TLDR: Add X-FRAME-Options: SAMEORIGIN to all HTTP(S) pages (server config), and test that nothing breaks.
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Description: Current HTTP headers do not contain the X-FRAME-Option, which helps prevents against Clickjacking attacks. A Clickjacking attack is similar to CSRF in which attacker can hijack a "click" on a web application from another "invisible" frame in the browser. Essentially, an attacker can force a user to click on a button that is invisible to him/her.

Exploit Scenario: An attacker crafts a malicious page such that when their victim clicks, they are actually clicking on the link or button in the vulnerable application hosted in an iframe. Thus, an attacker tricks the user into performing an action of the attacker's choosing by directing mouse input to the target application.