Web & mobile sessions for akka-http
akka-http is a great toolkit for building backends for single-page or mobile applications. In almost all apps there
is a need to maintain user sessions, make sure session data is secure and cannot be tampered with.
akka-http-session provides directives for client-side session management in web and mobile applications, using cookies
or custom headers + local storage, with optional Json Web Tokens format support.
A comprehensive FAQ is available, along with code examples (in Java, but easy to translate to Scala) which answers many common questions on how sessions work, how to secure them and implement using akka-http.
What is a session?
Session data typically contains at least the
username of the logged in user. This id must be secured so that a
session cannot be "stolen" or forged easily.
Sessions can be stored on the server, either in-memory or in a database, with the session
id sent to the client,
or entirely on the client in a serialized format. The former approach requires sticky sessions or additional shared
storage, while using the latter (which is supported by this library) sessions can be easily deserialized on any server.
A session is a string token which is sent to the client and should be sent back to the server on every request.
To prevent forging, serialized session data is signed using a server secret. The signature is appended to the session data that is sent to the client, and verified when the session token is received back.
- type-safe client-side sessions
- sessions can be encrypted
- sessions contain an expiry date
- cookie or custom header transport
- support for JWT
- refresh token support (e.g. to implement "remember me")
- CSRF tokens support
- Java & Scala APIs
SessionManager & configuration
All directives require an (implicit for scala) instance of a
SessionManager<T>), which can be created by providing a server
secret (via a
SessionConfig). The secret should be a long, random string unique to each environment your app is
running in. You can generate one with
SessionUtil.randomServerSecret(). Note that when you change the secret,
all sessions will become invalid.
SessionConfig instance can be created using Typesafe config.
The only value that you need to provide is
application.conf (then you can safely call
SessionConfig.fromConfig) or by using
You can customize any of the default config options
either by modifying them through
application.conf or by modifying the
SessionConfig case class. If a value has
Option, you can set it to
None by using a
none value in the config file (for both java and scala).
When using cookies, by default the
secure attribute of cookies is not set (for development), however it is
recommended that all sites use
https and all cookies have this attribute set.
All session-related directives take at least two parameters:
- session continuity:
refreshable; specifies what should happen when the session expires. If
refreshableand a refresh token is present, the session will be re-created. See below for details.
- session transport:
Typically, you would create aliases for the session-related directives which use the right parameters basing on the current request and logic specific to your application.
Cookies vs header
Session data can be sent to the client using cookies or custom headers. The first approach is the simplest to use, as cookies are automatically sent to the server on each request.
However, cookies have some security vulnerabilities, and are typically not used in mobile applications. For these scenarios, session data can be transported using custom headers (the names of the headers are configurable in the config).
When using headers, you need to store the session (and, if used, refresh-) tokens yourself. These tokens can be stored in-memory, or persistently e.g. using the browser's local storage.
You can dynamically decide which transport to use, basing e.g. on the user-agent or other request properties.
Sessions are typed. The
T type parameter in
SessionManager<T>) determines what data is stored in the session.
Basic types like
Map[String, String] (
Map<String, String>) are supported out-of-the box.
Support for other types can be added by providing a (an implicit for scala)
SessionSerializer[T, String] (
SessionSerializer<T, String>). For case classes, it's most
convenient to use a
MultiValueSessionSerializer[T] or (
MultiValueSessionSerializer<T>) which should convert the instance into a
String -> String map
(nested types are not supported on purpose, as session data should be small & simple). Examples of
usage can be found here for scala and here for java.
The basic directives enable you to set, read and invalidate the session. To create a new client-side session (create
and set a new session cookie), you need to use the
setSession directive. See how it's done in java and scala.
Note that when using cookies, their size is limited to 4KB, so you shouldn't put too much data in there (the signature takes about 50 characters).
Encrypting the session
It is possible to encrypt the session data by modifying the
akka.http.session.encrypt-data config option. When
sessions are encrypted, it's not possible to read their content on the client side.
The key used for encrypting will be calculated basing on the server secret.
By default, sessions expire after a week. This can be disabled or changed with the
Note that when using cookies, even though the cookie sent will be a session cookie, it is possible that the client will have the browser open for a very long time, uses Chrome or FF, or if an attacker steals the cookie, it can be re-used. Hence having an expiry date for sessions is highly recommended.
JWT: encoding sessions
By default, sessions are encoded into a string using a custom format, where expiry/data/signature parts are separated using
-, and data fields are separated using
= and url-encoded.
When using JWT, you need to provide a serializer which serializes session data to a
JValue instead of a
A number of serializers for the basic types are present in
JValueSessionSerializer, as well as a generic serializer for case classes (used above).
You may also find it helpful to include the json4s-ext library which provides serializers for common Java types such as
org.joda.time._ and Java enumerations.
There are many tools available to read JWT session data using various platforms, e.g. for Angular.
It is also possible to customize the session data content generated by overriding appropriate methods in
JwtSessionEncoder (e.g. provide additional claims in the payload).
CSRF protection (cookie transport only)
CSRF is a kind of an attack where an attacker issues a
POST request on behalf of a user, if the user e.g.
clicks on a specially constructed link. See the OWASP page
or the Play! docs for a thorough introduction.
- assumes that
GETrequests are non-mutating (have no side effects)
- uses double-submit cookies to verify requests
- requires the token to be set in a custom header or (optionally) in a form field
- generates a new token on the first
GETrequest that doesn't have the token cookie set
Note that if the token is passed in a form field, the website isn't protected by HTTPS or you don't control all subdomains, this scheme can be broken. Currently, setting a custom header seems to be a secure solution, and is what a number of projects do (that's why, when using custom headers to send session data, no additional protection is needed).
It is recommended to generate a new CSRF token after logging in, see this SO question.
A new token can be generated using the
By default the name of the CSRF cookie and the custom header matches what AngularJS expects and sets. These can be customized in the config.
Refresh tokens (a.k.a "remember me")
If you'd like to implement persistent, "remember-me" sessions, you should use
refreshable instead of
sessions. This is especially useful in mobile applications, where you log in once, and the session is remembered for
a long time. Make sure to adjust the
akka.http.session.refresh-token.max-age config option appropriately
(defaults to 1 month)!
You can dynamically decide, basing on the request properties (e.g. a query parameter), if a session should be
refreshable or not. Just pass the right parameter to
When using refreshable sessions, in addition to an (implicit)
SessionManager instance, you need to provide an
implementation of the
RefreshTokenStorage trait. This trait has methods to lookup, store and delete refresh tokens.
Typically it would use some persistent storage.
The tokens are never stored directly, instead only token hashes are passed to the storage. That way even if the token database is leaked, it won't be possible to forge sessions using the hashes. Moreover, in addition to the token hash, a selector value is stored. That value is used to lookup stored hashes; tokens are compared using a special constant-time comparison method, to prevent timing attacks.
When a session expires or is not present, but the refresh token is (sent from the client using either a cookie,
or a custom header), a new session will be created (using the
and a new refresh token will be created.
Note that you can differentiate between sessions created from refresh tokens and from regular authentication by storing appropriate information in the session data. That way, you can force the user to re-authenticate if the session was created by a refresh token before crucial operations.
It is of course possible to read
requiredSession(refreshable, ...). If a session was created
refreshable has no additional effect.
The semantics of
touch[Required|Optional]Session() are a bit subtle. You can still use expiring client
sessions when using refresh tokens. You will then have 2 stages of expiration: expiration of the client session
(should be shorter), and expiry of the refresh token. That way you can have strongly-authenticated sessions
which expire fast, and weaker-authenticated re-creatable sessions (as described in the paragraph above).
When touching an existing session, the refresh token will not be re-generated and extended, only the session cookie.
- Bootzooka, a web application template project using
- Spray session, similar project for spray.io
- Spray SPA, a single-page-application demo built using spray.io, also containing an implementation of client-side sessions
- Play framework, a full web framework, from which parts of the session encoding/decoding code was taken
- Rails security guide, a description of how sessions are stored in Rails
- Akka-Http issue 114 for implementing similar functionality straight in Akka-Http
- Implementing remember me
- The definitive guide to form-based website authorization
- The Anatomy of a JSON Web Token
- Cookies vs tokens
Using from SBT
libraryDependencies += "com.softwaremill.akka-http-session" %% "core" % "0.5.5" libraryDependencies += "com.softwaremill.akka-http-session" %% "jwt" % "0.5.5" // optional
Certain releases changed the client token encoding/serialization. In those cases, it's important to enable the appropriate token migrations, otherwise existing client sessions will be invalid (and your users will be logged out).
When updating from a version before 0.5.3, set
akka.http.session.token-migration.v0-5-3.enabled = true.
When updating from a version before 0.5.2, set
akka.http.session.token-migration.v0-5-2.enabled = true.
Note that when updating through multiple releases, be sure to enable all the appropriate migrations.
For versions prior to 0.5.0, no migration path is provided. However, you can implement your own encoders/serializers to support migrating from whatever version you are using.
Since token changes may be security related, migrations should be enabled for the shortest period of time after which the vast majority of client tokens have been migrated.