Validate manifest entry paths on deserialize (closes #61)

Untrusted .mf files were parsed with no path validation, so an entry
like ../../etc/passwd flowed into filepath.Join against the checker's
base path. ValidatePath ran only on the write side (Builder). Enforce it
on every entry in deserializeInner, so all consumers — Checker now, any
restore/extract path later — are protected. Reject the whole manifest on
the first offender rather than dropping entries, which could hide files
from a check; failures wrap a new errInvalidManifestPath sentinel and
name the path.

A path that is not valid UTF-8 is refused at load time by the proto3
string decoder before validation runs; the test covers that plus the
traversal, absolute, backslash, and empty cases, and a valid round-trip.

Model: opus-4-8

TODO.md

@@ -24,6 +24,8 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-09-21: validate manifest entry paths on deserialize so untrusted `.mf`
+  files cannot make `Checker` stat or read outside `basePath` (#61)
 - 2026-08-09: added `.prettierrc`/`.prettierignore`, gave `script/fmt` and
   `script/fmt-check` one shared prettier file set via `script/prettier`, dropped
   the `|| true` that hid prettier failures, and added a node-based Dockerfile

mfer/checker.go

@@ -312,6 +312,10 @@ func (c *Checker) FindExtraFiles(ctx context.Context, results chan<- Result) err
 }
 
 func (c *Checker) checkFile(entry *MFFilePath, checkedBytes *FileSize) Result {
+	// entry.GetPath() is safe to join here: a manifest's entry paths are
+	// validated against the path invariants when it is loaded (see
+	// deserializeInner) or built (see Builder.AddFile), so a traversal or
+	// absolute path can never reach this point.
 	absPath := filepath.Join(string(c.basePath), entry.GetPath())
 	relPath := RelFilePath(entry.GetPath())
 

mfer/deserialize.go

@@ -25,6 +25,7 @@ var (
 	errDecompressedTooLarge = errors.New("decompressed data exceeds maximum allowed size")
 	errUUIDMismatch         = errors.New("outer and inner UUID mismatch")
 	errInvalidFileFormat    = errors.New("invalid file format")
+	errInvalidManifestPath  = errors.New("manifest contains invalid path")
 )
 
 // validateUUID checks that the byte slice is a valid UUID (16 bytes, parseable).
@@ -181,6 +182,19 @@ func (m *manifest) deserializeInner() error {
 		return errUUIDMismatch
 	}
 
+	// Enforce the manifest path invariants on every entry as it is loaded,
+	// so that no consumer of a manifest — Checker today, any restore or
+	// extract path tomorrow — acts on a traversal or absolute path from an
+	// untrusted .mf. Reject loudly on the first offender rather than
+	// dropping entries, which would let a hostile manifest hide files from a
+	// check.
+	for _, f := range m.pbInner.GetFiles() {
+		err = ValidatePath(f.GetPath())
+		if err != nil {
+			return fmt.Errorf("%w: %w", errInvalidManifestPath, err)
+		}
+	}
+
 	log.Infof("loaded manifest with %d files", len(m.pbInner.GetFiles()))
 
 	return nil

mfer/deserialize_path_test.go

@@ -0,0 +1,145 @@
+//nolint:testpackage // white-box tests exercise unexported internals
+package mfer
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/klauspost/compress/zstd"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/encoding/protowire"
+	"google.golang.org/protobuf/proto"
+)
+
+// craftInnerBytes builds the wire bytes of an inner MFFile holding a single
+// file entry whose path is exactly pathBytes. It writes the wire form by hand
+// so a hostile path — including one that is not valid UTF-8 — can be embedded
+// without proto.Marshal's own UTF-8 enforcement rejecting it first.
+func craftInnerBytes(id uuid.UUID, pathBytes string) []byte {
+	entry := protowire.AppendTag(nil, 1, protowire.BytesType) // MFFilePath.path
+	entry = protowire.AppendString(entry, pathBytes)
+
+	inner := protowire.AppendTag(nil, 100, protowire.VarintType) // MFFile.version
+	inner = protowire.AppendVarint(inner, uint64(MFFile_VERSION_ONE))
+	inner = protowire.AppendTag(inner, 101, protowire.BytesType) // MFFile.files
+	inner = protowire.AppendBytes(inner, entry)
+	inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
+	inner = protowire.AppendBytes(inner, id[:])
+
+	return inner
+}
+
+// wrapInner wraps inner MFFile wire bytes in a complete, well-formed .mf
+// envelope (magic prefix, zstd-compressed payload, matching hash and UUID) so
+// that deserialization reaches path validation rather than failing earlier on
+// an integrity check.
+func wrapInner(t *testing.T, id uuid.UUID, innerData []byte) []byte {
+	t.Helper()
+
+	var cbuf bytes.Buffer
+
+	zw, err := zstd.NewWriter(&cbuf, zstd.WithEncoderLevel(zstd.SpeedBestCompression))
+	require.NoError(t, err)
+
+	_, err = zw.Write(innerData)
+	require.NoError(t, err)
+	require.NoError(t, zw.Close())
+
+	compressed := cbuf.Bytes()
+	sum := sha256.Sum256(compressed)
+
+	outer := &MFFileOuter{
+		InnerMessage:    compressed,
+		Size:            int64(len(innerData)),
+		Sha256:          sum[:],
+		Uuid:            id[:],
+		Version:         MFFileOuter_VERSION_ONE,
+		CompressionType: MFFileOuter_COMPRESSION_ZSTD,
+	}
+
+	ob, err := proto.Marshal(outer)
+	require.NoError(t, err)
+
+	return append([]byte(MAGIC), ob...)
+}
+
+func TestDeserializeRejectsInvalidEntryPaths(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		path string
+	}{
+		{"parent traversal", "../escape"},
+		{"interior traversal", "a/../../escape"},
+		{"absolute path", "/etc/passwd"},
+		{"backslash path", `a\b`},
+		{"double slash", "a//b"},
+		{"empty path", ""},
+		{"invalid utf-8", "abc\xff"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			id := uuid.New()
+			data := wrapInner(t, id, craftInnerBytes(id, tt.path))
+
+			_, err := NewManifestFromReader(bytes.NewReader(data))
+			require.Error(t, err)
+
+			if tt.path == "abc\xff" {
+				// A path that is not valid UTF-8 cannot survive the proto3
+				// string decoder, which rejects it before path validation
+				// runs; the manifest is still refused at load time.
+				return
+			}
+
+			require.ErrorIs(t, err, errInvalidManifestPath)
+
+			if tt.path != "" {
+				// ValidatePath quotes the path with %q; assert against the
+				// same rendering so escaped characters (e.g. a backslash)
+				// still match.
+				assert.Contains(t, err.Error(), fmt.Sprintf("%q", tt.path),
+					"error must name the offending path")
+			}
+		})
+	}
+}
+
+func TestDeserializeValidManifestRoundTrips(t *testing.T) {
+	t.Parallel()
+
+	hash := make([]byte, 34) // multihash: 2-byte prefix + 32-byte SHA-256
+
+	b := NewBuilder()
+	require.NoError(t, b.AddFileWithHash("dir/file.txt", 123, ModTime{}, hash))
+
+	var buf bytes.Buffer
+	require.NoError(t, b.Build(&buf))
+
+	m, err := NewManifestFromReader(bytes.NewReader(buf.Bytes()))
+	require.NoError(t, err)
+
+	files := m.Files()
+	require.Len(t, files, 1)
+	assert.Equal(t, "dir/file.txt", files[0].GetPath())
+	assert.Equal(t, int64(123), files[0].GetSize())
+}
+
+// TestValidatePathRejectsInvalidUTF8 pins the ValidatePath rule that a manifest
+// path must be valid UTF-8, independent of the proto decoder that also enforces
+// it on the wire.
+func TestValidatePathRejectsInvalidUTF8(t *testing.T) {
+	t.Parallel()
+
+	err := ValidatePath("abc\xff")
+	require.ErrorIs(t, err, errPathNotUTF8)
+	assert.Contains(t, err.Error(), "UTF-8")
+}