diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/releases/1.8.10.txt | 33 | ||||
| -rw-r--r-- | docs/topics/auth/passwords.txt | 30 |
2 files changed, 63 insertions, 0 deletions
diff --git a/docs/releases/1.8.10.txt b/docs/releases/1.8.10.txt index 73c7cc04a4..d57afc470d 100644 --- a/docs/releases/1.8.10.txt +++ b/docs/releases/1.8.10.txt @@ -22,6 +22,39 @@ redirecting to this URL sends the user to ``attacker.com``. Also, if a developer relies on ``is_safe_url()`` to provide safe redirect targets and puts such a URL into a link, they could suffer from an XSS attack. +CVE-2016-2513: User enumeration through timing difference on password hasher work factor upgrade +================================================================================================ + +In each major version of Django since 1.6, the default number of iterations for +the ``PBKDF2PasswordHasher`` and its subclasses has increased. This improves +the security of the password as the speed of hardware increases, however, it +also creates a timing difference between a login request for a user with a +password encoded in an older number of iterations and login request for a +nonexistent user (which runs the default hasher's default number of iterations +since Django 1.6). + +This only affects users who haven't logged in since the iterations were +increased. The first time a user logs in after an iterations increase, their +password is updated with the new iterations and there is no longer a timing +difference. + +The new ``BasePasswordHasher.harden_runtime()`` method allows hashers to bridge +the runtime gap between the work factor (e.g. iterations) supplied in existing +encoded passwords and the default work factor of the hasher. This method +is implemented for ``PBKDF2PasswordHasher`` and ``BCryptPasswordHasher``. +The number of rounds for the latter hasher hasn't changed since Django 1.4, but +some projects may subclass it and increase the work factor as needed. + +A warning will be emitted for any :ref:`third-party password hashers that don't +implement <write-your-own-password-hasher>` a ``harden_runtime()`` method. + +If you have different password hashes in your database (such as SHA1 hashes +from users who haven't logged in since the default hasher switched to PBKDF2 +in Django 1.4), the timing difference on a login request for these users may be +even greater and this fix doesn't remedy that difference (or any difference +when changing hashers). You may be able to :ref:`upgrade those hashes +<wrapping-password-hashers>` to prevent a timing attack for that case. + Bugfixes ======== diff --git a/docs/topics/auth/passwords.txt b/docs/topics/auth/passwords.txt index 29da3ae1d1..15d9a56ba2 100644 --- a/docs/topics/auth/passwords.txt +++ b/docs/topics/auth/passwords.txt @@ -194,6 +194,14 @@ sure never to *remove* entries from this list. If you do, users using unmentioned algorithms won't be able to upgrade. Passwords will be upgraded when changing the PBKDF2 iteration count. +Be aware that if all the passwords in your database aren't encoded in the +default hasher's algorithm, you may be vulnerable to a user enumeration timing +attack due to a difference between the duration of a login request for a user +with a password encoded in a non-default algorithm and the duration of a login +request for a nonexistent user (which runs the default hasher). You may be able +to mitigate this by :ref:`upgrading older password hashes +<wrapping-password-hashers>`. + .. _wrapping-password-hashers: Password upgrading without requiring a login @@ -283,6 +291,28 @@ Include any other hashers that your site uses in this list. .. _bcrypt: https://en.wikipedia.org/wiki/Bcrypt .. _`bcrypt library`: https://pypi.python.org/pypi/bcrypt/ +.. _write-your-own-password-hasher: + +Writing your own hasher +----------------------- + +.. versionadded:: 1.8.10 + +If you write your own password hasher that contains a work factor such as a +number of iterations, you should implement a +``harden_runtime(self, password, encoded)`` method to bridge the runtime gap +between the work factor supplied in the ``encoded`` password and the default +work factor of the hasher. This prevents a user enumeration timing attack due +to difference between a login request for a user with a password encoded in an +older number of iterations and a nonexistent user (which runs the default +hasher's default number of iterations). + +Taking PBKDF2 as example, if ``encoded`` contains 20,000 iterations and the +hasher's default ``iterations`` is 30,000, the method should run ``password`` +through another 10,000 iterations of PBKDF2. + +If your hasher doesn't have a work factor, implement the method as a no-op +(``pass``). Manually managing a user's password =================================== |