bpo-36993: Improve error detection of extra field in ZipFile

Lib/test/test_zipfile.py

@@ -1739,6 +1739,20 @@ def test_seek_tell(self):
                 fp.seek(0, os.SEEK_SET)
                 self.assertEqual(fp.tell(), 0)
 
+    def test_malformatted_zip64_extra_field(self):
+        zipdata = (
+            b'PK\x03\x04\xfb\x03PK\x01\x02\x1e\xfb\x13\x00\x00\x00\xd3'
+            b'\x00\x978\xa2NwS\x17\x88\xfa\x00\x00\x00\xff\xff\xff\xff'
+            b'\x12\x00\x18\x00\x00\x00\x00\x00\x01\x00\x00\x00\xb4\x81'
+            b'\x00\x00\x00\x00zipfile_extract/\x00\x00'
+            # Start of the extra field.
+            b'\x01\x00\x00\x00//(///\xff\x00\x00\x00//////\xe8\x03\x00 '
+            b'PK\x05\x06\x00\x00\x00\x00\x01\x00\x01\x00X\x00\x00\x00F'
+            b'\x01\x00\xee\xff\x01'
+        )
+        with self.assertRaises(zipfile.BadZipFile):
+            zipfile.ZipFile(io.BytesIO(zipdata))
+
     def tearDown(self):
         unlink(TESTFN)
         unlink(TESTFN2)

Lib/zipfile.py

@@ -463,15 +463,14 @@ def _decodeExtra(self):
             tp, ln = unpack('<HH', extra[:4])
             if ln+4 > len(extra):
                 raise BadZipFile("Corrupt extra field %04x (size=%d)" % (tp, ln))
+            # Zip64 extended information extra field
             if tp == 0x0001:
                 if ln >= 24:
                     counts = unpack('<QQQ', extra[4:28])
                 elif ln == 16:
                     counts = unpack('<QQ', extra[4:20])
                 elif ln == 8:
                     counts = unpack('<Q', extra[4:12])
-                elif ln == 0:
-                    counts = ()
                 else:
                     raise BadZipFile("Corrupt extra field %04x (size=%d)" % (tp, ln))
 

Misc/NEWS.d/next/Library/2019-06-01-19-10-57.bpo-36993.9nIngf.rst

@@ -0,0 +1,2 @@
+Improve error detection of malformed extra field in
+:class:`zipfile.ZipFile`.