Cis Nginx V300 5 3 1

CIS 5.3.1 — Ensure X-Content-Type-Options header is configured and enabled

Profile Applicability

• Level 1 - Webserver

Description

The X-Content-Type-Options header instructs the browser to strictly follow the MIME types declared in the Content-Type headers and not to guess ("sniff") the content type based on the file's actual content.

Rationale

Implementing the X-Content-Type-Options header with the nosniff directive helps to prevent drive-by download attacks where a user agent is sniffing content types in responses.

This header prevents "MIME type confusion" attacks. Without this header, browsers might interpret a file declared as text (e.g., snippet.txt) as executable if it contains script code. Setting the nosniff directive forces the browser to reject the file if the declared type doesn't match the context in which it's loaded (e.g., loading a text file as a script).

Impact

Low Risk: In rare cases, legacy applications or misconfigured servers might rely on the browser's ability to correct wrong Content-Type headers (e.g., serving JavaScript with a text/plain header). Enabling nosniff will break these applications because the browser will refuse to execute the script. Ensuring correct MIME types in the NGINX configuration (mime.types) is a prerequisite.

Audit Procedure

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

nginx -T 2>/dev/null | grep -i 'X-Content-Type-Options'

Verify the output:

• Verify that the configuration contains the line: add_header X-Content-Type-Options "nosniff" always;
• Specifically check for the always parameter at the end of the directive. Without it, the header will be missing on error pages.

Remediation

Open the NGINX configuration file that contains your server blocks. Add the below line into your server block to add X-Content-Type-Options header and direct your user agent to not sniff content types.

add_header X-Content-Type-Options "nosniff" always;

Default Value

This header is not implemented by default.

References

1. https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/X-Content-Type-Options
2. https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#x-content-type-options
3. https://nginx.org/en/docs/http/ngx_http_headers_module.html

Additional Information

The always parameter ensures that the header is applied to the HTTP response no matter what the HTTP response code is.

CIS Controls

MITRE ATT&CK Mappings

Profile

• Level 1 - Webserver