Pow Mnesia cache doesn't invalidate all expired keys on startup

Use of Pow.Store.Backend.MnesiaCache is susceptible to session hijacking as expired keys are not being invalidated correctly on startup. A cache key may become expired when all Pow.Store.Backend.MnesiaCache instances have been shut down for a period that is longer than the keys’ remaining TTL and the expired key won’t be invalidated on startups.

Workarounds

The expired keys, including all expired sessions, can be manually invalidated by running:

:mnesia.sync_transaction(fn ->
  Enum.each(:mnesia.dirty_select(Pow.Store.Backend.MnesiaCache, [{{Pow.Store.Backend.MnesiaCache, :_, :_}, [], [:"$_"]}]), fn {_, key,  {_value, expire}} ->
    ttl = expire - :os.system_time(:millisecond)
    if ttl < 0, do: :mnesia.delete({Pow.Store.Backend.MnesiaCache, key})
  end)
end)

References

https://github.com/pow-auth/pow/commit/15dc525be03c466daa5d2119ca7acdec7b24ed17
https://github.com/pow-auth/pow/issues/713
https://github.com/pow-auth/pow/pull/714