Raise on overlapping file blocks

Lib/test/test_zipfile/test_core.py

@@ -2024,6 +2024,21 @@ def test_repack_data_descriptor_no_sig_and_zip64(self):
                 # check file size
                 self.assertEqual(os.path.getsize(TESTFN), expected_size)
 
+    def test_repack_overlapping_blocks(self):
+        for ii in ([0], [1], [2]):
+            with self.subTest(remove=ii):
+                self._prepare_zip_from_test_files(TESTFN, self.test_files)
+                with open(TESTFN, 'r+b') as fh:
+                    with zipfile.ZipFile(fh, 'a') as zh:
+                        zh.writestr('file.txt', b'dummy')
+                        for i in ii:
+                            zh.infolist()[i].file_size += 50
+                            zh.infolist()[i].compress_size += 50
+
+                with zipfile.ZipFile(TESTFN, 'a') as zh:
+                    with self.assertRaises(zipfile.BadZipFile):
+                        zh.repack()
+
     @mock.patch('zipfile._ZipRepacker')
     def test_repack_closed(self, m_repack):
         self._prepare_zip_from_test_files(TESTFN, self.test_files)

Lib/zipfile/__init__.py

@@ -1452,25 +1452,40 @@ def _repack(self, zfile):
         # doesn't match the actual entry order
         filelist = sorted(zfile.filelist, key=lambda x: x.header_offset)
 
-        try:
-            data_offset = filelist[0].header_offset
-        except IndexError:
-            data_offset = zfile.start_dir
-
-        # calculate the starting entry offset (bytes to skip)
-        entry_offset = self._calc_initial_entry_offset(fp, data_offset)
-
-        # move file entries
+        # calculate each entry size and validate
+        entry_size_list = []
+        used_entry_size_list = []
         for i, zinfo in enumerate(filelist):
-            # get the total size of the entry
             try:
                 offset = filelist[i + 1].header_offset
             except IndexError:
                 offset = zfile.start_dir
             entry_size = offset - zinfo.header_offset
 
+            # may raise on an invalid local file header
             used_entry_size = self._calc_local_file_entry_size(fp, zinfo)
 
+            self._debug(3, i, zinfo.orig_filename, entry_size, used_entry_size)
+            if used_entry_size > entry_size:
+                raise BadZipFile(
+                    f"Overlapped entries: {zinfo.orig_filename!r} "
+                    f"(possible zip bomb)")
+
+            entry_size_list.append(entry_size)
+            used_entry_size_list.append(used_entry_size)
+
+        # calculate the starting entry offset (bytes to skip)
+        try:
+            data_offset = filelist[0].header_offset
+        except IndexError:
+            data_offset = zfile.start_dir
+        entry_offset = self._calc_initial_entry_offset(fp, data_offset)
+
+        # move file entries
+        for i, zinfo in enumerate(filelist):
+            entry_size = entry_size_list[i]
+            used_entry_size = used_entry_size_list[i]
+
             # update the header and move entry data to the new position
             if entry_offset > 0:
                 old_header_offset = zinfo.header_offset