The Portal for ArcGIS Security 2018 Update 1 Patch for 10.5.1 is now live on the support site. This is a multiple version patch. 10.4.1 and 10.3.1 will release on this page as they become available. Currently they are marked as coming soon. The URL is:
This security patch addresses multiple security vulnerabilities found in Portal for ArcGIS. Esri recommends that all customers using Portal for ArcGIS 10.5.1, 10.4.1 and 10.3.1 apply this patch.
Please see the download page for a list of issues addressed for each version.