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# lab-6

## File Path Traversal, Validation of File Extension with Null Byte Bypass

**Difficulty:** Practitioner

### Lab Description

This lab contains a path traversal vulnerability in the display of product images. The application validates that the supplied filename ends with the expected file extension.

**Objective:** Retrieve the contents of the `/etc/passwd` file.

### Root Cause

There are two security flaws working together.

**Flaw 1 — User input is used directly as a file path:**

```php
$file = $_GET['filename'];
readfile($file);
```

The application allows the user to decide which file to open.

**Flaw 2 — The extension check is performed on the raw input:**

```php
if (endsWith($filename, ".png")) {
    openFile($filename);
}
```

The developer validates that the filename ends with `.png` before opening it. The vulnerable assumption is:

```
Validation
    │
filename ends with .png
    │
    ▼
Filesystem opens exactly that filename
```

But that assumption is false when a null byte is involved.

### Exploitation

**Payload:**

```
../../../etc/passwd%00.png
```

**Step 1 — URL decoding:**

`%00` decodes to `\x00` (the null byte). The string becomes:

```
../../../etc/passwd\0.png
```

**Step 2 — Extension validation:**

The application checks: *does it end with `.png`?* Yes — the `.png` suffix is present and the check passes.

**Step 3 — Native filesystem/API call:**

Historically, many C libraries and older language runtimes treat the null byte as the string terminator. The OS or underlying C function receives the path up to the `\0` and ignores everything after it:

```
../../../etc/passwd\0.png  →  ../../../etc/passwd
```

The file that actually gets opened is `/etc/passwd`.

**Visual flow:**

```
Attacker
    │
    ▼
../../../etc/passwd%00.png
    │
    ▼
URL Decode
    │
    ▼
../../../etc/passwd\0.png
    │
    ▼
Extension Check
Ends with ".png" ✔
    │
    ▼
Native C API
Stops at null byte
    │
    ▼
../../../etc/passwd
    │
    ▼
OS opens /etc/passwd
```

### Lesson Learned

* **Never trust user-controlled paths.** Use allowlists of expected filenames or opaque identifiers (e.g. an image ID mapped server-side to a real path) instead of accepting arbitrary path input from the user.
* **Validate after canonicalisation.** Resolve the full canonical path first, then verify it remains inside the intended directory before opening any file.
* The null byte bypass works because the validation layer and the filesystem layer interpret the string differently. Ensuring a single, consistent parsing step for both validation and file access eliminates this class of mismatch.


---

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