On Mon, 17 Sep 2018, an anonymous encrypted disclosure message was sent to the BitcoinCore team by a user under the pseudonymous of beardnbobies. The message disclosed a Denial Of Service bug and a potential inflation bug

IMPORTANT The bug fixed in 0.16.3 is more severe than was previously made public. Full node users are advised to upgrade as soon as possible to Bitcoin Core 0.16.3. The releases can be found on bitcoin core website. Make sure to verify the binaries before using them. Stored funds are not at risk, there is however a small probability for a chainsplit. In the event of a chainsplit, transactions could be reversed even after a long time after they were fully confirmed.