Web pages must be protected from being embedded in malicious sites (clickjacking).
Clickjacking is a malicious technique often used in phishing attacks. By using it, attackers may deceive users and make them perform actions other than the ones they perceive, which may cause them to reveal confidential information. Web servers can protect pages they host to a great extent by including headers in their responses that prevent the pages from being embedded in malicious sites (e.g., "X-Frame-Options").
CWE-346: Origin Validation Error: The software does not properly verify that the source of data or communication is valid.
CWE-451: User Interface (UI) Misrepresentation of Critical Information: The user interface (UI) does not properly represent critical information to the user, allowing the information - or its source - to be obscured or spoofed. This is often a component in phishing attacks.
OWASP-ASVS v4.0.1 V14.4 HTTP Security Headers Requirements.(14.4.7): Verify that a suitable X-Frame-Options or Content-Security-Policy: frame-ancestors header is in use for sites where content should not be embedded in a third-party site.