Resolve RCE vulnerability in local WordNet browser (nltk#3100)

* Restrict wordnet app pickle loading: disallow functions and classes

* Resolve typo: where -> were

In an error message for the wordnet browser

Author: tomaarsen

nltk/app/wordnet_app.py

@@ -47,11 +47,10 @@
 
 import base64
 import copy
-import datetime
 import getopt
+import io
 import os
 import pickle
-import re
 import sys
 import threading
 import time
@@ -60,17 +59,12 @@
 from http.server import BaseHTTPRequestHandler, HTTPServer
 
 # Allow this program to run inside the NLTK source tree.
-from sys import argv, path
+from sys import argv
 from urllib.parse import unquote_plus
 
 from nltk.corpus import wordnet as wn
 from nltk.corpus.reader.wordnet import Lemma, Synset
 
-# now included in local file
-# from util import html_header, html_trailer, \
-#    get_static_index_page, get_static_page_by_path, \
-#    page_from_word, page_from_href
-
 firstClient = True
 
 # True if we're not also running a web browser.  The value f server_mode
@@ -659,6 +653,16 @@ def make_synset_html(db_name, disp_name, rels):
     return html
 
 
+class RestrictedUnpickler(pickle.Unpickler):
+    """
+    Unpickler that prevents any class or function from being used during loading.
+    """
+
+    def find_class(self, module, name):
+        # Forbid every function
+        raise pickle.UnpicklingError(f"global '{module}.{name}' is forbidden")
+
+
 class Reference:
     """
     A reference to a page that may be generated by page_word
@@ -694,7 +698,7 @@ def decode(string):
         Decode a reference encoded with Reference.encode
         """
         string = base64.urlsafe_b64decode(string.encode())
-        word, synset_relations = pickle.loads(string)
+        word, synset_relations = RestrictedUnpickler(io.BytesIO(string)).load()
         return Reference(word, synset_relations)
 
     def toggle_synset_relation(self, synset, relation):
@@ -794,7 +798,7 @@ def page_from_reference(href):
                 except KeyError:
                     pass
     if not body:
-        body = "The word or words '%s' where not found in the dictionary." % word
+        body = "The word or words '%s' were not found in the dictionary." % word
     return body, word