cis-nginx-v300-5-2-3

name	cis-nginx-v300-5-2-3
description	Ensure the maximum buffer size for URIs is defined (Manual)
category	cis-nginx
version	3.0
author	cyberstrike-official
tags	["cis","nginx","web-server","reverse-proxy","request-limits","request-filtering"]
cis_id	5.2.3
cis_benchmark	CIS NGINX Benchmark v3.0.0
tech_stack	["nginx","linux","web-server"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS 5.2.3 — Ensure the maximum buffer size for URIs is defined

Profile Applicability

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

Description

The large_client_header_buffers directive allocates the maximum number and size of buffers for reading the entire client request header, which includes both the request line (e.g., GET /path HTTP/1.1) and all subsequent header fields (e.g., Host:, Cookie:, Authorization:). If a client's request header exceeds the total allocated buffer space, NGINX immediately rejects the request with a 400 Bad Request error. Also, the client's request line cannot exceed the size of one buffer, otherwise the 414 Request-URI Too Large error will be returned to the client.

Rationale

While NGINX itself is not vulnerable to buffer overflows from large headers, this directive serves two critical purposes:

1. Denial of Service (DoS) Mitigation: It prevents malicious clients from attempting to exhaust server memory by sending excessively large headers.

2. Downstream Protection: It acts as a gatekeeper, ensuring that malformed or oversized headers do not reach backend applications that might be more fragile or could exhibit undefined behavior when processing them.

The default value is intentionally generous to handle legitimate modern web traffic, and lowering it without reason can be counterproductive.

Impact

Setting this value too low is a common cause of difficult-to-diagnose application failures. Legitimate requests from modern web applications, especially those using complex authentication (OAuth/SAML), or analytics, can easily generate headers larger than a few kilobytes. An overly restrictive value will cause these legitimate requests to fail with a generic 400 Bad Request error, which is often mistakenly attributed to the application itself rather than the NGINX configuration.

Audit Procedure

This is a manual check that requires context.

1. Run the following command to see if the directive is explicitly set:

nginx -T 2>/dev/null | grep 'large_client_header_buffers'

2. Evaluate the result:

• No output: The server is using the default value of . This is considered compliant and secure for general use.