use a constant time compare when verifying confirmation links

1 files changed, 27 insertions, 1 deletions

diff --git a/utils.py b/utils.py
index 7071284..7e5fd85 100644
--- a/utils.py
+++ b/utils.py
@@ -13,6 +13,7 @@ from flask import current_app, flash, g, redirect, render_template, request, ses
 from flask import url_for as flask_url_for
 from flask.ext.wtf import ValidationError
 from hashlib import sha1
+from itertools import izip
 from random import randint
 from time import time
 from werkzeug.exceptions import Forbidden
@@ -148,7 +149,7 @@ def verify_confirmation(realm, token, timeout=None):
     tokentime = struct.unpack('>i', token[20:24])[0]
     payload = token[24:]
 
-    if mac != hmac.new(key, token[20:], sha1).digest():
+    if not constant_time_compare(mac, hmac.new(key, token[20:], sha1).digest()):
         raise ConfirmationInvalid('MAC does not match')
 
     if timeout is not None and time() > tokentime + timeout:
@@ -170,6 +171,31 @@ def http_verify_confirmation(*args, **kwargs):
         raise Forbidden(u'Bestätigungslink ist zu alt.')
 
 
+# Shamelessly stolen from https://github.com/mitsuhiko/itsdangerous/
+# (C) 2011 by Armin Ronacher and the Django Software Foundation. 3-clause BSD.
+def constant_time_compare(val1, val2):
+    """Returns True if the two strings are equal, False otherwise.
+
+    The time taken is independent of the number of characters that match. Do
+    not use this function for anything else than comparision with known
+    length targets.
+
+    This is should be implemented in C in order to get it completely right.
+    """
+    if _builtin_constant_time_compare is not None:
+        return _builtin_constant_time_compare(val1, val2)
+    len_eq = len(val1) == len(val2)
+    if len_eq:
+        result = 0
+        left = val1
+    else:
+        result = 1
+        left = val2
+    for x, y in izip(bytearray(left), bytearray(val2)):
+        result |= x ^ y
+    return result == 0
+
+
 def send_mail(recipient, subject, body, sender=None):
     if sender is None:
         sender = current_app.config['MAIL_DEFAULT_SENDER']