Secure "Remember Me" functionality

security, cookie, remember


Secure "Remember Me"

This library implements the best practices for implementing a secure "Remember Me" functionality on web sites. Login information and unique secure tokens are stored in a cookie. If the user visits the site, the login information from the cookie is compared to information stored on the server. If the tokens match, the user is logged in. A user can have login cookies on several computers/browsers.

This library is heavily inspired by Barry Jaspan's article "Improved Persistent Login Cookie Best Practice". The library protects against the following attack scenarios:

  • The computer of a user is stolen or compromised, enabling the attacker to log in with the existing "Remember Me" cookie. The user knows this has happened. The user can remotely invalidate all login cookies.
  • An attacker has obtained the "Remember Me" cookie and has logged in with it. The user does not know this. The next time he tries to log in with the cookie that was stolen, he gets a warning and all login cookies are invalidated.
  • An attacker has obtained the database of login tokens from the server. The stored tokens are hashed so he can't use them without computational effort (rainbow tables or brute force).
  • An attacker tries to log in with brute force, by systematically generating "Remember Me" cookies. With the default security settings and 100 tries per second (a very high number which would probably show up in the server logs), it would take 8 months for a 50% chance to guess a cookie value right.


composer require birke/rememberme

Usage example

See the example directory for an example. You can run it on your local machine with the command

php -S -t example

To understand the basic application structure, have a look at index.php and the user_is_looged_in.php template.

The example uses the file system to store the tokens on the server side. In most cases it's better to swap the storage with the PDOStorage class.

Cookie configuration

By default the cookie is valid for one week and for all paths in the domain it was set. It cannot be accessed/changed via JavaScript and will be transmitted on HTTP connections. If your application requires a different configuration (for example, if you are using HTTPS and want to enhance security by only allowing transmission of the cookie over the secure connection), you can create your own PHPCookie instance:

$expire = strtotime("1 week", 0);
$cookie = new PHPCookie("REMEMBERME", $expire, "/", "", true, true);
$auth = new Authenticator($storage, null, $cookie);

Token security

This library uses the random_bytes function by default to generate a 16-byte token (a 32 char hexadecimal string). That should be sufficiently secure for most applications.

If you need more security, instantiate the Authenticator class with a custom token generator. The following example generates Base64-encoded tokens with 128 characters:

$tokenGenerator = new DefaultToken(94, DefaultToken::FORMAT_BASE64);
$auth = new Authenticator($storage, $tokenGenerator);

If you like even more control over the generation of your random tokens, have a look at the RandomLib. Rememberme has a RandomLibToken class that can use it.

Cleaning up expired tokens

The best way to clean expired tokens from your storage (file system or database) is to write a small script that initializes your token storage class and calls its cleanExpiredTokens method. Run this script regularly with a cron job or other worker method.

If you can't run the cleanup script regularly and have a low-traffic site, you can clean the storage on every page call by initializing the Authenticator class like this:

 $auth = new Authenticator($storage);

Updating from Version 1.x

The first you'll have to do is update the result checking of the Authenticator::login method. It no longer returns a boolean/the credentials, but instead returns a result object that must be queried for success, failure and credentials. See the example for how it is done.

If you did subclass Authenticator with a custom createToken method, you need to implement your token generation in a custom class that implements TokenInterface and pass it as a constructor argument.

The less secure pseudo-random tokens of the old version will be replaced by more secure tokens whenever a login occurs. For better security (and less convenience of your users) you could completely clear your token storage once after updating.