CVE-2018-17144 is a fix for which was released on September 18th in Bitcoin Core versions 0.16.3 and 0.17.0rc4, it includes both a Denial of Service component and a critical inflation vulnerability. It was originally reported to several developers working on Bitcoin Core, as well as projects supporting other cryptocurrencies.
If you are running an older version, shut it down. Wait until it has completely shut down (which might take a few minutes for older versions), then run the installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on Mac) or `bitcoind`/`bitcoin-qt` (on Linux).
The first time you run version 0.15.0 or newer, your chainstate database will be converted to a new format, which will take anywhere from a few minutes to half an hour, depending on the speed of your machine. Note that the block database format also changed in version 0.8.0 and there is no automatic upgrade code from before version 0.8 to version 0.15.0 or higher. Upgrading directly from 0.7.x and earlier without re-downloading the blockchain is not supported. However, as usual, old wallet versions are still supported.
Wallets created in 0.16 and later are not compatible with versions prior to 0.16 and will not work if you try to use newly created wallets in older versions. Existing wallets that were created with older versions are not affected by this.
Bitcoin Core is extensively tested on multiple operating systems using the Linux kernel, macOS 10.8+, and Windows Vista and later. Windows XP is not supported. Bitcoin Core should also work on most other Unix-like systems but is not frequently tested on them.
A denial-of-service vulnerability (CVE-2018-17144) exploitable by miners has been discovered in Bitcoin Core versions 0.14.0 up to 0.16.2. It is recommended to upgrade any of the vulnerable versions to 0.16.3 as soon as possible.
Further information – https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2018-17144