#21022 Switch to using bcrypt for hashing passwords

[johnbillion]

Latest approach to switching from phpass to bcrypt via the native PHP password hashing functions for the following features:

• User passwords
• Post passwords

The following features are switched from phpass to the cryptographically secure BLAKE2b hash via Sodium:

• Application passwords
• User password reset keys
• Personal data request keys
• The recovery mode key

Lots more info about the change can be found in the draft for the make/core announcement post.

Tickets

• Trac ticket: https://core.trac.wordpress.org/ticket/21022
• Trac ticket: https://core.trac.wordpress.org/ticket/50027
• Trac ticket: https://core.trac.wordpress.org/ticket/44628
• Related: https://core.trac.wordpress.org/ticket/39499

Notes

• wp_check_password() remains compatible with phpass hashes, so password checks will continue to succeed when checking a password against its existing hash. There is no need for users to change or resave their password.
• User passwords are rehashed using bcrypt after successful authentication in wp_authenticate_username_password() or wp_authenticate_email_password().
• wp_check_password() and the rehashing that occurs in the above functions are forward-compatible with future changes to the default bcrypt cost (the default bcrypt cost was increased in PHP 8.4), so password checks will continue to succeed when checking a password against its existing hash.
• wp_hash_password() and wp_check_password() retain support for a global $wp_hasher object which can be used to override the password hashing and checking mechanisms.

Elsewhere

• Laravel uses bcrypt
• Drupal uses bcrypt
• Symfony uses bcrypt
• Joomla uses bcrypt
• Laminas uses bcrypt
• Ghost uses bcrypt
• The Bedrock ecosystem uses bcrypt
• OWASP recommends bcrypt when scrypt and Argon2id aren't available

FAQs

What about salting?

Salting a bcrypt hash is handled by password_hash() and password_verify(). There is no need to implement salting in userland code.

What about peppering?

Peppering effectively eliminates the ability to crack a password given only its hash by introducing a secret which needs to be stored outside of the database and is used as part of the value that's hashed, as long as the pepper remains secret. This is compelling, however the portability of a password hash is also eliminated and if the secret pepper is lost, changed, or differs between environments, then all password hashes are invalidated. All users will need to reset their passwords in order to log in, and all application passwords and post passwords will need to be changed.

While a secret pepper does prevent an attacker from being able to crack a password hash if they gain access only to the database, the potential usability tradeoff is high. In addition, the intention of switching to bcrypt is to switch to a password hashing algorithm that is highly resistant to cracking in the first place, thus reducing the benefit gained from peppering.

What about layering hashes to immediately protect legacy hashes?

Hash layering is the process of taking an existing password hash (for example one hashed with phpass) and applying the new hashing algorithm on top of it (bcrypt in this case). The intention is to immediately protect stored passwords with the new hashing algorithm instead of waiting for users to log in or change their passwords in order to rehash the password.

A concern is the length of time it could take to rehash all passwords in the database during the upgrade routine. Handling would need to be implemented to cover usage of passwords (for example logging in or changing passwords) while the password upgrade routine runs.

Additional risks include null byte characters present in the raw output of other hashing algorithms, and password shucking. OWASP warns that layering hashes can make the password easier to crack.

None of this is insurmountable, but it adds complexity for what is in most cases a short term benefit. For this reason, hash layering has not been implemented.

What about the 72 byte limit of bcrypt?

This is addressed by pre-hashing the password with SHA-384 and base64 encoding it prior to hashing with bcrypt. There is discussion about this in the comments here and on #21022.

Supporting references:

• This separate PR concluded the decision
• OWASP recommends pre-hashing with SHA-384 when using bcrypt
  • This was recently changed as a result of this very discussion
• Paragon Initiative Enterprises recommends pre-hashing with SHA-384 when using bcrypt
• Dropbox uses SHA-512 pre-hashing with bcrypt
• Password Lock by Paragon Initiative Enterprises uses SHA-384 pre-hashing with bcrypt
• Symfony uses SHA-512 pre-hashing, but only for passwords greater than 72 bytes

What about DoS attacks from long passwords?

The bcrypt implementation in PHP is not susceptible to a DoS attack from long passwords because only the first 72 bytes of the password value are read, therefore there is no need to guard against a long password value prior to it being hashed or checked.

While phpass can be susceptible to a DoS attack via a very long password value, the phpass implementation in WordPress protects against this via a 4096 byte limit when hashing a password and when checking a password. This is unrelated to the password length limit discussed above.

What about the cost factor?

The default cost factor will be used. This is 10 in PHP up to 8.3 and has been increased to 12 in PHP 8.4. Hashes remain portable between installations of PHP that use different cost factors because the cost factor is encoded in the hash output.

It's beyond the scope of WordPress to make adjustments to the cost factor used by bcrypt. If you are planning on updating to PHP 8.4 then you should consider whether the default cost is appropriate for the resources available on your server. The wp_hash_password_options filter is available to change the cost factor should it be needed.

What about using PASSWORD_DEFAULT instead of PASSWORD_BCRYPT?

The intention of the PASSWORD_DEFAULT constant in PHP is to take advantage of future changes to the default algorithm that's used to hash passwords. There are currently no public plans to change this algorithm (at least, I haven't found any), but for safety it makes sense to be explicit about the use of bcrypt in WordPress. This can easily be changed in the future.

What about Argon2?

If your server supports the argon2id algorithm, switching to it is now a one-liner:

add_filter( 'wp_hash_password_algorithm', fn() => PASSWORD_ARGON2ID );

Unfortunately it's not possible to rely on argon2 being available because it requires both libargon2 to be available on the server and for PHP to be built with argon2 support enabled. Using argon2 via sodium_compat still requires the optional libsodium extension to be installed. Conditionally using argon2i, argon2id, or bcrypt depending on what is available on the server would increase complexity and limit the portability of hashes.

What about scrypt?

There is no native support for scrypt in PHP, and using scrypt via sodium_compat still requires the optional libsodium extension to be installed

Is this change compatible with existing plugins that implement bcrypt hashing?

Yes. If you've used such a plugin to hash passwords with bcrypt then those hashes are compatible with this bcrypt implementation and you should be able to remove the plugin.

What effect will this have on my database when a user logs in?

The first time that each user subsequently logs in after this change is deployed to your site, their password will be rehashed using bcrypt and the value stored in the database. This will result in an additional UPDATE query to update their user_pass field in the users table.

The query will look something like this:

UPDATE wp_users
SET user_pass = '<hash>', user_activation_key = ''
WHERE ID = <id>

This query performs an UPDATE but makes use of the primary key to target the row that needs updating, therefore it should remain very performant. The query only runs once for each user. When they subsequently log in again at a later date their password will not need to be rehashed again.

Note that when a user logs in, WordPress already writes to the database via an UPDATE query on the usermeta table to store their updated user session information in the session_tokens meta field. This happens every time a user logs in.

How do I use an algorithm other than bcrypt on my website?

If your server supports the argon2id algorithm, switching to it is now a one-liner:

add_filter( 'wp_hash_password_algorithm', fn() => PASSWORD_ARGON2ID );

If necessary, the password_algos() function can be used to check for argon2id support.

Alternatively, the wp_hash_password(), wp_check_password(), and wp_password_needs_rehash() functions are all pluggable. See wp-password-bcrypt as an example of overwriting them in a plugin.

Alternatively again, if you need to temporarily or permanently stick with phpass for user passwords and post passwords then you can instantiate the $wp_hasher global to restore the previous behaviour. This will not affect application passwords or security keys.

add_action(
  'plugins_loaded',
  function(): void {
    global $wp_hasher;
    require_once ABSPATH . WPINC . '/class-phpass.php';
    $wp_hasher = new PasswordHash( 8, true );
  }
);

Why switch to BLAKE2b for application passwords and security keys?

Switching from phpass to the cryptographically secure but fast BLAKE2b algorithm via Sodium is safe for application passwords and security keys which are randomly generated with sufficiently high entropy. Security keys and application passwords are all randomly generated with high entropy via wp_generate_password() from an alpha-numeric character set of size 62. BLAKE2b is highly resistant to preimage attacks (being able to reverse a hash to determine its input) while having a low computational cost.

Feature	Chars	Entropy (b)	Expires	Storage	Field type
Application passwords	24	142	Never	usermeta.meta_value	longtext
Recovery mode key	22	130	1 week	options.option_value	longtext
Password reset keys	20	119	1 day	users.user_activation_key	varchar(255)
User request keys	20	119	1 day	posts.post_password	varchar(255)

These hashes are generated via the new wp_fast_hash() function which in turn uses sodium_crypto_generichash() to generate the hash.

Existing hashes generated prior to WordPress x.y will remain valid as they are checked via the new wp_verify_fast_hash() function which includes fallback support for phpass portable hashes.

Is the Sodium extension now required?

No. The Sodium functions that are used are included in the sodium_compat library which is loaded when the libsodium extension isn't installed. This library has been bundled with WordPress since version 5.2.

Todo

• Figure out why the performance tests are failing
• Triple check that 1227f17 doesn't cause users to get logged out the first time they change their password after the switch from phpass to bcrypt
• The use of hash_hmac( 'sha382' ) means that the hash extension is required on PHP 7.2 and 7.3 because the hash_hmac() compat function in compat.php only supports md5 and sha1.
• Make the decision on SHA-256 or BLAKE2d via Sodium for security key HMACs
• Make a decision on the increased DoS impact on XML-RPC that results from switching from phpass to bcrypt for user passwords
  • See https://core.trac.wordpress.org/ticket/62789
• Make a decision on the 72 byte limit imposed by bcrypt
• Decide whether the options array for password_hash() should be filterable.
• Decide whether wp_hash_password() and wp_check_password() need to retain back-compat support for the global $wp_hasher
• Fully support changes to the default bcrypt cost
• Tests for the hash upgrade routine for a user password
• Tests for the hash upgrade routine for an application password
• Tests for handling post password hashes
• Address md5 hashing of passwords in repair.php

Testing

There's good test coverage for this change. Here are some manual testing steps that can be taken.

Remaining logged in after the update

• Start on the trunk branch
• Log in to WordPress
• Switch to this branch
• Ensure you have remained logged in
• Log out and back in again
• Confirm it works as expected
• Confirm that the user_pass field for your user account in the wp_users table in the database has been updated -- it should be prefixed with $wp$2y$ instead of $P$

Post passwords

• Start on the trunk branch
• Publish a post and set a password for it
• View the post and enter the password to view it
• Switch to this branch
• Confirm you can still see the post without having to re-enter its password
• Edit the post and change its password
• View the post and confirm that you can re-enter the new password and view it

Password resets

• Start with the "Lost your password?" link on the login screen
• Click the confirmation link sent to your email
• Follow the process of resetting your user password
• Confirm you can log in with your new password

Personal data requests

• Start on the trunk branch
• Log in to WordPress as an Administrator
• Initiate a data export from Tools -> Export Personal Data
• Switch to this branch
• Click the confirmation link sent to the email address and confirm that it still works
• Initiate another data export from Tools -> Export Personal Data
• Click the confirmation link sent to the email address and confirm that it works

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