800 2b2f 4f5a 0040 1909 6edf cf11 .3..+/OZ.@..n...
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/38281.zip
