There's a possible information leak / session hijack vulnerability in Rack.

Attackers may be able to find and hijack sessions by using timing attacks
targeting the session id. Session ids are usually stored and indexed in a
database that uses some kind of scheme for speeding up lookups of that
session id. By carefully measuring the amount of time it takes to look up
a session, an attacker may be able to find a valid session id and hijack
the session.

The session id itself may be generated randomly, but the way the session is
indexed by the backing store does not use a secure comparison.

Impact:

The session id stored in a cookie is the same id that is used when querying
the backing session storage engine. Most storage mechanisms (for example a
database) use some sort of indexing in order to speed up the lookup of that
id. By carefully timing requests and session lookup failures, an attacker
may be able to perform a timing attack to determine an existing session id
and hijack that session.

See the full diff on Github. The new version differs by 14 commits:

• Bumping version
• Introduce a new base class to avoid breaking when upgrading
• Add a version prefix to the private id to make easier to migrate old values
• Fallback to the public id when reading the session in the pool adapter
• Also drop the session with the public id when destroying sessions
• Fallback to the legacy id when the new id is not found
• Add the private id
• revert conditionals to master
• remove NullSession
• remove || raise and get closer to master
• store hashed id, send public id
• use session id objects
• remove more nils
• try to ensure we always have some kind of object

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