Rack session gets restored after deletion

### Summary 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. ### Details [Rack session middleware](https://github.com/rack/rack/blob/v2.2.13/lib/rack/session/abstract/id.rb#L263-L270) 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. ### Impact 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. ## Mitigation - Update to the latest version of `rack`, or - Ensure your 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. ### Related As this code was moved to `rack-session` in Rack 3+, see <https://github.com/rack/rack-session/security/advisories/GHSA-9j94-67jr-4cqj> for the equivalent advisory in `rack-session` (affecting Rack 3+ only).

Quoted source text, attributed separately from HOL analysis.