cis-nginx-v300-4-1-10

name	cis-nginx-v300-4-1-10
description	Ensure the upstream traffic server certificate is trusted (Manual)
category	cis-nginx
version	3.0
author	cyberstrike-official
tags	["cis","nginx","web-server","reverse-proxy","tls-ssl","encryption"]
cis_id	4.1.10
cis_benchmark	CIS NGINX Benchmark v3.0.0
tech_stack	["nginx","linux","web-server"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS 4.1.10 — Ensure the upstream traffic server certificate is trusted

Profile Applicability

• Level 2 - Proxy
• Level 2 - Loadbalancer

Description

When acting as a reverse proxy, NGINX must be configured to function as a secure TLS client. This requires validating the identity of the upstream server by verifying its certificate against a trusted Certificate Authority (CA). Furthermore, NGINX must ensure that the hostname of the upstream server matches the name (Subject Name / SAN) within the certificate itself.

Rationale

Without proper validation, NGINX blindly trusts the identity of the upstream server, making it vulnerable to man-in-the-middle (MitM) attacks within the internal network. An attacker could impersonate a legitimate backend service and intercept sensitive traffic. By enforcing certificate validation (proxy_ssl_verify) and hostname verification (proxy_ssl_name), NGINX guarantees that it is communicating with the intended, authentic upstream server.

Impact

Enabling strict upstream verification introduces a dependency on certificate lifecycle management. If the upstream server's certificate expires or is misconfigured, or if the CA certificate file on the NGINX server is outdated, NGINX will refuse to connect to the upstream, resulting in a service outage. This behavior is intentional and secures the connection at the cost of requiring careful certificate management.

Audit Procedure

Run the following command to inspect the fully loaded NGINX configuration for the three required directives within the relevant proxy location block:

nginx -T 2>/dev/null | grep -E \
'^\s*(proxy_ssl_verify|proxy_ssl_trusted_certificate|proxy_ssl_name)'

Verify that the output contains the following directives:

• proxy_ssl_verify on;
• proxy_ssl_trusted_certificate /path/to/ca.crt;
• proxy_ssl_name your-upstream-hostname.com;

If any of these three directives are missing, this recommendation is not fully implemented. Additionally, manually verify that the certificate referenced by proxy_ssl_trusted_certificate is the correct, valid CA certificate for your upstream services.

Remediation

To securely configure upstream validation, you must obtain the CA certificate that signed your upstream server's certificate.

1. Place the CA certificate (e.g., upstream_ca.crt) in a secure directory on the NGINX server (e.g., ).