cis-nginx-v300-2-5-2

DevOps & Security

Ensure default error and index.html pages do not reference NGINX (Manual)

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CIS 2.5.2 — Ensure default error and index.html pages do not reference NGINX

Profile Applicability

Level 1 - Webserver
Level 1 - Proxy
Level 1 - Loadbalancer

Description

Default error pages (e.g., 404, 500) and the default welcome page often contain NGINX branding or signatures. These pages should be removed or replaced with generic or custom-branded pages that do not disclose the underlying server technology.

Rationale

Standard NGINX error pages visually identify the server software, even if headers are suppressed. By gathering information about the underlying technology stack, attackers can tailor their exploits to known vulnerabilities of NGINX. Replacing default pages with generic or branded content removes this information leakage vector and increases the effort required for successful reconnaissance.

Impact

Creating and maintaining custom error pages requires additional administrative effort. Ensure that custom error pages are simple and do not themselves introduce vulnerabilities.

Audit Procedure

1. Verify Config:

Check if error_page directives are active:

nginx -T 2>/dev/null | grep -i "error_page"

2. Verify Content (Functional Test):

Trigger an error (e.g., request a non-existent page) and inspect the body:

curl -k https://127.0.0.1/non-existent-page | grep -i "nginx"

Evaluation:

PASS: The output does not contain "nginx" (or grep returns nothing).
FAIL: The HTML body contains "nginx".

Remediation

Instead of editing the default files (which may be overwritten by package updates), configure NGINX to use custom error pages.

1. Create Custom Error Pages:

Create a directory (e.g., /var/www/html/errors) and place generic HTML files there (e.g., 404.html, 50x.html) without NGINX branding.

2. Configure NGINX:

Add the error_page directive to your http or server blocks:

error_page 404 /404.html;
error_page 500 502 503 504 /50x.html;

location = /50x.html {
    root /var/www/html/errors;
    internal;
}

Default Value

Default error pages identify the server as NGINX.

References

https://nginx.org/en/docs/http/ngx_http_core_module.html#error_page

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
Reconnaissance	T1592 - Gather Victim Host Information

Profile

Level 1 - Webserver
Level 1 - Proxy
Level 1 - Loadbalancer