diff --git a/Lib/test/test_zipfile.py b/Lib/test/test_zipfile.py
index 52831a7bd7ce00..a0bca62ab88ac3 100644
--- a/Lib/test/test_zipfile.py
+++ b/Lib/test/test_zipfile.py
@@ -887,6 +887,8 @@ def make_zip64_file(
         self, file_size_64_set=False, file_size_extra=False,
         compress_size_64_set=False, compress_size_extra=False,
         header_offset_64_set=False, header_offset_extra=False,
+        extensible_data=b'',
+        end_of_central_dir_size=None, offset_to_end_of_central_dir=None,
     ):
         """Generate bytes sequence for a zip with (incomplete) zip64 data.
 
@@ -940,6 +942,12 @@ def make_zip64_file(
 
         central_dir_size = struct.pack('<Q', 58 + 8 * len(central_zip64_fields))
         offset_to_central_dir = struct.pack('<Q', 50 + 8 * len(local_zip64_fields))
+        if end_of_central_dir_size is None:
+            end_of_central_dir_size = 44 + len(extensible_data)
+        if offset_to_end_of_central_dir is None:
+            offset_to_end_of_central_dir = (108
+                                            + 8 * len(local_zip64_fields)
+                                            + 8 * len(central_zip64_fields))
 
         local_extra_length = struct.pack("<H", 4 + 8 * len(local_zip64_fields))
         central_extra_length = struct.pack("<H", 4 + 8 * len(central_zip64_fields))
@@ -968,14 +976,17 @@ def make_zip64_file(
             + filename
             + central_extra
             # Zip64 end of central directory
-            + b"PK\x06\x06,\x00\x00\x00\x00\x00\x00\x00-\x00-"
-            + b"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00"
+            + b"PK\x06\x06"
+            + struct.pack('<Q', end_of_central_dir_size)
+            + b"-\x00-\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00"
             + b"\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00"
             + central_dir_size
             + offset_to_central_dir
+            + extensible_data
             # Zip64 end of central directory locator
-            + b"PK\x06\x07\x00\x00\x00\x00l\x00\x00\x00\x00\x00\x00\x00\x01"
-            + b"\x00\x00\x00"
+            + b"PK\x06\x07\x00\x00\x00\x00"
+            + struct.pack('<Q', offset_to_end_of_central_dir)
+            + b"\x01\x00\x00\x00"
             # end of central directory
             + b"PK\x05\x06\x00\x00\x00\x00\x01\x00\x01\x00:\x00\x00\x002\x00"
             + b"\x00\x00\x00\x00"
@@ -1006,6 +1017,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_file_size_extra))
         self.assertIn('file size', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_file_size_extra)))
 
         # zip64 file size present, zip64 compress size present, one field in
         # extra, expecting two, equals missing compress size.
@@ -1017,6 +1029,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_compress_size_extra))
         self.assertIn('compress size', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_compress_size_extra)))
 
         # zip64 compress size present, no fields in extra, expecting one,
         # equals missing compress size.
@@ -1026,6 +1039,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_compress_size_extra))
         self.assertIn('compress size', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_compress_size_extra)))
 
         # zip64 file size present, zip64 compress size present, zip64 header
         # offset present, two fields in extra, expecting three, equals missing
@@ -1040,6 +1054,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_header_offset_extra))
         self.assertIn('header offset', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_header_offset_extra)))
 
         # zip64 compress size present, zip64 header offset present, one field
         # in extra, expecting two, equals missing header offset
@@ -1052,6 +1067,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_header_offset_extra))
         self.assertIn('header offset', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_header_offset_extra)))
 
         # zip64 file size present, zip64 header offset present, one field in
         # extra, expecting two, equals missing header offset
@@ -1064,6 +1080,7 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_header_offset_extra))
         self.assertIn('header offset', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_header_offset_extra)))
 
         # zip64 header offset present, no fields in extra, expecting one,
         # equals missing header offset
@@ -1075,6 +1092,63 @@ def test_bad_zip64_extra(self):
         with self.assertRaises(zipfile.BadZipFile) as e:
             zipfile.ZipFile(io.BytesIO(missing_header_offset_extra))
         self.assertIn('header offset', str(e.exception).lower())
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(missing_header_offset_extra)))
+
+    def test_bad_zip64_end_of_central_dir(self):
+        zipdata = self.make_zip64_file(end_of_central_dir_size=0)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'Corrupt.*record'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+        zipdata = self.make_zip64_file(end_of_central_dir_size=100)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'Corrupt.*record'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+        zipdata = self.make_zip64_file(offset_to_end_of_central_dir=0)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'Corrupt.*record'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+        zipdata = self.make_zip64_file(offset_to_end_of_central_dir=1000)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'Corrupt.*locator'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(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)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'record not found'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+        zipdata = self.make_zip64_file(
+            extensible_data=b'\xca\xfe\x04\x00\x00\x00data')
+        zipdata = zipdata.replace(b"PK\x06\x06", b'\x00'*4)
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'record not found'):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+    def test_zip64_extensible_data(self):
+        # These values are what is set in the make_zip64_file method.
+        expected_file_size = 8
+        expected_compress_size = 8
+        expected_header_offset = 0
+        expected_content = b"test1234"
+
+        zipdata = self.make_zip64_file(
+            extensible_data=b'\xca\xfe\x04\x00\x00\x00data')
+        with zipfile.ZipFile(io.BytesIO(zipdata)) as zf:
+            zinfo = zf.infolist()[0]
+            self.assertEqual(zinfo.file_size, expected_file_size)
+            self.assertEqual(zinfo.compress_size, expected_compress_size)
+            self.assertEqual(zinfo.header_offset, expected_header_offset)
+            self.assertEqual(zf.read(zinfo), expected_content)
+        self.assertTrue(zipfile.is_zipfile(io.BytesIO(zipdata)))
+
+        with self.assertRaisesRegex(zipfile.BadZipFile, 'record not found'):
+            zipfile.ZipFile(io.BytesIO(b'prepended' + zipdata))
+        self.assertFalse(zipfile.is_zipfile(io.BytesIO(b'prepended' + zipdata)))
 
     def test_generated_valid_zip64_extra(self):
         # These values are what is set in the make_zip64_file method.
diff --git a/Lib/zipfile.py b/Lib/zipfile.py
index 6a4cf7a972bf62..64289ebd674659 100644
--- a/Lib/zipfile.py
+++ b/Lib/zipfile.py
@@ -236,7 +236,7 @@ def is_zipfile(filename):
         else:
             with open(filename, "rb") as fp:
                 result = _check_zipfile(fp)
-    except OSError:
+    except (OSError, BadZipFile):
         pass
     return result
 
@@ -244,16 +244,15 @@ def _EndRecData64(fpin, offset, endrec):
     """
     Read the ZIP64 end-of-archive records and use that to update endrec
     """
-    try:
-        fpin.seek(offset - sizeEndCentDir64Locator, 2)
-    except OSError:
-        # If the seek fails, the file is not large enough to contain a ZIP64
+    offset -= sizeEndCentDir64Locator
+    if offset < 0:
+        # The file is not large enough to contain a ZIP64
         # end-of-archive record, so just return the end record we were given.
         return endrec
-
+    fpin.seek(offset)
     data = fpin.read(sizeEndCentDir64Locator)
     if len(data) != sizeEndCentDir64Locator:
-        return endrec
+        raise OSError("Unknown I/O error")
     sig, diskno, reloff, disks = struct.unpack(structEndArchive64Locator, data)
     if sig != stringEndArchive64Locator:
         return endrec
@@ -261,16 +260,33 @@ def _EndRecData64(fpin, offset, endrec):
     if diskno != 0 or disks > 1:
         raise BadZipFile("zipfiles that span multiple disks are not supported")
 
-    # Assume no 'zip64 extensible data'
-    fpin.seek(offset - sizeEndCentDir64Locator - sizeEndCentDir64, 2)
+    offset -= sizeEndCentDir64
+    if reloff > offset:
+        raise BadZipFile("Corrupt zip64 end of central directory locator")
+    # First, check the assumption that there is no prepended data.
+    fpin.seek(reloff)
+    extrasz = offset - reloff
     data = fpin.read(sizeEndCentDir64)
     if len(data) != sizeEndCentDir64:
-        return endrec
+        raise OSError("Unknown I/O error")
+    if not data.startswith(stringEndArchive64) and reloff != offset:
+        # Since we already have seen the Zip64 EOCD Locator, it's
+        # possible we got here because there is prepended data.
+        # Assume no 'zip64 extensible data'
+        fpin.seek(offset)
+        extrasz = 0
+        data = fpin.read(sizeEndCentDir64)
+        if len(data) != sizeEndCentDir64:
+            raise OSError("Unknown I/O error")
+    if not data.startswith(stringEndArchive64):
+        raise BadZipFile("Zip64 end of central directory record not found")
+
     sig, sz, create_version, read_version, disk_num, disk_dir, \
         dircount, dircount2, dirsize, diroffset = \
         struct.unpack(structEndArchive64, data)
-    if sig != stringEndArchive64:
-        return endrec
+    if (diroffset + dirsize != reloff or
+        sz + 12 != sizeEndCentDir64 + extrasz):
+        raise BadZipFile("Corrupt zip64 end of central directory record")
 
     # Update the original endrec using data from the ZIP64 record
     endrec[_ECD_SIGNATURE] = sig
@@ -280,6 +296,7 @@ def _EndRecData64(fpin, offset, endrec):
     endrec[_ECD_ENTRIES_TOTAL] = dircount2
     endrec[_ECD_SIZE] = dirsize
     endrec[_ECD_OFFSET] = diroffset
+    endrec[_ECD_LOCATION] = offset - extrasz
     return endrec
 
 
@@ -313,7 +330,7 @@ def _EndRecData(fpin):
         endrec.append(filesize - sizeEndCentDir)
 
         # Try to read the "Zip64 end of central directory" structure
-        return _EndRecData64(fpin, -sizeEndCentDir, endrec)
+        return _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"
@@ -337,8 +354,7 @@ def _EndRecData(fpin):
         endrec.append(maxCommentStart + start)
 
         # Try to read the "Zip64 end of central directory" structure
-        return _EndRecData64(fpin, maxCommentStart + start - filesize,
-                             endrec)
+        return _EndRecData64(fpin, maxCommentStart + start, endrec)
 
     # Unable to find a valid end of central directory structure
     return None
@@ -1386,9 +1402,6 @@ def _RealGetContents(self):
 
         # "concat" is zero, unless zip was concatenated to another file
         concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
-        if endrec[_ECD_SIGNATURE] == stringEndArchive64:
-            # If Zip64 extension structures are present, account for them
-            concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
 
         if self.debug > 2:
             inferred = concat + offset_cd
@@ -1989,7 +2002,7 @@ def _write_end_record(self):
                                    " would require ZIP64 extensions")
             zip64endrec = struct.pack(
                 structEndArchive64, stringEndArchive64,
-                44, 45, 45, 0, 0, centDirCount, centDirCount,
+                sizeEndCentDir64 - 12, 45, 45, 0, 0, centDirCount, centDirCount,
                 centDirSize, centDirOffset)
             self.fp.write(zip64endrec)
 
diff --git a/Misc/NEWS.d/next/Security/2025-10-07-19-31-34.gh-issue-139700.vNHU1O.rst b/Misc/NEWS.d/next/Security/2025-10-07-19-31-34.gh-issue-139700.vNHU1O.rst
new file mode 100644
index 00000000000000..a8e7a1f1878c6b
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2025-10-07-19-31-34.gh-issue-139700.vNHU1O.rst
@@ -0,0 +1,3 @@
+Check consistency of the zip64 end of central directory record. Support
+records with "zip64 extensible data" if there are no bytes prepended to the
+ZIP file.