Rack is a modular Ruby web server interface. Prior to version 2.2.14, when using the Rack::Session::Pool middleware, simultaneous rack requests can restore a deleted rack session, which allows the unauthenticated user to occupy that session. Rack session middleware prepares the session at the beginning of request, then saves is back to the store with possible changes applied by host rack application. This way the session becomes to be a subject of race conditions in general sense over concurrent rack requests. When using the Rack::Session::Pool middleware, and provided the attacker can acquire a session cookie (already a major issue), the session may be restored if the attacker can trigger a long running request (within that same session) adjacent to the user logging out, in order to retain illicit access even after a user has attempted to logout. Version 2.2.14 contains a patch for the issue. Some other mitigations are available. Either ensure the application invalidates sessions atomically by marking them as logged out e.g., using a logged_out flag, instead of deleting them, and check this flag on every request to prevent reuse.

or implement a custom session store that tracks session invalidation timestamps and refuses to accept session data if the session was invalidated after the request began.