fix: validate tar entry paths against escaping the target directory in bundle Unpack

pkg/bundle/bundle.go

@@ -270,10 +270,10 @@ func Unpack(in io.Reader, targetPath string) (*UnpackDetails, error) {
 			continue
 		}
 
-		// #nosec G305 path traversal is checked by validating that the resulting path does not contain unexpected special elements
+		// #nosec G305 path traversal is checked by validating that the resulting path does not escape targetPath
 		var target = filepath.Join(targetPath, header.Name)
-		if strings.Contains(target, "/../") {
-			return nil, fmt.Errorf("targetPath validation failed, path contains unexpected special elements")
+		if rel, err := filepath.Rel(targetPath, target); err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
+			return nil, fmt.Errorf("targetPath validation failed, %q escapes the target directory", header.Name)
 		}
 
 		switch header.Typeflag {

pkg/bundle/bundle_test.go

@@ -5,6 +5,8 @@
 package bundle_test
 
 import (
+	"archive/tar"
+	"bytes"
 	"fmt"
 	"log"
 	"net/http/httptest"
@@ -85,6 +87,41 @@ var _ = Describe("Bundle", func() {
 				})
 			})
 		})
+
+		DescribeTable("should reject a tar entry that attempts to escape the target directory",
+			func(entryName string) {
+				withTempDir(func(outerDir string) {
+					// nested four levels deep so that the "../../../../etc/canary"
+					// payload still escapes targetDir while resolving to a path
+					// inside outerDir, not the host's real /etc
+					targetDir := filepath.Join(outerDir, "a", "b", "c", "target")
+					Expect(os.MkdirAll(targetDir, os.FileMode(0755))).To(Succeed())
+
+					// where the malicious entry would land if it were not rejected
+					escapedPath := filepath.Join(targetDir, entryName)
+
+					var buf bytes.Buffer
+					tw := tar.NewWriter(&buf)
+					Expect(tw.WriteHeader(&tar.Header{
+						Name:     entryName,
+						Typeflag: tar.TypeReg,
+						Mode:     0644,
+						Size:     int64(len("owned")),
+					})).To(Succeed())
+					_, err := tw.Write([]byte("owned"))
+					Expect(err).ToNot(HaveOccurred())
+					Expect(tw.Close()).To(Succeed())
+
+					_, err = Unpack(&buf, targetDir)
+					Expect(err).To(HaveOccurred())
+					Expect(err.Error()).To(ContainSubstring("escapes the target directory"))
+
+					Expect(escapedPath).ToNot(BeAnExistingFile())
+				})
+			},
+			Entry("single level traversal", "../canary"),
+			Entry("deep traversal matching the reported issue", "../../../../etc/canary"),
+		)
 	})
 
 	Context("packing/pushing and pulling/unpacking", func() {