Patch malicious tarballs

Tarballs with files containing directory traversal components can write
files to unintended locations. This change ensures the Untar function
will error when a given tarball has a traversal component (..).

See https://cwe.mitre.org/data/definitions/22.html

pkg/tar/untar.go

@@ -2,8 +2,10 @@ package tar
 
 import (
 	"archive/tar"
+	"fmt"
 	"io"
 	"os"
+	"strings"
 )
 
 func UntarFile(tarFile, dstFolder string) error {
@@ -32,6 +34,12 @@ func Untar(source io.Reader, router Router) error {
 			continue
 		}
 
+		// Prevent malicous directory traversals.
+		// https://cwe.mitre.org/data/definitions/22.html
+		if strings.Contains(header.Name, "..") {
+			return fmt.Errorf("file in tarball contains a directory traversal component (..): %v", header.Name)
+		}
+
 		info := header.FileInfo()
 		if info.IsDir() {
 			if err = os.MkdirAll(path, info.Mode()); err != nil {

pkg/tar/untar_test.go

@@ -1,6 +1,10 @@
 package tar_test
 
 import (
+	stdtar "archive/tar"
+	"bytes"
+	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"testing"
@@ -30,3 +34,43 @@ func TestUntarFile(t *testing.T) {
 	g.Expect(filepath.Join(untarFolder, "dummy3")).To(BeADirectory())
 	g.Expect(filepath.Join(untarFolder, "dummy3", "dummy4")).To(BeARegularFile())
 }
+
+func TestUntarFile_DirTraversalComponents(t *testing.T) {
+	// This test ensures Untar fails when a tarball contains paths with directory traversal
+	// components. It addresses https://cwe.mitre.org/data/definitions/22.html.
+	g := NewWithT(t)
+
+	dir := t.TempDir()
+	tarPath := filepath.Join(dir, "test")
+	fh, err := os.Create(tarPath)
+	g.Expect(err).To(Succeed())
+
+	createArbitraryTarball(t, fh)
+
+	g.Expect(tar.UntarFile(tarPath, dir)).ToNot(Succeed())
+}
+
+func createArbitraryTarball(t *testing.T, w io.Writer) {
+	t.Helper()
+
+	tb := stdtar.NewWriter(w)
+
+	data := bytes.NewBufferString("Hello, world!")
+	header := stdtar.Header{
+		Name:     "../foobar",
+		Mode:     int64(fs.ModePerm),
+		Typeflag: stdtar.TypeReg,
+		Size:     int64(data.Len()),
+	}
+
+	if err := tb.WriteHeader(&header); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := io.Copy(tb, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tb.Close()
+}