rack -- information leak / session hijack vulnerability

National Vulnerability Database:

There’s a possible information leak / session hijack vulnerability in
Rack (RubyGem rack). This vulnerability is patched in versions 1.6.12
and 2.0.8. 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.