Request Smuggling in Chi

Request smuggling in Chi applications typically occurs when the framework's HTTP request parsing interacts poorly with upstream proxies or load balancers. Chi uses the standard net/http package for request handling, which can be vulnerable to smuggling when content-length and transfer-encoding headers conflict.

The most common Chi-specific smuggling scenario involves the Context middleware chain. When a malicious request contains both Content-Length and Transfer-Encoding headers with conflicting values, Chi's underlying http.Request parsing may interpret the body boundaries differently than the proxy. This creates a desynchronization where the proxy forwards more or less data than Chi expects.

package main

import (
	"fmt"
	"net/http"
	"github.com/go-chi/chi/v5"
)

func main() {
	r := chi.NewRouter()
	
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			// Chi processes the request body here
			next.ServeHTTP(w, r)
		})
	})
	
	r.Get("/api/data", func(w http.ResponseWriter, r *http.Request) {
		header := r.Header.Get("Content-Type")
		fmt.Fprintf(w, "Received: %s", header)
	})
	
	http.ListenAndServe(":8080", r)
}

In this Chi setup, a request like:

POST /api/data HTTP/1.1
Host: example.com
Content-Length: 10
Transfer-Encoding: chunked

0

GET /api/secret HTTP/1.1
Host: example.com

The proxy might forward only the first 10 bytes, while Chi parses the chunked encoding, causing the smuggled GET request to be processed as part of the original request body. This can lead to unauthorized access to protected endpoints.

Chi's chi.Router also doesn't automatically validate header consistency, making it susceptible to CL.TE (Content-Length header, Transfer-Encoding body) and TE.CL (Transfer-Encoding header, Content-Length body) attacks. The framework's minimalist design means these low-level HTTP parsing issues fall through to the application layer.

Detecting request smuggling in Chi applications requires both static analysis and dynamic testing. The static approach involves examining how your Chi application handles HTTP headers and request bodies.

Start by reviewing your Chi middleware chain. Any middleware that processes request bodies without validating header consistency is a potential vulnerability point. Use this detection function to check for risky patterns:

package main

import (
	"net/http"
	"github.com/go-chi/chi/v5"
)

func detectSmugglingVulnerabilities(r *chi.Mux) []string {
	vulnerabilities := []string{}
	
	// Check for middleware that reads request bodies
	for _, m := range r.Middlewares() {
		if middlewareReadsBody(m) {
			vulnerabilities = append(vulnerabilities, "Middleware reads body without header validation")
		}
	}
	
	// Check for handlers that don't validate Content-Length vs Transfer-Encoding
	for _, route := range r.Routes() {
		if route.SubRoutes != nil {
			vulnerabilities = append(vulnerabilities, detectSmugglingVulnerabilities(route.SubRoutes)...)
		} else {
			for _, method := range route.Handlers.Keys() {
				if handlerVulnerable(route.Handlers.Get(method)) {
					vulnerabilities = append(vulnerabilities, fmt.Sprintf("Handler %s %s may be vulnerable", method, route.Pattern))
				}
			}
		}
	}
	
	return vulnerabilities
}

For dynamic testing, use middleBrick's API security scanner which includes request smuggling detection specifically for Go/chi applications. The scanner tests for header inconsistencies and attempts controlled smuggling attacks to verify vulnerability.

middleBrick's approach for Chi applications:

1. Scans your API endpoints for inconsistent header handling
2. Tests CL.TE and TE.CL attack patterns against your routes

3. Verifies if smuggled requests reach protected endpoints
4. Provides severity scoring based on potential impact

Run middleBrick from your terminal:

npx middlebrick scan https://your-chiapp.com/api

The scanner will identify if your Chi application is vulnerable to request smuggling and provide specific remediation guidance based on the detected patterns.

Remediating request smuggling in Chi applications requires a multi-layered approach. The most effective strategy is to ensure consistent request parsing across your entire application stack.

First, implement strict header validation middleware in your Chi application:

package main

import (
	"net/http"
	"github.com/go-chi/chi/v5"
	"strings"
)

func validateHeadersMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Check for conflicting headers
		contentLength := r.Header.Get("Content-Length")
		transferEncoding := r.Header.Get("Transfer-Encoding")
		
		if contentLength != "" && transferEncoding != "" {
			http.Error(w, "Conflicting headers detected", http.StatusBadRequest)
			return
		}
		
		// Check for multiple Transfer-Encoding values
		if transferEncoding != "" {
			values := strings.Split(transferEncoding, ",")
			if len(values) > 1 {
				http.Error(w, "Multiple Transfer-Encoding values not allowed", http.StatusBadRequest)
				return
			}
		}
		
		next.ServeHTTP(w, r)
	})
}

func main() {
	r := chi.NewRouter()
	
	r.Use(validateHeadersMiddleware)
	
	r.Get("/api/data", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("Request processed securely"))
	})
	
	http.ListenAndServe(":8080", r)
}

This middleware prevents the most common smuggling vectors by rejecting requests with conflicting or malformed headers before they reach your application logic.

Second, configure your reverse proxy to use a single, consistent transfer encoding. If you're using Nginx in front of Chi:

server {
	listen 80;
	server_name example.com;
	
	# Normalize all requests to Content-Length
	proxy_http_version 1.1;
	proxy_pass_request_headers on;
	proxy_set_header Transfer-Encoding "";
	proxy_set_header Content-Length $request_content_length;
	
	location / {
		proxy_pass http://localhost:8080;
	}
}

For Apache, use:

<VirtualHost *:80>
	ServerName example.com
	
	# Force Content-Length encoding
	SetEnv proxy-sendcl
	
	ProxyPass / http://localhost:8080/ keepalive=On
	ProxyPassReverse / http://localhost:8080/
</VirtualHost>

Third, use middleBrick's continuous monitoring to verify your fixes. The Pro plan includes scheduled scans that will alert you if smuggling vulnerabilities reappear after deployment.

middleBrick's remediation guidance for Chi applications includes:

• Specific header validation patterns for your route structure
• Proxy configuration templates for common setups
• Testing procedures to verify fixes
• Compliance mapping to OWASP API Security Top 10

By combining strict header validation, consistent proxy configuration, and continuous monitoring, you can effectively eliminate request smuggling vulnerabilities in your Chi applications.