Treat an MPO with too few MP entries as a JPEG

Tests/test_file_jpeg.py

@@ -2,6 +2,7 @@
 
 import os
 import re
+import struct
 import warnings
 from io import BytesIO
 from pathlib import Path
@@ -766,6 +767,18 @@ def test_bad_mpo_header(self) -> None:
 
         im.close()
 
+    def test_mp_entry_count_too_high(self) -> None:
+        """Treat an MPO with fewer MP entries than images as JPEG"""
+        with open("Tests/images/sugarshack.mpo", "rb") as fp:
+            data = fp.read()
+
+        # Change the number of images to three, without adding a third MP entry
+        data = data[:6048] + struct.pack(">L", 3) + data[6052:]
+
+        with pytest.warns(UserWarning, match="malformed MPO file"):
+            with Image.open(BytesIO(data)) as im:
+                assert im.format == "JPEG"
+
     @pytest.mark.parametrize("mode", ("1", "L", "RGB", "RGBX", "CMYK", "YCbCr"))
     def test_save_correct_modes(self, mode: str) -> None:
         out = BytesIO()

src/PIL/JpegImagePlugin.py

@@ -605,7 +605,7 @@ def _getmp(self: JpegImageFile) -> dict[int, Any] | None:
             mpentry["Attribute"] = mpentryattr
             mpentries.append(mpentry)
         mp[0xB002] = mpentries
-    except KeyError as e:
+    except (KeyError, struct.error) as e:
         msg = "malformed MP Index (bad MP Entry)"
         raise SyntaxError(msg) from e
     # Next we should try and parse the individual image unique ID list;