symfony · weaverryan · Sep 26, 2013 · Sep 25, 2013
diff --git a/book/security.rst b/book/security.rst
@@ -1410,6 +1410,13 @@ can always be determined in the following way from a controller::
     $password = $encoder->encodePassword('ryanpass', $user->getSalt());
     $user->setPassword($password);
 
+.. caution::
+
+    When you allow a user to submit a plaintext password (e.g. registration
+    form, change password form), you *must* have validation that guarantees
+    that the password is 4096 characters or less. Read more details in
+    :ref:`How to implement a simple Registration Form <cookbook-registration-password-max>`.
+
 Retrieving the User Object
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 

diff --git a/components/security/authentication.rst b/components/security/authentication.rst
@@ -190,8 +190,21 @@ Each encoder should implement :class:`Symfony\\Component\\Security\\Core\\Encode
 or be an array with a ``class`` and an ``arguments`` key, which allows the
 encoder factory to construct the encoder only when it is needed.
 
-Password Encoders
-~~~~~~~~~~~~~~~~~
+Creating a Custom Password Encoder
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are many built-in password encoders. But if you need to create your
+own, it just needs to follow these rules:
+
+#. The class must implement :class:`Symfony\\Component\\Security\\Core\\Encoder\\PasswordEncoderInterface`;
+
+#. The first line in ``encodePassword`` and ``isPasswordValid`` must check
+   to make sure the password is not too long (e.g. 4096). This is for security
+   (see `CVE-2013-5750`_), and you can copy the `BasePasswordEncoder::checkPasswordLength`_
+   implementation from Symfony 2.4.
+
+Using Password Encoders
+~~~~~~~~~~~~~~~~~~~~~~~
 
 When the :method:`Symfony\\Component\\Security\\Core\\Encoder\\EncoderFactory::getEncoder`
 method of the password encoder factory is called with the user object as
@@ -213,3 +226,6 @@ which should be used to encode this user's password::
         $user->getPassword(),
         $password,
         $user->getSalt());
+
+.. _`CVE-2013-5750`: http://symfony.com/blog/cve-2013-5750-security-issue-in-fosuserbundle-login-form
+.. _`BasePasswordEncoder::checkPasswordLength`: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Security/Core/Encoder/BasePasswordEncoder.php
diff --git a/cookbook/doctrine/registration_form.rst b/cookbook/doctrine/registration_form.rst
@@ -45,6 +45,7 @@ You have a simple ``User`` entity mapped to the database::
         /**
          * @ORM\Column(type="string", length=255)
          * @Assert\NotBlank()
+         * @Assert\Length(max = 4096)
          */
         protected $plainPassword;
 
@@ -85,6 +86,21 @@ the class.
     to implement the :ref:`UserInterface<book-security-user-entity>` of the
     security component.
 
+.. _cookbook-registration-password-max:
+
+.. sidebar:: Why the 4096 Password Limit?
+
+    Notice that the ``plainPassword`` has a max length of ``4096`` characters.
+    For security purposes (`CVE-2013-5750`_), Symfony limits the plain password
+    length to 4096 characters when encoding it. Adding this constraint makes
+    sure that your form will give a validation error if anyone tries a super-long
+    password.
+
+    You'll need to add this constraint anywhere in your application where
+    your user submits a plaintext password (e.g. change password form). The
+    only place where you don't need to worry about this is your login form,
+    since Symfony's Security component handles this for you.
+
 Create a Form for the Model
 ---------------------------
 
@@ -346,3 +362,5 @@ That's it! Your form now validates, and allows you to save the ``User``
 object to the database. The extra ``terms`` checkbox on the ``Registration``
 model class is used during validation, but not actually used afterwards when
 saving the User to the database.
+
+.. _`CVE-2013-5750`: http://symfony.com/blog/cve-2013-5750-security-issue-in-fosuserbundle-login-form