diff --git a/tests/core/test_get_boxes.py b/tests/core/test_get_boxes.py
new file mode 100644
index 00000000..546a41c4
--- /dev/null
+++ b/tests/core/test_get_boxes.py
@@ -0,0 +1,44 @@
+"""get_boxes must give each thread its own box when tracks read their init segments at the same time."""
+
+from __future__ import annotations
+
+import struct
+import threading
+from uuid import UUID, uuid4
+
+from unshackle.core.utilities import get_boxes
+
+
+def tenc_init(kid: UUID, version: int, crypt_skip: int) -> bytes:
+    body = struct.pack(">B3s", version, b"\0\0\0") + bytes([0, crypt_skip if version else 0]) + b"\x01\x10" + kid.bytes
+    return b"\x00" * 8 + struct.pack(">I", 8 + len(body)) + b"tenc" + body
+
+
+def test_get_boxes_finds_the_tenc() -> None:
+    kid = uuid4()
+    (box,) = get_boxes(tenc_init(kid, 1, 0x19), b"tenc")
+    assert box.key_ID == kid
+
+
+def test_get_boxes_is_right_when_tracks_parse_at_the_same_time() -> None:
+    cases = [(uuid4(), version, crypt_skip) for version, crypt_skip in ((0, 0), (1, 0x19), (1, 0x91), (1, 0x55))] * 2
+    failures: list[str] = []
+
+    def parse(kid: UUID, version: int, crypt_skip: int) -> None:
+        data = tenc_init(kid, version, crypt_skip)
+        for _ in range(1500):
+            try:
+                boxes = list(get_boxes(data, b"tenc"))
+            except Exception as e:
+                failures.append(repr(e))
+                continue
+            if [box.key_ID for box in boxes] != [kid]:
+                failures.append("wrong or missing tenc")
+
+    threads = [threading.Thread(target=parse, args=case) for case in cases]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join(timeout=120)
+    assert not failures, failures[:3]
+    assert not any(thread.is_alive() for thread in threads)
diff --git a/unshackle/core/tracks/subtitle.py b/unshackle/core/tracks/subtitle.py
index b362e7c9..de8fb22f 100644
--- a/unshackle/core/tracks/subtitle.py
+++ b/unshackle/core/tracks/subtitle.py
@@ -25,7 +25,7 @@ from subtitle_filter import Subtitles
 from unshackle.core import binaries
 from unshackle.core.config import config
 from unshackle.core.tracks.track import Track
-from unshackle.core.utilities import try_ensure_utf8, with_script
+from unshackle.core.utilities import PYMP4_LOCK, try_ensure_utf8, with_script
 from unshackle.core.utils.subprocess import log_tool_run
 from unshackle.core.utils.webvtt import merge_segmented_webvtt, rebase_webvtt
 
@@ -737,10 +737,10 @@ class Subtitle(Track):
                 caption_set = pycaption.SRTReader().read(text)
             elif codec == Subtitle.Codec.fTTML:
                 caption_lists: dict[str, pycaption.CaptionList] = defaultdict(pycaption.CaptionList)
+                with PYMP4_LOCK:
+                    boxes = MP4.parse_stream(BytesIO(data))
                 for segment in (
-                    Subtitle.parse(box.data, Subtitle.Codec.TimedTextMarkupLang)
-                    for box in MP4.parse_stream(BytesIO(data))
-                    if box.type == b"mdat"
+                    Subtitle.parse(box.data, Subtitle.Codec.TimedTextMarkupLang) for box in boxes if box.type == b"mdat"
                 ):
                     for lang in segment.get_languages():
                         caption_lists[lang].extend(segment.get_captions(lang))
@@ -923,7 +923,9 @@ class Subtitle(Track):
                 child["type"] = child.get("type") or child.get("format")
                 yield child
 
-        for box in flatten_boxes(MP4.parse_stream(BytesIO(data))):
+        with PYMP4_LOCK:
+            boxes = MP4.parse_stream(BytesIO(data))
+        for box in flatten_boxes(boxes):
             # init
             if box.type == b"mdhd":
                 timescale = box.timescale
@@ -963,7 +965,8 @@ class Subtitle(Track):
                 if not saw_trun_box:
                     raise ValueError("The TRUN box was not found in the Segmented WebVTT.")
 
-                vttc_boxes = MP4.parse_stream(BytesIO(box.data))
+                with PYMP4_LOCK:
+                    vttc_boxes = MP4.parse_stream(BytesIO(box.data))
                 current_time = base_time + period_start
 
                 for sample, vttc_box in zip(samples, vttc_boxes):
diff --git a/unshackle/core/utilities.py b/unshackle/core/utilities.py
index 932c461c..1a768d06 100644
--- a/unshackle/core/utilities.py
+++ b/unshackle/core/utilities.py
@@ -9,6 +9,7 @@ import os
 import re
 import socket
 import sys
+import threading
 import time
 import traceback
 import unicodedata
@@ -456,6 +457,11 @@ def resolve_sort_langs(
     return resolved
 
 
+# construct 2.8 gives each bit-field struct one shared stream, so two threads in a pymp4
+# parse or build of a box with bit fields (tenc, senc, trun, sidx, ...) read each other's bytes
+PYMP4_LOCK = threading.Lock()
+
+
 def get_boxes(data: bytes, box_type: bytes, as_bytes: bool = False) -> Box:  # type: ignore
     """
     Scan a byte array for a wanted MP4/ISOBMFF box, then parse and yield each find.
@@ -506,14 +512,13 @@ def get_boxes(data: bytes, box_type: bytes, as_bytes: bool = False) -> Box:  # t
         box_start = pos - 4
 
         try:
-            box = Box.parse(data[box_start:])
-            if as_bytes:
-                box = Box.build(box)
+            with PYMP4_LOCK:
+                box = Box.parse(data[box_start:])
+                built = Box.build(box)
 
-            yield box
+            yield built if as_bytes else box
 
-            box_size = len(Box.build(box))
-            offset = box_start + box_size
+            offset = box_start + len(built)
 
         except IOError:
             break