Recent security scans of ArcGIS Portal and ArcGIS Server raised security concerns around use of these values being assigned in cookies to protect and validate their use.
"HTTPOnly" - Cookies without the "HTTPOnly" attribute are permitted to be accessed via JavaScript. Cross-site scripting attacks can steal cookies which could lead to user impersonation or compromise of the application account.
"secure" - Cookies with the "secure" attribute are only permitted to be sent via HTTPS. Cookies sent via HTTP expose an unsuspecting user to sniffing attacks that could lead to user impersonation or compromise of the application account.