If you are currently running Bitcoin Core, then you must upgrade to 0.16.3 as soon as possible. You can download it from bitcoin.org or bitcoincore.org or via BitTorrent, and as always, make sure that you verify the download.
If you only occasionally run Bitcoin Core, then it is less urgent, though it would be best to upgrade as soon as convenient.
A bug was found which allows anyone capable of mining a sufficient-PoW block to crash Bitcoin Core nodes running versions 0.14.0 to 0.16.2. Stored funds are not at risk.
Since most altcoins are based on Bitcoin Core, I would guess that many of them are also affected, though I don’t have any specific info on that at this time.
Main release thread: https://bitcointalk.org/index.php?topic=5032424.0