diff --git a/go_backend/ac4_config.go b/go_backend/ac4_config.go
index efeeea0e..cdf82a3e 100644
--- a/go_backend/ac4_config.go
+++ b/go_backend/ac4_config.go
@@ -87,22 +87,26 @@ func findBoxBySignature(data []byte, start, end int64, typ string) (mp4Box, bool
 }
 
 // audioSampleEntryHeaderLen returns the byte length of the fixed audio sample
-// entry header (from the box body start) before child boxes begin.
-func audioSampleEntryHeaderLen(data []byte, entry mp4Box) int64 {
+// entry header (from the box body start) before child boxes begin. ok is false
+// for malformed/truncated entries whose declared header is not fully present.
+func audioSampleEntryHeaderLen(data []byte, entry mp4Box) (hdrLen int64, ok bool) {
 	// 6 bytes reserved + 2 bytes data_reference_index, then the audio fields.
 	base := entry.body()
 	if base+10 > entry.end() {
-		return 8 + 20
+		return 0, false
 	}
 	version := binary.BigEndian.Uint16(data[base+8 : base+10])
+	hdrLen = 8 + 20
 	switch version {
 	case 1:
-		return 8 + 20 + 16
+		hdrLen += 16
 	case 2:
-		return 8 + 20 + 36
-	default:
-		return 8 + 20
+		hdrLen += 36
 	}
+	if base+hdrLen > entry.end() {
+		return 0, false
+	}
+	return hdrLen, true
 }
 
 type ac4Location struct {
@@ -232,13 +236,16 @@ func normalizeQuickTimeAudioToMP4(data []byte) []byte {
 		return data // already v0 (or v2, left untouched)
 	}
 
-	binary.BigEndian.PutUint16(data[verPos:verPos+2], 0)
 	// The v1 QuickTime sound extension is the 16 bytes following the 20-byte v0
 	// audio fields (samplesPerPacket, bytesPerPacket, bytesPerFrame, bytesPerSample).
 	extStart := entry.body() + 8 + 20
 	extEnd := extStart + 16
+	if extEnd > entry.end() {
+		return data
+	}
 	delta := int64(-16)
 
+	binary.BigEndian.PutUint16(data[verPos:verPos+2], 0)
 	shiftChunkOffsets(data, loc.chain[0], extStart, delta)
 	for _, b := range loc.chain {
 		growBoxSize(data, b, delta)
@@ -273,7 +280,10 @@ func EnsureAC4ConfigBox(decryptedPath, sourcePath string) error {
 		return nil
 	}
 
-	hdrLen := audioSampleEntryHeaderLen(dst, loc.entry)
+	hdrLen, ok := audioSampleEntryHeaderLen(dst, loc.entry)
+	if !ok {
+		return fmt.Errorf("malformed ac-4 sample entry")
+	}
 	childStart := loc.entry.body() + hdrLen
 	if _, has := findChildMP4(dst, childStart, loc.entry.end(), "dac4"); has {
 		// Already has dac4; still persist any normalization changes.
diff --git a/go_backend/ac4_config_test.go b/go_backend/ac4_config_test.go
new file mode 100644
index 00000000..53dacd23
--- /dev/null
+++ b/go_backend/ac4_config_test.go
@@ -0,0 +1,76 @@
+package gobackend
+
+import (
+	"bytes"
+	"encoding/binary"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func mp4TestBox(typ string, body []byte) []byte {
+	out := make([]byte, 8+len(body))
+	binary.BigEndian.PutUint32(out[:4], uint32(len(out)))
+	copy(out[4:8], typ)
+	copy(out[8:], body)
+	return out
+}
+
+func mp4TestAC4Tree(entryBody []byte) []byte {
+	entry := mp4TestBox("ac-4", entryBody)
+	stsdBody := append([]byte{
+		0, 0, 0, 0, // version/flags
+		0, 0, 0, 1, // entry_count
+	}, entry...)
+	stsd := mp4TestBox("stsd", stsdBody)
+	stbl := mp4TestBox("stbl", stsd)
+	minf := mp4TestBox("minf", stbl)
+	mdia := mp4TestBox("mdia", minf)
+	trak := mp4TestBox("trak", mdia)
+	moov := mp4TestBox("moov", trak)
+	return moov
+}
+
+func shortAC4SampleEntryBody(version uint16) []byte {
+	body := make([]byte, 10)
+	binary.BigEndian.PutUint16(body[8:10], version)
+	return body
+}
+
+func TestNormalizeQuickTimeAudioToMP4IgnoresTruncatedAC4Entry(t *testing.T) {
+	input := mp4TestAC4Tree(shortAC4SampleEntryBody(1))
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("normalizeQuickTimeAudioToMP4 panicked: %v", r)
+		}
+	}()
+
+	got := normalizeQuickTimeAudioToMP4(append([]byte{}, input...))
+	if !bytes.Equal(got, input) {
+		t.Fatal("truncated QuickTime AC-4 entry should be left unchanged")
+	}
+}
+
+func TestEnsureAC4ConfigBoxRejectsTruncatedAC4Entry(t *testing.T) {
+	dir := t.TempDir()
+	decryptedPath := filepath.Join(dir, "decrypted.mp4")
+	sourcePath := filepath.Join(dir, "source.mp4")
+
+	if err := os.WriteFile(decryptedPath, mp4TestAC4Tree(shortAC4SampleEntryBody(2)), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(sourcePath, mp4TestBox("moov", mp4TestBox("dac4", []byte{1, 2, 3, 4})), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("EnsureAC4ConfigBox panicked: %v", r)
+		}
+	}()
+
+	if err := EnsureAC4ConfigBox(decryptedPath, sourcePath); err == nil {
+		t.Fatal("expected malformed AC-4 sample entry error")
+	}
+}