Why password_needs_rehash May Inaccurately Determine Password Hash Updates: Common Configuration Pitfalls and Fixes

gitbox 2025-08-05

1. How password_needs_rehash Works

The password_needs_rehash function checks whether the given password hash needs to be updated based on the specified algorithm, cost, and other configuration options. Its function signature is as follows:

<span><span><span class="hljs-keyword">bool</span></span><span> </span><span><span class="hljs-title function_ invoke__">password_needs_rehash</span></span><span> ( </span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$hash</span></span><span> , </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$algo</span></span><span> , </span><span><span class="hljs-keyword">array</span></span><span> </span><span><span class="hljs-variable">$options</span></span><span> )
</span></span>

$hash: The password hash to be checked.
$algo: The hashing algorithm used (e.g., PASSWORD_DEFAULT or PASSWORD_BCRYPT).
$options: A set of configuration options, typically including the hashing cost.

This function determines whether the current password hash meets the configuration criteria. If not, it returns true, indicating the hash should be updated.

2. Common Configuration Issues

(1) PASSWORD_DEFAULT May Change Over Time

PASSWORD_DEFAULT is PHP's default hashing algorithm, which currently uses bcrypt, but may switch to a different algorithm in future versions. If your PHP version is upgraded or the underlying cryptographic library changes, PASSWORD_DEFAULT may begin using a new algorithm.

For example, assume you're using PASSWORD_DEFAULT to hash and store passwords in PHP 5.6. Upon upgrading to PHP 7.x, PASSWORD_DEFAULT may switch from bcrypt to something else (e.g., argon2). Then, when using password_needs_rehash to check hashes, the following could happen:

The stored password uses the old hashing algorithm.
The system expects a new algorithm (like argon2), but password_needs_rehash fails to detect the mismatch.

Solution

To avoid this, explicitly define the hashing algorithm rather than relying on PASSWORD_DEFAULT—especially in production. This ensures consistency even if PHP changes the default algorithm in future versions.

<span><span><span class="hljs-variable">$hash</span></span><span> = </span><span><span class="hljs-title function_ invoke__">password_hash</span></span><span>(</span><span><span class="hljs-variable">$password</span></span><span>, PASSWORD_BCRYPT);
</span></span>

(2) Changes in Cost Factor

password_needs_rehash also checks the cost factor (e.g., for bcrypt). If you change the cost factor in your system (e.g., from 10 to 12), but the stored hash uses the old cost, password_needs_rehash may fail to recognize that the hash needs to be updated.

Solution

Always generate password hashes using the current cost factor. To avoid outdated hashes, consider checking and updating the hash during user logins.

<span><span><span class="hljs-variable">$options</span></span><span> = [</span><span><span class="hljs-string">&#039;cost&#039;</span></span><span> =&gt; </span><span><span class="hljs-number">12</span></span><span>];  </span><span><span class="hljs-comment">// Use an appropriate cost</span></span><span>
</span><span><span class="hljs-variable">$hash</span></span><span> = </span><span><span class="hljs-title function_ invoke__">password_hash</span></span><span>(</span><span><span class="hljs-variable">$password</span></span><span>, PASSWORD_BCRYPT, </span><span><span class="hljs-variable">$options</span></span><span>);
</span></span>

(3) Incorrect Usage of password_needs_rehash

password_needs_rehash requires three parameters: the hash, the algorithm, and the configuration options. Incorrectly passing any of these may cause the function to misbehave. Common issues include:

Failing to correctly pass the $options array, resulting in missing cost or other configuration settings.
Passing a hash that doesn’t match the algorithm used to generate it.

Solution

Ensure that all parameters passed to password_needs_rehash are correct. Here's an example:

<span><span><span class="hljs-variable">$hash</span></span><span> = </span><span><span class="hljs-title function_ invoke__">password_hash</span></span><span>(</span><span><span class="hljs-variable">$password</span></span><span>, PASSWORD_BCRYPT, [</span><span><span class="hljs-string">&#039;cost&#039;</span></span><span> =&gt; </span><span><span class="hljs-number">12</span></span><span>]);
<p></span>if (password_needs_rehash($hash, PASSWORD_BCRYPT, ['cost' => 12])) {<br>
$hash = password_hash($password, PASSWORD_BCRYPT, ['cost' => 12]);<br>
}<br>
</span>

Ensure that $hash matches the chosen algorithm and configuration.

3. Additional Considerations

(1) Regularly Update Password Hashes

Hashing algorithms and configurations evolve and may become outdated. In addition to using password_needs_rehash, it’s a good practice to periodically rehash passwords. This can be enforced through scheduled password changes or rechecking during login events.

(2) Compatibility Issues

Sometimes, you might need to support multiple PHP versions or environments. In such cases, the behavior of password_needs_rehash may vary. Always ensure compatibility with the PHP versions your application supports and make necessary adjustments accordingly.

4. Conclusion

password_needs_rehash is a powerful tool for checking if a password hash meets current configuration requirements. However, configuration issues—such as algorithm changes, cost factor updates, or incorrect parameter usage—can lead to inaccurate results. Solutions include explicitly setting the algorithm and cost, consistently applying the latest configuration, and regularly reviewing your password policies.

By understanding these common issues and how to resolve them, you can better protect your users' passwords and mitigate potential security risks.

password_needs_rehash