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Secure, modular server-side sessions.

This repo contains many packages that together implement traditional server-side sessions. Users who don't have a session yet are assigned a random 144-bit session ID that is the key on a storage backend. All session data is saved on a storage backend. Includes 110+ unit and property tests.

The serversession package (Hackage) implements the core logic. It needs to be paired up with two companion packages:

  • Backend (storage), in order to store the session data. Currently we support:

    • serversession-backend-persistent (Hackage): Storage backend using persistent's SQL backend. Works with PostgreSQL, MySQL, SQLite.
    • serversession-backend-acid-state (Hackage): Storage backend using acid-state. This backend keeps sessions in memory but provides ACID guarantees using a transaction log. It can also be used without durability as a memory-only backend.
    • serversession-backend-redis (Hackage): Storage backend using Redis via the hedis package.
  • Frontend, bindings for your web framework of choice. Currently we support:

    • serversession-frontend-yesod (Hackage): Support for the Yesod framework. Replaces the default clientsession.
    • serversession-frontend-snap (Hackage): Support for the Snap framework. Replaces the default Snap.Snaplet.Session.Backends.CookieSession based on clientsession.
    • serversession-frontend-wai (Hackage): Generic support for WAI applications via the wai-session package by using a vault.

If your favorite storage backend or framework is not listed above, please send us a pull request! The serversession package should work for any session that may be represented as a mapping of keys to values.


  • Using Yesod frontend + Persistent backend: GitHub link.

Security notes

The session ID is generated via the nonce package, which in turn uses a CPRNG created from AES on CTR mode. The CPRNG is reseed automatically from /dev/urandom (or equivalent) periodically. We use the base64url variant, thus providing 144 bits of entropy, which is more than enough to make guessing session IDs impossible.

The session ID stays fixed most of the time. Anonymous users receive session IDs unless their session remains empty (as an optimization). The session ID can be invalidated in order to prevent session fixation attacks, either automatically (see below) or manually (via forceInvalidate). On a session fixation attack, the attacker convinces the victim to use the same session ID as his and asks the victim to log in. If the session is not invalidated upon login, the attacker will now be in possession of a session ID that is logged in as the victim. If the session is invalidated, the victim receives a new session ID that the attacker doesn't have any knowledge of.

We support both idle timeouts and absolute timeouts. Idle timeouts invalidate the session if a given amount of time has passed since the last request was made for a session. Absolute timeouts invalidate the session if a given amount of time has passed since the session was created, no matter the activity.

Authentication integration

We have special support for authentication plugins that save information about the logged in user on a session variable:

  • The session key used by authentication plugin (e.g., _ID for yesod-auth) is recognized and saved separately on the database. This allows you to quickly identify all sessions of a given user. For example, you're able to implement a "log out everywhere" button.

  • Whenever the logged in user changes, the backend will also invalidate the current session ID and migrate the session data to a new ID. This prevents session fixation attacks while still allowing you to maintain session state accross login/logout boundaries.

Any authentication mechanism is supported as long as it uses a session variable.

Storage optimizations

We provide the following storage optimizations:

  • Empty sessions are not saved. This is done transparently: just insert a session variable and the session will materialize. Note that if your framework always creates a CSRF token (e.g., Snap), then this optimization will not apply you.

  • You can set the timeout resolution. Requests made within the timeout resolution that do not change any session variables will not update the session on the database. By default the timeout resolution is set to 10 minutes.

Session data types

The serversession family is generalized with respect to the session data type. We provide SessionMap, a newtype of Map Text ByteString, that is guaranteed to work with every backend and frontend. However, you're able to create a custom data type and use it as well, as long as you implement the typeclasses needed by your backend and frontend.

The storage backends support basically anything they're able to serialize. The persistent backend accepts anything that can be used as a persistent field. The acid-state backend accepts anything that can be serialized via SafeCopy. The redis backend accepts anything that can be stored as a Redis hash.

The frontends unfortunately are quite more limited. Yesod insists that sessions be Maps of Text to ByteString, while Snap insists on Maps of Text to Text. The wai-session package allows any types of keys and values, but still expect a map. Until the frameworks get generalized as well, it's a bit difficult to take advantage of custom session data types.

Current limitations

These limitations may be addressed in the future. Right now, though, please bear in mind that:

  • There's no support for cleaning old sessions from the storage backends.

  • There's no way of setting timeouts and cookie persistence on a per-session basis, only on a global basis.

  • The Redis backend is not set to expire sessions yet.

  • The persistent backend does not automatically create an index for the auth ID. Thus, by default the deleteAllSessionsOfAuthId operation will take linear time.

  • We stress test our backends to ensure they support almost every conceivable use case for sessions, but we only guarantee to support:

    • Every possible character and byte for keys and values, respectively.
    • At least one million keys per session.
    • Session values of at least 100 MiB.
    • Session keys of At least 1 MiB.
    • At least 200 independent sessions per user.


Yesod has always support client-side sessions via the clientsession package: the session data is encrypted, signed, encoded and sent to the client inside a cookie. When receiving a request, the cookie is decoded, verified and decrypted. The server does not have to maintain any state, so the client-side session backend is as fast as the cryptographic primitives.

However, there are some disadvantages to client-side sessions:

  • Replay attacks. It's not possible to invalidate a session, for example. When logging out, a new cookie is sent with logged out session data. However, as the server doesn't maintain state about sessions, it will still accept the old, logged in cookie until it expires. One could set very small expiration times to mitigate this, but this would force users to relogin frequently. This server-side backend allows you to maintain long expiration times while still having secure logouts.

  • Cookie size. As the cookie contains the whole session data plus some overhead, care must be taken not to create too much session data. Yesod already saves the logged in user ID via yesod-auth and a XSRF token via yesod-form. This server-side backend uses a cookie of fixed size (24 bytes).

  • No remote logout. In many instances it is desirable to invalidate sessions other than the current one. For example, the user may have changed their password, or the the site provides a button to cancel all logged in sessions besides the current one. This server-side backend allows you to invalidate sessions other than the current one via forceInvalidate.

  • Missing key rotation. Ideally, clientsession's keys should be rotated periodically. In practice, support for key rotation has never been implemented on clientsession. This server-side backend does not need to do key rotations, and the session ID CPRNG is automatically reseeded.

The serversession package is clientsession's rival, each has their own advantages and disadvantages. However, both of them can be used on different ecosystems and take security from the ground up.

Comparision to other packages

At the time of writing (2015-05-22), these are the session packages that do not use either clientsession or serversession:

  • mysnapsession (via Memory module, also supports clientsession mode): Server-side sessions. Works for snap. Weak session ID generation. Vulnerable to session fixation attacks. Cannot invalidate other sessions.

  • salvia-sessions: Server-side sessions. Works only for salvia. No built-in support for DB-backed sessions, only memory-backed ones. Weak session ID generation. Vulnerable to session fixation attacks. Cannot invalidate other sessions.

  • simple-session: Client-side sessions. Works for simple framework. No encryption. Authentication vulnerable to timing attacks.

  • Spock (formely scotty-session): Server-side sessions. Works for Spock (code is not packaged separately). Only supports memory-backed sessions persisted on a file. Weak session ID generation. Vulnerable to session fixation attacks. Cannot invalidate other sessions.

  • wai-session: Server-side sessions. Works for wai applications. Weak session ID generation. Vulnerable to session fixation. Cannot invalidate other sessions. Out-of-the-box support for TokyoCabinet only.

  • yesod-session-redis: Server-side sessions. Works for Yesod and Redis. Weak session ID generation via random. Vulnerable to session fixation. Cannot invalidate other sessions.

We apologize in advance if any information above is incorrect. Please contact us about any errors.


Assuming a version of MAJOR.MINOR.PATCH, the following conventions are used by the serversession family of packages:

  • The PVP's versioning scheme is followed without changes. The important part is that either MAJOR or MINOR needs to be increased whenever a breaking change is made, while only PATCH needs to be changed otherwise.

  • In addition to the PVP, we increment MAJOR only when the serversession core package makes a breaking change. This means that:

    • The serversession core package always has MINOR equal to 0.
    • The other serversession-* packages have the same MAJOR as serversession, while being free to have any MINOR.

The scheme above is used to ensure consistent versioning of all packages even if they're not always released at the same time.