Harden the security where hash values are compared

Problem/Motivation

Currently the equality operator is used throughout Drupal core when hash values are compared. As pointed out elsewhere this can result in false positives under certain circumstances. This can be the case when a hashing operation results in values which are interpreted as a number by PHP (type juggling).

The following grep turns up all relevant hash/token comparisons in the code base excluding vendor directory and clear false positives (for example from the text token replacement system, aka token module):

$ find . -type f -and -not -name '*.js' -and -not -name '*.tokens.inc' -and -not -name 'token.api.php' -and -not -name PoHeader.php -and -not -path '*/vendor/*' | xargs git grep '[^=!]==[^=]' | grep -E 'token|hash' | cat -n
     1	core/lib/Drupal/Core/Password/PhpassHashedPassword.php:    return ($hash && $stored_hash == $hash);
     2	core/modules/toolbar/src/Controller/ToolbarController.php:    return AccessResult::allowedIf($this->currentUser()->hasPermission('access toolbar') && $hash == _toolbar_get_subtrees_hash($langcode))->cachePerPermissions();
     3	core/modules/user/src/AccountForm.php:        $user_pass_reset = $pass_reset = isset($_SESSION['pass_reset_' . $account->id()]) && (\Drupal::request()->query->get('pass-reset-token') == $_SESSION['pass_reset_' . $account->id()]);
     4	core/modules/user/src/Controller/UserController.php:      if ($timestamp <= $current && $current - $timestamp < $timeout && $user->id() && $timestamp >= $user->getLastLoginTime() && $hashed_pass == user_pass_rehash($user->getPassword(), $timestamp, $user->getLastLoginTime(), $user->id())) {

Proposed resolution

• Introduce a new static method \Drupal\Component\Utility\Crypt::hashEquals() wrapping around the constant time hash_equals() function.
• Replace all relevant hash/token comparisons with the new Crypt::hashEquals() method.

Remaining tasks

• Review

User interface changes

None.

API changes

API addition (Crypt::hashEquals()).

Beta phase evaluation

Original report

As of PHP 5.6, the function hash_equals() was added to help prevent timing attack security issues.

All this does is provide a string comparison for the use inside of php.

I have made a patch that adds forward compatibility by checking the php version constants.

see: http://us3.php.net/manual/en/function.hash-equals.php
see: http://us3.php.net/manual/en/migration56.new-features.php

<?php
  // use more secure hash_equal() that was introduced in php 5.6 and greater.
  // see: http://php.net/manual/en/function.hash-equals.php.
  if (PHP_MAJOR_VERSION > 4 && PHP_MINOR_VERSION > 5) {
    return ($hash && hash_equals($stored_hash, $hash));
  }
?>

I am checking the php constants instead of using a function_exists() to prevent the possibility of some attacker implementing their own hash_equals() function for php versions < 5.6.