esm.sh is a nobuild content delivery network(CDN) for modern web development. Prior to version 136, the esm.sh CDN service is vulnerable to path traversal during NPM package tarball extraction. An attacker can craft a malicious NPM package containing specially crafted file paths (e.g., package/../../tmp/evil.js). When esm.sh downloads and extracts this package, files may be written to arbitrary locations on the server, escaping the intended extraction directory. This issue has been patched in version 136.

Affected: Go: github.com/esm-dev/esm.sh

### Summary The esm.sh CDN service is vulnerable to a Path Traversal (CWE-22) vulnerability during NPM package tarball extraction. An attacker can craft a malicious NPM package containing specially crafted file paths (e.g., `package/../../tmp/evil.js`). When esm.sh downloads and extracts this package, files may be written to arbitrary locations on the server, escaping the intended extraction directory.

Uploading files containing `../` in the path is not allowed on official registries (npm, GitHub), but the `X-Npmrc` header allows specifying any arbitrary registry. By setting the registry to an attacker-controlled server via the `X-Npmrc` header, this vulnerability can be triggered.

### Details **file:** `server/npmrc.go` **line:** 552-567

```go func extractPackageTarball(installDir string, pkgName string, tarball io.Reader) (err error) { pkgDir := path.Join(installDir, "node_modules", pkgName) tr := tar.NewReader(unziped) for { h, err := tr.Next() // ... // Strip tarball root directory _, name := utils.SplitByFirstByte(h.Name, '/') // "package/../../tmp/evil" → "../../tmp/evil" filename := path.Join(pkgDir, name) // ← No validation if h.Typeflag != tar.TypeReg { continue } // Extension filtering extname := path.Ext(filename) if !(extname != "" && (allowed_extensions)) { continue // Only extract .js,…