The vulnerability describes a race condition where a deleted Rack session can be restored by simultaneous requests when using Rack::Session::Pool. The commit c48e52f7c57e99e1e1bf54c8760d4f082cd1c89d addresses this issue by modifying two functions in lib/rack/session/pool.rb: write_session and delete_session.

The core of the vulnerability lies in the ability to restore a session after it has been deleted. This restoration happens when session data is written back to the store. The write_session function is responsible for this action. The patch introduces a crucial check (return false unless get_session_with_fallback(session_id)) in write_session before it attempts to store the session data. The absence of this check in the vulnerable version allowed a concurrent request (that had loaded session data before its deletion) to write this data back, effectively 'restoring' the deleted session. The commit message directly confirms that the change to write_session is to prevent it from succeeding if the session doesn't exist, fixing the concurrent deletion issue.

The delete_session function was also modified to ensure that when a new session ID is generated after a deletion (if options[:drop] is false), an empty session is immediately stored for this new ID. This is a hardening measure to prevent the old, deleted session data from being associated with a potentially reused SID. However, the actual act of 'restoring' the session, which is the crux of the vulnerability, occurs within write_session due to the missing check.

Therefore, Rack::Session::Pool#write_session is identified as the vulnerable function because its previous implementation allowed the restoration of a deleted session under race conditions.