gh-155639: Reject ZIP files with inconsistent EOCD disk information

Author: abdulm5

Lib/test/test_zipfile/test_core.py

@@ -1136,6 +1136,54 @@ def test_bad_zip64_end_of_central_dir(self):
             zipfile.ZipFile(io.BytesIO(zipdata))
         self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
 
+    def test_multi_disk_zip64_end_of_central_dir(self):
+        zipdata = self.make_zip64_file()
+        eocd64 = zipdata.rfind(zipfile.stringEndArchive64)
+        disk_fields_offset = eocd64 + struct.calcsize('<4sQ2H')
+
+        for disk_number, disk_start in ((1, 0), (0, 1), (1, 1)):
+            corrupted = bytearray(zipdata)
+            struct.pack_into(
+                '<LL', corrupted, disk_fields_offset,
+                disk_number, disk_start,
+            )
+            with self.subTest(
+                disk_number=disk_number,
+                disk_start=disk_start,
+            ):
+                with self.assertRaisesRegex(zipfile.BadZipFile,
+                                            'multiple disks'):
+                    zipfile.ZipFile(io.BytesIO(corrupted))
+                self.assertFalse(zipfile.is_zipfile(io.BytesIO(corrupted)))
+
+    def test_multi_disk_end_of_central_dir_with_zip64(self):
+        zipdata = self.make_zip64_file()
+        eocd = zipdata.rfind(zipfile.stringEndArchive)
+
+        for disk_number, disk_start in ((1, 0), (0, 1), (1, 1)):
+            corrupted = bytearray(zipdata)
+            struct.pack_into(
+                '<HH', corrupted, eocd + 4,
+                disk_number, disk_start,
+            )
+            with self.subTest(
+                disk_number=disk_number,
+                disk_start=disk_start,
+            ):
+                with self.assertRaisesRegex(zipfile.BadZipFile,
+                                            'multiple disks'):
+                    zipfile.ZipFile(io.BytesIO(corrupted))
+                self.assertFalse(zipfile.is_zipfile(io.BytesIO(corrupted)))
+
+    def test_zip64_end_of_central_dir_disk_sentinels(self):
+        zipdata = bytearray(self.make_zip64_file())
+        eocd = zipdata.rfind(zipfile.stringEndArchive)
+        struct.pack_into('<HH', zipdata, eocd + 4, 0xffff, 0xffff)
+
+        with zipfile.ZipFile(io.BytesIO(zipdata)) as zf:
+            self.assertEqual(zf.namelist(), ['test.txt'])
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
     def test_zip64_end_of_central_dir_record_not_found(self):
         zipdata = self.make_zip64_file()
         zipdata = zipdata.replace(b"PK\x06\x06", b'\x00'*4)
@@ -4246,6 +4294,34 @@ def test_damaged_zipfile(self):
             fp = io.BytesIO(zipfiledata[:N])
             self.assertRaises(zipfile.BadZipFile, zipfile.ZipFile, fp)
 
+    def test_multi_disk_end_of_central_dir(self):
+        for comment in (b'', b'comment'):
+            archive = io.BytesIO()
+            with zipfile.ZipFile(archive, 'w') as zf:
+                zf.writestr('entry.txt', b'payload')
+                zf.comment = comment
+            zipdata = archive.getvalue()
+            eocd = zipdata.rfind(zipfile.stringEndArchive)
+
+            for disk_number, disk_start in ((1, 0), (0, 1), (1, 1)):
+                corrupted = bytearray(zipdata)
+                struct.pack_into(
+                    '<HH', corrupted, eocd + 4,
+                    disk_number, disk_start,
+                )
+                with self.subTest(
+                    comment=comment,
+                    disk_number=disk_number,
+                    disk_start=disk_start,
+                ):
+                    with self.assertRaisesRegex(zipfile.BadZipFile,
+                                                'multiple disks'):
+                        zipfile.ZipFile(io.BytesIO(corrupted))
+                    fp = io.BytesIO(corrupted)
+                    pos = fp.seek(5)
+                    self.assertFalse(zipfile.is_zipfile(fp))
+                    self.assertEqual(fp.tell(), pos)
+
     def test_is_zip_valid_file(self):
         """Check that is_zipfile() correctly identifies zip files."""
         # - passing a filename

Lib/zipfile/__init__.py

@@ -247,7 +247,7 @@ def _check_zipfile(fp):
                         centdir = struct.unpack(structCentralDir, data) # CD is the right size
                         if centdir[_CD_SIGNATURE] == stringCentralDir:
                             return True         # First central directory entry  has correct magic number
-    except OSError:
+    except (OSError, BadZipFile):
         pass
     return False
 
@@ -282,6 +282,15 @@ def _handle_prepended_data(endrec, debug=0):
 
     return offset_cd, concat
 
+
+def _validate_end_record(endrec, *, allow_zip64_sentinels=False):
+    valid_disk_numbers = (0, 0xffff) if allow_zip64_sentinels else (0,)
+    if (endrec[_ECD_DISK_NUMBER] not in valid_disk_numbers or
+        endrec[_ECD_DISK_START] not in valid_disk_numbers):
+        raise BadZipFile("zipfiles that span multiple disks are not supported")
+    return endrec
+
+
 def _EndRecData64(fpin, offset, endrec):
     """
     Read the ZIP64 end-of-archive records and use that to update endrec
@@ -372,7 +381,10 @@ def _EndRecData(fpin):
         endrec.append(filesize - sizeEndCentDir)
 
         # Try to read the "Zip64 end of central directory" structure
-        return _EndRecData64(fpin, filesize - sizeEndCentDir, endrec)
+        _validate_end_record(endrec, allow_zip64_sentinels=True)
+        return _validate_end_record(
+            _EndRecData64(fpin, filesize - sizeEndCentDir, endrec)
+        )
 
     # Either this is not a ZIP file, or it is a ZIP file with an archive
     # comment.  Search the end of the file for the "end of central directory"
@@ -396,7 +408,10 @@ def _EndRecData(fpin):
         endrec.append(maxCommentStart + start)
 
         # Try to read the "Zip64 end of central directory" structure
-        return _EndRecData64(fpin, maxCommentStart + start, endrec)
+        _validate_end_record(endrec, allow_zip64_sentinels=True)
+        return _validate_end_record(
+            _EndRecData64(fpin, maxCommentStart + start, endrec)
+        )
 
     # Unable to find a valid end of central directory structure
     return None

Misc/NEWS.d/next/Library/2026-08-14-16-29-48.gh-issue-155639.Lm3kQw.rst

@@ -0,0 +1,2 @@
+Reject ZIP archives whose end of central directory records indicate that
+the archive spans multiple disks.