cis-nginx-v300-5-3-2

CIS 5.3.2 — Ensure that Content Security Policy (CSP) is enabled and configured properly

Profile Applicability

• Level 2 - Webserver

Description

Content Security Policy (CSP) is an HTTP response header that allows site administrators to declare approved sources of content that browsers are allowed to load on that page. It is a mechanism to detect and mitigate certain types of attacks, including Cross-Site Scripting (XSS) and data injection attacks. Furthermore, CSP's frame-ancestors directive is the modern replacement for the X-Frame-Options header to prevent Clickjacking.

Rationale

A robust CSP significantly reduces the attack surface of a web application. By restricting the domains from which scripts, styles, images, and other resources can be loaded, it effectively neutralizes many XSS vectors. Additionally, by using the frame-ancestors directive, it explicitly controls which parent pages are allowed to embed the application (e.g., via <iframe>), providing a more flexible protection against Clickjacking than the legacy X-Frame-Options header.

Impact

Implementing a strict CSP is complex and carries a high risk of breaking application functionality. If a legitimate resource (e.g., a CDN script, a font file, or an inline script) is not whitelisted, the browser will block it, potentially making the site unusable. CSP implementation should always start in "Report-Only" mode (Content-Security-Policy-Report-Only) to gather violation reports before enforcing the policy.

Audit Procedure

1. Run the following command to inspect the CSP configuration:

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

2. Evaluate the policy:

• Is the header present?
• Does it include at least a restrictive default-src directive (e.g., 'self' or 'none')?
• Does it include the frame-ancestors directive to mitigate Clickjacking?
• Critically: Is unsafe-inline or unsafe-eval avoided in script-src? (Allowing these significantly weakens the protection).

Remediation

CSP must be tailored to the specific application. There is no single "correct" policy.

Step 1: The Baseline Policy (High Security)

Start with a policy that denies everything by default and only allows resources from the same origin. It also prevents the site from being framed by anyone (Clickjacking protection).

add_header Content-Security-Policy "default-src 'self'; frame-ancestors 'self'; form-action 'self';" always;

Step 2: Adaptation (Example)

If your application loads images from a CDN and needs to be embeddable by a specific partner site:

add_header Content-Security-Policy "default-src 'self'; img-src 'self' https://cdn.example.com; frame-ancestors 'self' https://partner-site.com;" always;

Note: Use Content-Security-Policy-Report-Only during the testing phase to debug your policy without breaking the site.

Default Value

By default, no Content Security Policy is sent. Browsers default to the standard Same-Origin Policy, which is much less restrictive.

References

1. https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html
2. https://cheatsheetseries.owasp.org/cheatsheets/Clickjacking_Defense_Cheat_Sheet.html
3. https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Security-Policy
4. https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Security-Policy-Report-Only
5. https://nginx.org/en/docs/http/ngx_http_headers_module.html#add_header
6. https://caniuse.com/?search=frame-ancestors

Additional Information

OWASP HTTP Headers Cheat Sheet states:

Content Security Policy (CSP) frame-ancestors directive obsoletes X-Frame-Options for supporting browsers (source).

X-Frame-Options header is only useful when the HTTP response where it is included has something to interact with (e.g. links, buttons). If the HTTP response is a redirect or an API returning JSON data, X-Frame-Options does not provide any security.

Recommendation: Use Content Security Policy (CSP) frame-ancestors directive if possible.

Almost all modern browsers do support CSP. A comprehensive overview of compatible browsers can be found here: https://caniuse.com/?search=frame-ancestors

CIS Controls

MITRE ATT&CK Mappings

Profile

• Level 2 - Webserver