[[servlet-rememberme]]
= Remember-Me Authentication

[[remember-me-overview]]
Remember-me or persistent-login authentication refers to web sites being able to remember the identity of a principal between sessions.
This is typically accomplished by sending a cookie to the browser, with the cookie being detected during future sessions and causing automated login to take place.
Spring Security provides the necessary hooks for these operations to take place and has two concrete remember-me implementations.
One uses hashing to preserve the security of cookie-based tokens and the other uses a database or other persistent storage mechanism to store the generated tokens.

Note that both implementations require a `UserDetailsService`.
If you use an authentication provider that does not use a `UserDetailsService` (for example, the LDAP provider), it does not work unless you also have a `UserDetailsService` bean in your application context.


[[remember-me-hash-token]]
== Simple Hash-Based Token Approach
This approach uses hashing to achieve a useful remember-me strategy.
In essence, a cookie is sent to the browser upon successful interactive authentication, with the cookie being composed as follows:

[source,txt]
----
base64(username + ":" + expirationTime + ":" + algorithmName + ":"
algorithmHex(username + ":" + expirationTime + ":" password + ":" + key))

username:          As identifiable to the UserDetailsService
password:          That matches the one in the retrieved UserDetails
expirationTime:    The date and time when the remember-me token expires, expressed in milliseconds
key:               A private key to prevent modification of the remember-me token
algorithmName:     The algorithm used to generate and to verify the remember-me token signature
----

The remember-me token is valid only for the period specified and only if the username, password, and key do not change.
Notably, this has a potential security issue, in that a captured remember-me token is usable from any user agent until such time as the token expires.
This is the same issue as with digest authentication.
If a principal is aware that a token has been captured, they can easily change their password and immediately invalidate all remember-me tokens on issue.
If more significant security is needed, you should use the approach described in the next section.
Alternatively, remember-me services should not be used at all.

If you are familiar with the topics discussed in the chapter on xref:servlet/configuration/xml-namespace.adoc#ns-config[namespace configuration], you can enable remember-me authentication by adding the `<remember-me>` element:

[source,xml]
----
<http>
...
<remember-me key="myAppKey"/>
</http>
----

The `UserDetailsService` is normally selected automatically.
If you have more than one in your application context, you need to specify which one should be used with the `user-service-ref` attribute, where the value is the name of your `UserDetailsService` bean.

[[remember-me-persistent-token]]
== Persistent Token Approach
This approach is based on the article https://web.archive.org/web/20180819014446/http://jaspan.com/improved_persistent_login_cookie_best_practice[Improved Persistent Login Cookie Best Practice] with some minor modifications  footnote:[Essentially, the username is not included in the cookie, to prevent exposing a valid login name unnecessarily.
There is a discussion on this in the comments section of this article.].
To use this approach with namespace configuration, you would supply a datasource reference:

[source,xml]
----
<http>
...
<remember-me data-source-ref="someDataSource"/>
</http>
----

The database should contain a `persistent_logins` table, created by using the following SQL (or equivalent):

[source,ddl]
----
create table persistent_logins (username varchar(64) not null,
								series varchar(64) primary key,
								token varchar(64) not null,
								last_used timestamp not null)
----

[[remember-me-impls]]
== Remember-Me Interfaces and Implementations
Remember-me is used with `UsernamePasswordAuthenticationFilter` and is implemented through hooks in the `AbstractAuthenticationProcessingFilter` superclass.
It is also used within `BasicAuthenticationFilter`.
The hooks invoke a concrete `RememberMeServices` at the appropriate times.
The following listing shows the interface:

[source,java]
----
Authentication autoLogin(HttpServletRequest request, HttpServletResponse response);

void loginFail(HttpServletRequest request, HttpServletResponse response);

void loginSuccess(HttpServletRequest request, HttpServletResponse response,
	Authentication successfulAuthentication);
----

See the Javadoc for javadoc:org.springframework.security.web.authentication.RememberMeServices[] for a fuller discussion on what the methods do, although note that, at this stage, `AbstractAuthenticationProcessingFilter` calls only the `loginFail()` and `loginSuccess()` methods.
The `autoLogin()` method is called by `RememberMeAuthenticationFilter` whenever the `SecurityContextHolder` does not contain an `Authentication`.
This interface, therefore, provides the underlying remember-me implementation with sufficient notification of authentication-related events and delegates to the implementation whenever a candidate web request might contain a cookie and wish to be remembered.
This design allows any number of remember-me implementation strategies.

We have seen earlier that Spring Security provides two implementations.
We look at each of these in turn.

[[token-based-remember-me-services]]
=== TokenBasedRememberMeServices
This implementation supports the simpler approach described in <<remember-me-hash-token>>.
`TokenBasedRememberMeServices` generates a `RememberMeAuthenticationToken`, which is processed by `RememberMeAuthenticationProvider`.
A `key` is shared between this authentication provider and the `TokenBasedRememberMeServices`.
In addition, `TokenBasedRememberMeServices` requires a `UserDetailsService`, from which it can retrieve the username and password for signature comparison purposes and generate the `RememberMeAuthenticationToken` to contain the correct `GrantedAuthority` instances.
`TokenBasedRememberMeServices` also implements Spring Security's `LogoutHandler` interface so that it can be used with `LogoutFilter` to have the cookie cleared automatically.

By default, this implementation uses the SHA-256 algorithm to encode the token signature.
To verify the token signature, the algorithm retrieved from `algorithmName` is parsed and used.
If no `algorithmName` is present, the default matching algorithm will be used, which is SHA-256.
You can specify different algorithms for signature encoding and for signature matching, this allows users to safely upgrade to a different encoding algorithm while still able to verify old ones if there is no `algorithmName` present.
To do that you can specify your customized `TokenBasedRememberMeServices` as a Bean and use it in the configuration.

include-code::./CustomAlgorithmRememberMeServicesConfiguration[tag=snippet,indent=0]

The following beans are required in an application context to enable remember-me services:

include-code::./DefaultAlgorithmRememberMeServicesConfiguration[tag=snippet,indent=0]

Remember to add your `RememberMeServices` implementation to your `UsernamePasswordAuthenticationFilter.setRememberMeServices()` property, include the `RememberMeAuthenticationProvider` in your `AuthenticationManager.setProviders()` list, and add `RememberMeAuthenticationFilter` into your `FilterChainProxy` (typically immediately after your `UsernamePasswordAuthenticationFilter`).


=== PersistentTokenBasedRememberMeServices
You can use this class in the same way as `TokenBasedRememberMeServices`, but it additionally needs to be configured with a `PersistentTokenRepository` to store the tokens.

* `InMemoryTokenRepositoryImpl` which is intended for testing only.
* `JdbcTokenRepositoryImpl` which stores the tokens in a database.

See <<remember-me-persistent-token>> for the database schema.
