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.
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.