Cis Nginx V300 2 5 4

CIS 2.5.4 — Ensure the NGINX reverse proxy does not enable information disclosure

Profile Applicability

• Level 1 - Proxy
• Level 1 - Loadbalancer

Description

When NGINX acts as a reverse proxy, it forwards headers sent by the upstream application (e.g., "X-Powered-By: Custom_APP" or "Server: Apache/2.4"). These headers should be stripped before the response reaches the client to prevent information disclosure about the backend infrastructure.

Rationale

Attackers conduct reconnaissance by inspecting response headers to identify the technologies used in the backend (e.g., specific versions of PHP, Java/Tomcat, or Python frameworks). Knowing the exact version allows attackers to target specific CVEs associated with that software stack. Removing these headers reduces the information available for targeted attacks.

Impact

Removing upstream headers may affect debugging or monitoring tools that rely on these headers. Ensure that any required headers are logged internally before being stripped.

Audit Procedure

1. Configuration Check:

Search the loaded configuration for header hiding directives:

nginx -T 2>/dev/null | grep -Ei "(proxy|fastcgi)_hide_header"

Evaluation:

• Verify that directives exist to hide X-Powered-By and Server.

2. Functional Check (Recommended):

Send a request to a proxied endpoint and inspect the response headers:

curl -k -I https://127.0.0.1 | grep -Ei "^(Server|X-Powered-By)"

Evaluation:

• PASS: The output does not contain backend details (e.g., X-Powered-By: PHP/8.2). If Server is present, it should only be Server: nginx (controlled by Control 2.5.1).
• FAIL: The output contains backend information or unmasked version numbers.

Remediation

Configure NGINX to strip the sensitive headers. The directive depends on the upstream protocol (HTTP Proxy vs. FastCGI).

For Standard Reverse Proxy (proxy_pass):

Add the following directives to your http, server, or location block:

proxy_hide_header X-Powered-By;
proxy_hide_header Server;

For PHP/FastCGI (fastcgi_pass):

If you are using FastCGI (e.g., for PHP-FPM), use the fastcgi_hide_header directive instead:

fastcgi_hide_header X-Powered-By;

Default Value

By default, NGINX passes all headers received from the upstream server to the client unchanged.

References

1. https://nginx.org/en/docs/http/ngx_http_proxy_module.html
2. https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_hide_header

CIS Controls

MITRE ATT&CK Mappings

Profile

• Level 1 - Proxy
• Level 1 - Loadbalancer