safely join paths derived from archive headers

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/zip_file_traversal.go

@@ -24,6 +24,15 @@ const (
 
 const perFileReadLimit = 2 * GB
 
+type errZipSlipDetected struct {
+	Prefix   string
+	JoinArgs []string
+}
+
+func (e *errZipSlipDetected) Error() string {
+	return fmt.Sprintf("paths are not allowed to resolve outside of the root prefix (%q). Destination: %q", e.Prefix, e.JoinArgs)
+}
+
 type zipTraversalRequest map[string]struct{}
 
 func newZipTraverseRequest(paths ...string) zipTraversalRequest {
@@ -169,17 +178,12 @@ func ContentsFromZip(archivePath string, paths ...string) (map[string]string, er
 // UnzipToDir extracts a zip archive to a target directory.
 func UnzipToDir(archivePath, targetDir string) error {
 	visitor := func(file *zip.File) error {
-		// the zip-slip attack protection is still being erroneously detected
-		// nolint:gosec
-		expandedFilePath := filepath.Clean(filepath.Join(targetDir, file.Name))
-
-		// protect against zip slip attacks (traversing unintended parent paths from maliciously crafted relative-path entries)
-		if !strings.HasPrefix(expandedFilePath, filepath.Clean(targetDir)+string(os.PathSeparator)) {
-			return fmt.Errorf("potential zip slip attack: %q", expandedFilePath)
+		joinedPath, err := safeJoin(targetDir, file.Name)
+		if err != nil {
+			return err
 		}
 
-		err := extractSingleFile(file, expandedFilePath, archivePath)
-		if err != nil {
+		if err = extractSingleFile(file, joinedPath, archivePath); err != nil {
 			return err
 		}
 		return nil
@@ -188,6 +192,20 @@ func UnzipToDir(archivePath, targetDir string) error {
 	return TraverseFilesInZip(archivePath, visitor)
 }
 
+// safeJoin ensures that any destinations do not resolve to a path above the prefix path.
+func safeJoin(prefix string, dest ...string) (string, error) {
+	joinResult := filepath.Join(append([]string{prefix}, dest...)...)
+	cleanJoinResult := filepath.Clean(joinResult)
+	if !strings.HasPrefix(cleanJoinResult, filepath.Clean(prefix)) {
+		return "", &errZipSlipDetected{
+			Prefix:   prefix,
+			JoinArgs: dest,
+		}
+	}
+	// why not return the clean path? the called may not be expected it from what should only be a join operation.
+	return joinResult, nil
+}
+
 func extractSingleFile(file *zip.File, expandedFilePath, archivePath string) error {
 	zippedFile, err := file.Open()
 	if err != nil {

internal/file/zip_file_traversal_test.go

@@ -3,6 +3,8 @@ package file
 import (
 	"crypto/sha256"
 	"encoding/json"
+	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -12,6 +14,7 @@ import (
 	"testing"
 
 	"github.com/go-test/deep"
+	"github.com/stretchr/testify/assert"
 )
 
 func equal(r1, r2 io.Reader) (bool, error) {
@@ -173,3 +176,79 @@ func TestContentsFromZip(t *testing.T) {
 		t.Errorf("full result: %s", string(b))
 	}
 }
+
+// looks like there isn't a helper for this yet? https://github.com/stretchr/testify/issues/497
+func assertErrorAs(expectedErr interface{}) assert.ErrorAssertionFunc {
+	return func(t assert.TestingT, actualErr error, i ...interface{}) bool {
+		return errors.As(actualErr, &expectedErr)
+	}
+}
+
+func TestSafeJoin(t *testing.T) {
+	tests := []struct {
+		prefix       string
+		args         []string
+		expected     string
+		errAssertion assert.ErrorAssertionFunc
+	}{
+		// go cases...
+		{
+			prefix: "/a/place",
+			args: []string{
+				"somewhere/else",
+			},
+			expected:     "/a/place/somewhere/else",
+			errAssertion: assert.NoError,
+		},
+		{
+			prefix: "/a/place",
+			args: []string{
+				"somewhere/../else",
+			},
+			expected:     "/a/place/else",
+			errAssertion: assert.NoError,
+		},
+		{
+			prefix: "/a/../place",
+			args: []string{
+				"somewhere/else",
+			},
+			expected:     "/place/somewhere/else",
+			errAssertion: assert.NoError,
+		},
+		// zip slip examples....
+		{
+			prefix: "/a/place",
+			args: []string{
+				"../../../etc/passwd",
+			},
+			expected:     "",
+			errAssertion: assertErrorAs(&errZipSlipDetected{}),
+		},
+		{
+			prefix: "/a/place",
+			args: []string{
+				"../",
+				"../",
+			},
+			expected:     "",
+			errAssertion: assertErrorAs(&errZipSlipDetected{}),
+		},
+		{
+			prefix: "/a/place",
+			args: []string{
+				"../",
+			},
+			expected:     "",
+			errAssertion: assertErrorAs(&errZipSlipDetected{}),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("%+v:%+v", test.prefix, test.args), func(t *testing.T) {
+			actual, err := safeJoin(test.prefix, test.args...)
+			test.errAssertion(t, err)
+			assert.Equal(t, test.expected, actual)
+		})
+	}
+}