Http Request Smuggling in Chi with Basic Auth

HTTP request smuggling occurs when an intermediary (such as a proxy or load balancer) processes and forwards HTTP requests differently than the origin server does. The interaction between Chi routing and Basic Authentication headers can amplify confusion-based smuggling risks, especially when request parsing diverges between the frontend and backend.

Chi is a minimal HTTP router and middleware framework for Go. When Basic Authentication is enforced via middleware that inspects and validates the Authorization header before routing, certain malformed or ambiguous requests can be interpreted differently by the authentication layer versus the underlying server or reverse proxy. This discrepancy can allow a smuggler to craft a request that passes the Basic Auth check on one layer but is parsed differently downstream, potentially bypassing intended route matching or access controls.

For example, a request with multiple Content-Length headers or an inconsistent Transfer-Encoding header may be processed variably: the Basic Auth middleware might read the body or headers one way, while the Chi router or a backend handler interprets them differently. If the middleware consumes or modifies the request body (e.g., reading the raw header to extract credentials) before passing the request to Chi, the body may be partially consumed, leading to request splitting or request concatenation vulnerabilities.

In a typical Chi route setup, routes are matched based on method and path. If Basic Auth middleware reads the Authorization header but leaves the request body in an inconsistent state, a smuggled request might reach a different route than intended, or bypass authentication checks altogether. Consider a scenario where a POST to /admin is protected by Basic Auth middleware, but a smuggled request is interpreted by the backend as a GET to /public due to header parsing differences. The request may authenticate successfully at the middleware level but be routed according to a different path rule, exposing an unintended endpoint.

Because Chi does not inherently normalize or validate header inconsistencies, the onus is on the developer to ensure that authentication and routing layers agree on request framing. Without strict header validation and body handling discipline, an attacker can exploit differences in how chunked versus content-length-based requests are processed, even when credentials are verified.

To mitigate request smuggling when using Basic Authentication in Chi, ensure consistent request parsing, avoid consuming the body before routing, and enforce strict header validation. The following examples demonstrate secure patterns.

First, use a middleware that validates the Authorization header without modifying the request body, and ensure the request body is not read prematurely. Use io.Copy(io.Discard, r.Body) only if you must consume the body, and always restore it if needed for downstream handlers.

// Secure Basic Auth middleware for Chi
package main

import (
	"io"
	"net/http"
	"strings"
)

func basicAuthMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		user, pass, ok := r.BasicAuth()
		if !ok || !checkCredentials(user, pass) {
			w.Header().Set("WWW-Authenticate", `Basic realm="restricted", charset="UTF-8"`)
			http.Error(w, "Unauthorized", http.StatusUnauthorized)
			return
		}
		// Do not read r.Body here; pass request unchanged to router
		next.ServeHTTP(w, r)
	})
}

func checkCredentials(user, pass string) bool {
	// Validate against a secure store; this is a placeholder
	return user == "admin" && pass == "securepass"
}

Second, enforce consistent transfer encoding by rejecting requests with both Transfer-Encoding and Content-Length headers, and normalize headers before routing.

// Header normalization middleware to prevent smuggling
func headerSanitizationMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Header.Get("Transfer-Encoding") != "" && r.Header.Get("Content-Length") != "" {
			http.Error(w, "Invalid headers", http.StatusBadRequest)
			return
		}
		// Ensure headers are canonicalized; remove any duplicate headers
		r.Header.Del("X-Forwarded-For") // Example: remove potentially smuggled headers
		next.ServeHTTP(w, r)
	})
}

Third, use Chi with strict routing and avoid side-effectful header parsing that can desynchronize the request state. Define routes explicitly and keep middleware lean.

// Chi router with secure middleware stacking
package main

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

func main() {
	r := chi.NewRouter()
	r.Use(middleware.RequestID)
	r.Use(middleware.RealIP)
	r.Use(headerSanitizationMiddleware)
	r.Use(basicAuthMiddleware)

	r.Get("/public", publicHandler)
	r.Post("/admin", adminHandler)

	http.ListenAndServe(":8080", r)
}

func publicHandler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("public endpoint"))
}

func adminHandler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("admin endpoint"))
}

Finally, validate that no middleware or handler reads the request body before route matching. If you must inspect the body, wrap it with a tee reader or buffer to avoid desynchronization between authentication, routing, and downstream handlers.