> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/binary-exploitation/heap0.md).

# Heap0

### Heap0 — PicoCTF Walkthrough | Binary Exploitation

First thing we do is read the source code carefully and understand the memory layout.

The program allocates two buffers on the heap:

* `input_data` → 5 bytes → contains `"pico"`
* `safe_var` → 5 bytes → contains `"bico"`

These allocations happen one after another, so in memory they are placed **adjacent**:

```
[input_data][safe_var]
```

***

### Goal

The win condition is inside `check_win()`:

```
if (strcmp(safe_var, "bico") != 0)
```

So to win:

```
safe_var must NOT be "bico"
```

That means we need to overwrite it.

***

### Vulnerability

The bug is here:

```
scanf("%s", input_data);
```

This reads user input **without any bounds checking**.

Even though `input_data` is only 5 bytes, you can input way more → causing a **heap buffer overflow**.

***

### Memory Insight

At runtime, the program even shows heap addresses:

```
input_data: 0x...2b0
safe_var:   0x...2d0
```

Difference:

```
0x2d0 - 0x2b0 = 0x20 = 32 bytes
```

So:

* 32 bytes → gap between buffers
* After that → `safe_var`

***

### Exploit Idea

We overflow `input_data` until we reach `safe_var`, then overwrite it.

Payload structure:

```
"A" * 32 + "AAAA"
```

Even simpler:

```
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
```

(32+ bytes is enough to corrupt `safe_var`)

***

### Why This Works

* First 32 bytes → fill the gap
* Next bytes → overwrite `safe_var`
* `safe_var != "bico"` → condition becomes true
* Program prints the flag

***

### Exploitation Steps

1. Run the program
2. Choose option `2` (write buffer)
3. Enter payload:

```
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
```

4. Choose option `4` (print flag)

***

### Key Takeaway

Heap overflows are about **adjacent memory corruption**.

Instead of guessing, always calculate:

```
distance = target - source
payload = "A" * distance + overwrite_data
```

That’s it — simple overflow, clean win.


---

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