Use constant-time string comparison for auth.

r2/r2/lib/utils/utils.py

@@ -1342,3 +1342,19 @@ def thread_dump(*a):
                              % dict(filename=filename, lineno=lineno, fnname=fnname))
             sys.stderr.write('\t\t\t%(line)s\n' % dict(line=line))
 
+
+def constant_time_compare(actual, expected):
+    """
+    Returns True if the two strings are equal, False otherwise
+    
+    The time taken is dependent on the number of charaters provided
+    instead of the number of characters that match.
+    """
+    actual_len   = len(actual)
+    expected_len = len(expected)
+    result = actual_len ^ expected_len
+    if expected_len > 0:
+        for i in xrange(actual_len):
+            result |= ord(actual[i]) ^ ord(expected[i % expected_len])
+    return result == 0
+

r2/r2/models/account.py

@@ -25,6 +25,7 @@ from r2.lib.db.userrel   import UserRel
 from r2.lib.memoize      import memoize
 from r2.lib.utils        import modhash, valid_hash, randstr, timefromnow
 from r2.lib.utils        import UrlParser, set_last_visit, last_visit
+from r2.lib.utils        import constant_time_compare
 from r2.lib.cache        import sgm
 from r2.lib.log import log_text
 
@@ -550,9 +551,9 @@ def valid_cookie(cookie):
     except NotFound:
         return (False, False)
 
-    if cookie == account.make_cookie(timestr, admin = False):
+    if constant_time_compare(cookie, account.make_cookie(timestr, admin = False)):
         return (account, False)
-    elif cookie == account.make_cookie(timestr, admin = True):
+    elif constant_time_compare(cookie, account.make_cookie(timestr, admin = True)):
         return (account, True)
     return (False, False)
 
@@ -563,7 +564,7 @@ def valid_feed(name, feedhash, path):
         try:
             user = Account._by_name(name)
             if (user.pref_private_feeds and
-                feedhash == make_feedhash(user, path)):
+                constant_time_compare(feedhash, make_feedhash(user, path))):
                 return user
         except NotFound:
             pass
@@ -590,17 +591,21 @@ def valid_login(name, password):
 
 def valid_password(a, password):
     try:
-        if a.password == passhash(a.name, password, ''):
+        # A constant_time_compare isn't strictly required here
+        # but it is doesn't hurt
+        if constant_time_compare(a.password, passhash(a.name, password, '')):
             #add a salt
             a.password = passhash(a.name, password, True)
             a._commit()
             return a
         else:
             salt = a.password[:3]
-            if a.password == passhash(a.name, password, salt):
+            if constant_time_compare(a.password, passhash(a.name, password, salt)):
                 return a
     except AttributeError, UnicodeEncodeError:
         return False
+    # Python defaults to returning None
+    return False
 
 def passhash(username, password, salt = ''):
     if salt is True: