Rack::Session is a session management implementation for Rack. In versions starting from 2.0.0 to before 2.1.1, 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. This issue has been patched in version 2.1.1.
https://github.com/rack/rack/security/advisories/GHSA-vpfw-47h7-xj4g
https://github.com/rack/rack-session/security/advisories/GHSA-9j94-67jr-4cqj
https://github.com/rack/rack-session/commit/c28c4a8c1861d814e09f2ae48264ac4c40be2d3b