Possible information leak / session hijack vulnerability

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.