Fix rate limiter bypass via client-controlled X-Forwarded-For (#280)

clientIP() keyed rate-limit buckets on the rightmost X-Forwarded-For
entry, assuming traefik appends the real client IP. The deployed ingress
does not rewrite XFF, so rotating the header gave a fresh bucket per
request (pentest H1: 8 creates with rotating XFF -> 6x201).

Now the bucket keys on the actual peer address (RemoteAddr) by default;
every client-supplied IP header is ignored. Deployments whose ingress
overwrites a client-IP header can opt in via PALETTE_TRUSTED_IP_HEADER
(e.g. CF-Connecting-IP behind Cloudflare) to restore per-client limits.

Adds tests: rotating XFF no longer resets the bucket; the trusted header
is honored only when explicitly configured.

README.md

@@ -70,6 +70,7 @@ go build -o palette ./cmd/palette
 | `PALETTE_ADMIN_KEY` | generated | Admin key; if unset a 32-char hex key is generated and persisted to `<db-dir>/admin-key` (0600) |
 | `PALETTE_DEFAULT_DARK` | dark on | Default dark mode for new visitors. Set `false`, `0`, or `off` to default to light mode. Visitors who toggle dark mode keep their choice in their browser. |
 | `PALETTE_UNLOCK_SECRET` | random per start | HMAC secret for password-unlock cookies. Set a fixed value to keep unlock sessions across restarts or across replicas. |
+| `PALETTE_TRUSTED_IP_HEADER` | unset | Name of a proxy-controlled client-IP header to key API rate limits on (e.g. `CF-Connecting-IP` when Cloudflare is the ingress; Cloudflare strips any client-supplied value). Unset: rate limits key on the peer address only, and all client-supplied IP headers (X-Forwarded-For, X-Real-Ip) are ignored. (#280) |
 
 An `/admin` page exists for runtime settings, protected by a key set at
 install (`PALETTE_ADMIN_KEY` env var) and resettable locally. See

internal/api/clientip.go

@@ -0,0 +1,57 @@
+// clientIP extracts the client IP for rate-limit keying.
+//
+// Trust boundary (issue #280): the bucket key MUST NOT come from any header a
+// client can influence. The previous rightmost-X-Forwarded-For scheme (#85)
+// assumed Traefik appends the real client IP, but the deployed ingress does
+// not rewrite XFF, so a client rotating its own XFF value got a fresh bucket
+// per request and the limit was unenforceable (pentest H1: 6x201 across 8
+// rotating-XFF creates).
+//
+// Default: key on the actual peer address (RemoteAddr) only. Behind any
+// reverse proxy this is the proxy's address, so all clients share one bucket
+// per endpoint — coarse, but safe.
+//
+// Proxy-honoring mode: a deployment in front of a proxy that OVERWRITES (not
+// appends to) a client-IP header can set PALETTE_TRUSTED_IP_HEADER (e.g.
+// CF-Connecting-IP when Cloudflare is the ingress; Cloudflare strips any
+// client-supplied value). The header is honored ONLY when explicitly
+// configured at startup, and X-Forwarded-For / X-Real-Ip are never trusted.
+package api
+
+import (
+	"net"
+	"net/http"
+	"sync"
+)
+
+var (
+	trustedIPMu     sync.RWMutex
+	trustedIPHeader string // empty = never trust any client-IP header
+)
+
+// SetTrustedIPHeader configures the single proxy-controlled header whose
+// value may key rate-limit buckets. Called at startup; tests may reset it.
+func SetTrustedIPHeader(name string) {
+	trustedIPMu.Lock()
+	defer trustedIPMu.Unlock()
+	trustedIPHeader = name
+}
+
+func getTrustedIPHeader() string {
+	trustedIPMu.RLock()
+	defer trustedIPMu.RUnlock()
+	return trustedIPHeader
+}
+
+func clientIP(r *http.Request) string {
+	if name := getTrustedIPHeader(); name != "" {
+		if v := r.Header.Get(name); v != "" {
+			return v
+		}
+	}
+	host := r.RemoteAddr
+	if h, _, err := net.SplitHostPort(r.RemoteAddr); err == nil {
+		host = h
+	}
+	return host
+}

internal/api/ratelimit.go

@@ -3,7 +3,6 @@ package api
 import (
 	"net/http"
 	"strconv"
-	"strings"
 	"sync"
 	"time"
 )
@@ -48,35 +47,6 @@ func (l *limiter) allow(key string, rate, burst float64) bool {
 	return true
 }
 
-// clientIP extracts the client IP for rate-limit keying (#85).
-//
-// Trust boundary: palette runs behind exactly ONE trusted reverse proxy
-// (Traefik in the k3s pod network). Traefik APPENDS the real client IP to
-// X-Forwarded-For, so the RIGHTMOST entry is the last value the trusted
-// proxy observed and is unspoofable by the client (a client-supplied fake
-// entry only lands on the LEFT and is ignored). This matches chi's
-// middleware.RealIP semantics for a single trusted proxy hop.
-//
-// Direct connections (no XFF header) fall back to RemoteAddr. Directly
-// reachable deployments must NOT expose the app to untrusted networks
-// without a proxy in front, or attackers could forge the rightmost entry.
-func clientIP(r *http.Request) string {
-	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
-		if i := strings.LastIndex(xff, ","); i >= 0 {
-			return strings.TrimSpace(xff[i+1:])
-		}
-		return strings.TrimSpace(xff)
-	}
-	if xr := r.Header.Get("X-Real-Ip"); xr != "" {
-		return strings.TrimSpace(xr)
-	}
-	host := r.RemoteAddr
-	if i := strings.LastIndex(host, ":"); i > 0 {
-		host = host[:i]
-	}
-	return host
-}
-
 var globalLimiter = newLimiter()
 
 // globalSettingsFn is set at startup; tests can point it at fixed settings.

internal/api/ratelimit_xff_test.go

@@ -1,85 +1,57 @@
 package api
 
-// Issue #85: the rate limit key must use the rightmost X-Forwarded-For entry
-// (appended by the trusted Traefik proxy), never the raw/leftmost header
-// value a client can forge. A spoofed FIRST XFF entry must not bypass the
-// limit or rotate buckets.
-
 import (
-	"bytes"
+	"fmt"
 	"net/http/httptest"
 	"testing"
 )
 
-func TestClientIPTakesRightmostXFF(t *testing.T) {
+func TestClientIPUsesRemoteAddrNotXFF(t *testing.T) {
-	r := httptest.NewRequest("POST", "/", nil)
+	SetTrustedIPHeader("")
-	r.RemoteAddr = "10.42.0.7:51000" // trusted Traefik pod
+	defer SetTrustedIPHeader("")
+	r := httptest.NewRequest("POST", "/api/pastes", nil)
+	r.RemoteAddr = "203.0.113.7:4432"
 	r.Header.Set("X-Forwarded-For", "1.2.3.4, 1.2.3.5, 203.0.113.9")
-	if got := clientIP(r); got != "203.0.113.9" {
-		t.Fatalf("clientIP = %q, want rightmost 203.0.113.9", got)
-	}
-}
-
-func TestClientIPXRealIPFallback(t *testing.T) {
-	r := httptest.NewRequest("POST", "/", nil)
-	r.RemoteAddr = "10.42.0.7:51000"
 	r.Header.Set("X-Real-Ip", "203.0.113.10")
-	if got := clientIP(r); got != "203.0.113.10" {
+	if got := clientIP(r); got != "203.0.113.7" {
-		t.Fatalf("clientIP = %q, want 203.0.113.10", got)
+		t.Fatalf("clientIP = %q, want peer 203.0.113.7", got)
 	}
 }
 
-func TestClientIPDirectFallback(t *testing.T) {
+func TestClientIPTrustedHeaderOnlyWhenConfigured(t *testing.T) {
-	r := httptest.NewRequest("POST", "/", nil)
+	SetTrustedIPHeader("")
-	r.RemoteAddr = "198.51.100.5:51000"
+	defer SetTrustedIPHeader("")
-	if got := clientIP(r); got != "198.51.100.5" {
+	r := httptest.NewRequest("POST", "/api/pastes", nil)
-		t.Fatalf("clientIP = %q, want 198.51.100.5", got)
+	r.RemoteAddr = "10.0.1.47:9999"
+	r.Header.Set("CF-Connecting-IP", "198.51.100.9")
+	if got := clientIP(r); got != "10.0.1.47" {
+		t.Fatalf("unconfigured: clientIP = %q, want peer 10.0.1.47", got)
+	}
+	SetTrustedIPHeader("CF-Connecting-IP")
+	if got := clientIP(r); got != "198.51.100.9" {
+		t.Fatalf("configured: clientIP = %q, want CF-Connecting-IP value", got)
 	}
 }
 
-// TestRateLimitSpoofedFirstXFFDoesNotBypass: an attacker rotating a fake
+// Issue #280: rotating X-Forwarded-For must NOT reset the bucket. Pentest
-// leftmost XFF entry stays limited on their real (rightmost) IP.
+// repro was 8 creates with rotating XFF -> 6x201.
-func TestRateLimitSpoofedFirstXFFDoesNotBypass(t *testing.T) {
+func TestRotatingXFFDoesNotResetBucket(t *testing.T) {
-	srv := newTestServer(t)
+	globalLimiter = newLimiter()
-	h := srv.routes()
+	defer SetTrustedIPHeader("")
-	for i := 0; i < 5; i++ {
+	SetTrustedIPHeader("")
-		req := httptest.NewRequest("POST", "/api/pastes", bytes.NewReader([]byte(`{"content":"hi"}`)))
+	s := defaultSettings(Config{}) // burst/limit defaults; any header values are ignored anyway
-		req.RemoteAddr = "10.42.0.7:51000"
+	var allowed, limited int
-		// each request spoofs a DIFFERENT leftmost entry
+	for i := 0; i < 8; i++ {
-		req.Header.Set("X-Forwarded-For", spoofN(i)+", 203.0.113.9")
+		r := httptest.NewRequest("POST", "/api/pastes", nil)
-		rr := httptest.NewRecorder()
+		r.RemoteAddr = "198.51.100.1:5000"
-		h.ServeHTTP(rr, req)
+		r.Header.Set("X-Forwarded-For", fmt.Sprintf("9.9.9.%d", i))
-		if rr.Code != 201 {
+		if rateLimitCreate(r, s) {
-			t.Fatalf("req %d: want 201, got %d", i, rr.Code)
+			allowed++
+		} else {
+			limited++
 		}
 	}
-	// 6th request, still the same real IP, new spoofed prefix: must 429
+	if float64(allowed) != s.RateLimitBurst || limited != 8-int(s.RateLimitBurst) {
-	req := httptest.NewRequest("POST", "/api/pastes", bytes.NewReader([]byte(`{"content":"hi"}`)))
+		t.Fatalf("rotating XFF: allowed=%d limited=%d, want allowed=%v (burst), limited=%d", allowed, limited, s.RateLimitBurst, 8-int(s.RateLimitBurst))
-	req.RemoteAddr = "10.42.0.7:51000"
-	req.Header.Set("X-Forwarded-For", "9.9.9.9, 203.0.113.9")
-	rr := httptest.NewRecorder()
-	h.ServeHTTP(rr, req)
-	if rr.Code != 429 {
-		t.Fatalf("spoofed 6th req: want 429, got %d", rr.Code)
-	}
-}
-
-func spoofN(i int) string {
-	return "1.2.3." + string(rune('0'+i))
-}
-
-// Distinct real IPs must still get distinct buckets (no over-limiting).
-func TestRateLimitDistinctRightmostIPsIndependent(t *testing.T) {
-	srv := newTestServer(t)
-	h := srv.routes()
-	for _, ip := range []string{"203.0.113.20", "203.0.113.21"} {
-		req := httptest.NewRequest("POST", "/api/pastes", bytes.NewReader([]byte(`{"content":"hi"}`)))
-		req.RemoteAddr = "10.42.0.7:51000"
-		req.Header.Set("X-Forwarded-For", "6.6.6.6, "+ip)
-		rr := httptest.NewRecorder()
-		h.ServeHTTP(rr, req)
-		if rr.Code != 201 {
-			t.Fatalf("ip %s: want 201, got %d", ip, rr.Code)
-		}
 	}
 }

internal/api/server.go

@@ -28,6 +28,10 @@ type Config struct {
 	DBPath       string
 	MaxTextBytes int64
 	MaxItemBytes int64
+	// TrustedIPHeader optionally names a proxy-controlled client-IP header
+	// (e.g. CF-Connecting-IP behind Cloudflare) to key rate limits on. Empty
+	// (default) keys on the peer address only. See clientip.go (#280).
+	TrustedIPHeader string
 }
 
 type apiServer struct {
@@ -39,6 +43,7 @@ type apiServer struct {
 }
 
 func NewServer(st *store.Store, cfg Config, ui *web.UI, ss *settingsStore, adminKey string) *apiServer {
+	SetTrustedIPHeader(cfg.TrustedIPHeader) // #280
 	return &apiServer{store: st, cfg: cfg, ui: ui, settings: ss, adminKey: adminKey}
 }