https://wordpress.org/news/2017/11/wordpress-4-9-1-security-and-maintenance-release/
WordPress 4.9.1 is now available. This is a security and maintenance release for all versions since WordPress 3.7. We strongly encourage you to update your sites immediately.
WordPress versions 4.9 and earlier are affected by four security issues which could potentially be exploited as part of a multi-vector attack. As part of the core team’s ongoing commitment to security hardening,
Source: Security Feed