Impact

ActionDispatch::Session::CacheStore writes a session back to the cache at the end of each request that loaded it, without checking that the session's cache entry still exists. When a concurrent request destroys the session, through reset_session on logout or by deleting its cache entry on the server, a request that loaded the session earlier and finishes later restores it under its original session ID. Logout and server-side revocation could therefore fail to end a session, and an attacker who holds a stolen session cookie could keep that session valid across the victim's logout.

Applications that use config.session_store :cache_store are affected, with any cache backend. ActionDispatch::Session::CookieStore is not affected, nor is ActionDispatch::Session::MemCacheStore with Dalli 4.0.0 or later, whose session store does not write back a session that was deleted.

Details

The fixed releases record which session a request loaded and skip the write when that session's cache entry no longer exists. Changes that such a request makes to the destroyed session are discarded, and Rack logs a warning that the session could not be saved. Each request that writes back a session it loaded makes one additional cache read.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/d479192cf88a8b30a335a8bc25188a2d8e0f8d9f.patch
• 8.0: https://github.com/rails/rails/commit/5bb5ec66ed8faaf590ffe1222d7a5eb6e9d050f5.patch

Credit

Found by the team during internal code review.