cis-nginx-v300-5-3-3

DevOps & Security

Ensure the Referrer Policy is enabled and configured properly (Manual)

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CIS 5.3.3 — Ensure the Referrer Policy is enabled and configured properly

Profile Applicability

Level 2 - Webserver

Description

The Referrer-Policy HTTP header controls how much referrer information (sent via the Referer header) should be included with requests. It allows site administrators to restrict the data sent to upstream servers when a user clicks a link or loads a resource. This is a privacy control to prevent leaking sensitive URL parameters or internal path structures to third parties.

Rationale

URLs often contain sensitive information such as session tokens, search queries, or Personally Identifiable Information (PII) in their query parameters. Without a strict Referrer Policy, this full URL is transmitted to any third-party site the user visits from your page, potentially logging sensitive data on external servers. Configuring this header ensures that only the necessary information (e.g., just the origin domain) is shared, protecting user privacy and preventing data leakage.

Impact

Choosing an overly restrictive policy like no-referrer can break functionality that relies on knowing the source of traffic, such as web analytics, affiliate tracking, or multi-site authentication flows. Conversely, a loose policy (unsafe-url) leaks private data. The recommended policy strict-origin-when-cross-origin is a balance by preserving full referrer data for internal navigation while stripping sensitive path and query data when navigating to external sites.

Audit Procedure

Run the following command to inspect the fully loaded NGINX configuration:

nginx -T 2>/dev/null | grep -i 'Referrer-Policy'

Evaluate the findings:

Verify that the directive add_header Referrer-Policy "..." always; is present.
Check the configured value. While no-referrer is the most secure, strict-origin-when-cross-origin is the widely accepted standard for balancing security and functionality.
Ensure the always parameter is present.

Remediation

Add the Referrer-Policy header to your server block. The recommended value is strict-origin-when-cross-origin.

add_header Referrer-Policy "strict-origin-when-cross-origin" always;

Policy Options (from most to least restrictive):

Policy	Behavior
`no-referrer`	Never send the Referer header. Most private but breaks analytics.
`same-origin`	Send full referrer for same-origin requests only. No referrer for cross-origin.
`strict-origin-when-cross-origin`	Recommended. Full referrer for same-origin. Origin-only for cross-origin (HTTPS->HTTPS). No referrer on downgrade (HTTPS->HTTP).
`origin-when-cross-origin`	Full referrer for same-origin. Origin-only for cross-origin.
`unsafe-url`	Always send full referrer. Not recommended.

Default Value

By default, no Referrer-Policy header is sent by NGINX. Browsers typically default to strict-origin-when-cross-origin, but explicitly setting this header ensures consistent behavior across all browsers and versions.

References

CIS Controls

Controls Version	Control	IG 1	IG 2	IG 3
v8	16.1 Establish and Maintain a Secure Application Development Process	N	Y	Y
v7	18.1 Establish Secure Coding Practices	N	Y	Y

MITRE ATT&CK Mappings

Tactic	Technique
Collection	T1185 - Browser Session Hijacking
Exfiltration	T1041 - Exfiltration Over C2 Channel

Profile

Level 2 - Webserver