Gate forwarded-header trust behind trusted-proxy config (closes #88)

Every rate limiter keyed on httprate.KeyByRealIP, which believes
True-Client-IP, X-Real-IP and the first X-Forwarded-For entry from
any peer. A client could therefore mint a fresh bucket per request
by rotating a spoofed header, or drain another client's bucket by
claiming its address, which left the receiver, login and password
change limits with no value against a deliberate attacker.

The receiver, login and password change limiters now share one key
function: the connection's own address, unless the direct peer is
inside a network listed in the new TRUSTED_PROXIES CIDR list, in
which case the forwarded client address is used. The list is empty
by default, so nothing is trusted until an operator names their
proxy; a set-but-unparseable value aborts startup, matching the
handling of the other parsed variables.

X-Forwarded-For is the only forwarded header read, from any peer.
Reverse proxies append to it but pass other client headers through
verbatim, so believing a single-valued X-Real-IP or True-Client-IP
would hand a client behind the trusted proxy a fresh bucket per
request - the same bypass, inside the deployment TRUSTED_PROXIES
exists to serve. Within a trusted request the chain is walked right
to left, since the rightmost entry is the one the nearest proxy
appended, and the first hop that is not itself a trusted proxy is
taken as the client. A hop that is not a bare address - ip:port, a
bracketed IPv6 literal, the token unknown - ends the walk and the
peer address is used, rather than continuing left into entries the
client controls.

Trusted-proxy prefixes written in IPv4-mapped form are unmapped at
parse time, since peer addresses are unmapped before matching and
such a prefix would otherwise silently never match.

Also folds in two cleanups from the same review: the 429 responder
shared by all three limiters is extracted, and the
RECEIVER_RATE_LIMIT error-path tests now assert that the failure
names the variable and wraps ErrNonPositiveValue rather than only
that some error occurred.

internal/middleware/ratelimit_test.go

@@ -2,9 +2,11 @@ package middleware_test
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"net/netip"
 	"os"
 	"testing"
 
@@ -182,11 +184,22 @@ func TestLoginRateLimit_IndependentPerIP(t *testing.T) {
 	)
 }
 
-// receiverLimitedHandler builds a ReceiverRateLimit-wrapped
-// handler with the given per-minute limit.
-func receiverLimitedHandler(
-	t *testing.T, limit int,
-) http.Handler {
+// okHandler is the terminal handler the limiter middleware wraps
+// in these tests: it answers 200 to anything that reaches it.
+func okHandler() http.Handler {
+	return http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		},
+	)
+}
+
+// rateLimitMiddleware builds a Middleware around cfg, whose
+// TrustedProxies field is what the rate limit key function gates
+// forwarded-header trust on.
+func rateLimitMiddleware(
+	t *testing.T, cfg *config.Config,
+) *middleware.Middleware {
 	t.Helper()
 
 	log := slog.New(slog.NewTextHandler(
@@ -194,17 +207,53 @@ func receiverLimitedHandler(
 		&slog.HandlerOptions{Level: slog.LevelDebug},
 	))
 
-	m := middleware.NewForTest(
-		log,
-		&config.Config{ReceiverRateLimit: limit},
-		nil,
+	return middleware.NewForTest(log, cfg, nil)
+}
+
+// trustedProxies parses CIDR strings for a test Config.
+func trustedProxies(cidrs ...string) []netip.Prefix {
+	prefixes := make([]netip.Prefix, 0, len(cidrs))
+	for _, cidr := range cidrs {
+		prefixes = append(prefixes, netip.MustParsePrefix(cidr))
+	}
+
+	return prefixes
+}
+
+// postWithHeaders sends one POST to the handler from peer with the
+// given headers set and returns the recorder.
+func postWithHeaders(
+	handler http.Handler,
+	peer, path string,
+	headers map[string]string,
+) *httptest.ResponseRecorder {
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodPost, path, nil,
+	)
+	req.RemoteAddr = peer
+
+	for name, value := range headers {
+		req.Header.Set(name, value)
+	}
+
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, req)
+
+	return w
+}
+
+// receiverLimitedHandler builds a ReceiverRateLimit-wrapped
+// handler with the given per-minute limit and no trusted proxies.
+func receiverLimitedHandler(
+	t *testing.T, limit int,
+) http.Handler {
+	t.Helper()
+
+	m := rateLimitMiddleware(
+		t, &config.Config{ReceiverRateLimit: limit},
 	)
 
-	return m.ReceiverRateLimit()(http.HandlerFunc(
-		func(w http.ResponseWriter, _ *http.Request) {
-			w.WriteHeader(http.StatusOK)
-		},
-	))
+	return m.ReceiverRateLimit()(okHandler())
 }
 
 // receiverPost sends one POST to the handler from the given IP
@@ -311,3 +360,252 @@ func TestReceiverRateLimit_CountsEveryMethod(t *testing.T) {
 		"a GET over the limit must be rate-limited",
 	)
 }
+
+const (
+	loginPath  = "/pages/login"
+	headerXFF  = "X-Forwarded-For"
+	headerReal = "X-Real-IP"
+	headerTrue = "True-Client-IP"
+)
+
+// assertSharedBucket drives the login limiter from peer with the
+// trusted-proxy set proxies, sending one more request than the limit
+// allows and varying the headers on each with headers(i). Every
+// request must land in the same bucket, so the last one is rejected:
+// if any of the varying header values reached the key, the run would
+// have minted fresh buckets and nothing would be rejected.
+func assertSharedBucket(
+	t *testing.T,
+	proxies []netip.Prefix,
+	peer string,
+	headers func(i int) map[string]string,
+	msg string,
+) {
+	t.Helper()
+
+	m := rateLimitMiddleware(
+		t, &config.Config{TrustedProxies: proxies},
+	)
+	handler := m.LoginRateLimit()(okHandler())
+
+	for i := range middleware.LoginRateLimitConst {
+		w := postWithHeaders(handler, peer, loginPath, headers(i))
+		assert.Equal(
+			t, http.StatusOK, w.Code,
+			"request %d should pass", i,
+		)
+	}
+
+	w := postWithHeaders(
+		handler, peer, loginPath,
+		headers(middleware.LoginRateLimitConst),
+	)
+	assert.Equal(t, http.StatusTooManyRequests, w.Code, msg)
+}
+
+// TestRateLimitKey_SpoofedForwardedFromUntrustedPeer is the test
+// this gating exists for: with no trusted proxies configured (the
+// default), a client that rotates a forwarded header on every
+// request must stay in one bucket. If forwarded headers were
+// trusted unconditionally, each spoofed value would mint a fresh
+// bucket and the limit would stop no one.
+func TestRateLimitKey_SpoofedForwardedFromUntrustedPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	for _, header := range []string{
+		headerXFF, headerReal, headerTrue,
+	} {
+		t.Run(header, func(t *testing.T) {
+			t.Parallel()
+
+			assertSharedBucket(
+				t, nil, "203.0.113.9:44444",
+				func(i int) map[string]string {
+					return map[string]string{
+						header: fmt.Sprintf(
+							"198.51.100.%d", i+1,
+						),
+					}
+				},
+				"a spoofed "+header+" from an untrusted peer "+
+					"must not mint a fresh bucket",
+			)
+		})
+	}
+}
+
+// TestRateLimitKey_SingleValuedHeadersIgnoredFromTrustedPeer is the
+// regression test for the bypass hiding inside the trusted case.
+// Real reverse proxies (nginx, HAProxy, Caddy, ALB) set only
+// X-Forwarded-For and pass every other client header through
+// verbatim, so a client behind the configured proxy can send its own
+// X-Real-IP or True-Client-IP. Reading either would hand that client
+// a fresh bucket per request from inside exactly the deployment
+// TRUSTED_PROXIES exists to serve, so neither header is read at all.
+func TestRateLimitKey_SingleValuedHeadersIgnoredFromTrustedPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	for _, header := range []string{headerReal, headerTrue} {
+		t.Run(header, func(t *testing.T) {
+			t.Parallel()
+
+			assertSharedBucket(
+				t, trustedProxies("10.0.0.0/8"),
+				"10.0.0.1:44444",
+				func(i int) map[string]string {
+					return map[string]string{
+						header: fmt.Sprintf(
+							"198.51.100.%d", i+1,
+						),
+					}
+				},
+				header+" from a trusted peer must not mint a "+
+					"fresh bucket: only X-Forwarded-For is read",
+			)
+		})
+	}
+}
+
+// TestRateLimitKey_MalformedRightmostHopFallsBackToPeer covers the
+// other end of the chain walk. The rightmost X-Forwarded-For entry
+// is the one the trusted proxy appended; if it cannot be read as an
+// address the chain is not the shape the walk assumes, and every
+// entry to its left may have come from the client. The walk must
+// stop and fall back to the peer rather than select one of them.
+func TestRateLimitKey_MalformedRightmostHopFallsBackToPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	// Forms seen in the wild: host:port (Azure Application
+	// Gateway, IIS ARR), a bracketed IPv6 literal, and the
+	// RFC 7239 placeholder token.
+	for _, tail := range []string{
+		"198.51.100.7:1234", "[2001:db8::1]", "unknown",
+	} {
+		t.Run(tail, func(t *testing.T) {
+			t.Parallel()
+
+			assertSharedBucket(
+				t, trustedProxies("10.0.0.0/8"),
+				"10.0.0.1:44444",
+				func(i int) map[string]string {
+					return map[string]string{
+						headerXFF: fmt.Sprintf(
+							"9.9.9.%d, %s", i+1, tail,
+						),
+					}
+				},
+				"an unparseable rightmost hop must fall back "+
+					"to the peer address, not select a "+
+					"client-controlled entry",
+			)
+		})
+	}
+}
+
+// TestRateLimitKey_ForwardedHonouredFromTrustedPeer checks the
+// other half: when the direct peer is a configured trusted proxy,
+// the forwarded client address is what buckets are keyed on, so
+// one sender behind the proxy cannot exhaust another's limit.
+func TestRateLimitKey_ForwardedHonouredFromTrustedPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	m := rateLimitMiddleware(t, &config.Config{
+		TrustedProxies: trustedProxies("10.0.0.0/8"),
+	})
+	handler := m.LoginRateLimit()(okHandler())
+
+	const peer = "10.0.0.1:44444"
+
+	first := map[string]string{headerXFF: "198.51.100.7"}
+
+	for range middleware.LoginRateLimitConst {
+		postWithHeaders(handler, peer, loginPath, first)
+	}
+
+	w := postWithHeaders(handler, peer, loginPath, first)
+	assert.Equal(
+		t, http.StatusTooManyRequests, w.Code,
+		"the forwarded client's own bucket must fill up",
+	)
+
+	w = postWithHeaders(
+		handler, peer, loginPath,
+		map[string]string{headerXFF: "198.51.100.8"},
+	)
+	assert.Equal(
+		t, http.StatusOK, w.Code,
+		"a forwarded header from a trusted peer must be honoured",
+	)
+}
+
+// TestRateLimitKey_ChainWalkSkipsClientPrepended covers the
+// residual spoofing route behind a trusted proxy: the client
+// controls the leftmost X-Forwarded-For entries, so the key is the
+// rightmost hop that is not itself trusted. Rotating the prepended
+// entry must not create new buckets.
+func TestRateLimitKey_ChainWalkSkipsClientPrepended(t *testing.T) {
+	t.Parallel()
+
+	assertSharedBucket(
+		t, trustedProxies("10.0.0.0/8"), "10.0.0.1:44444",
+		func(i int) map[string]string {
+			return map[string]string{
+				headerXFF: fmt.Sprintf(
+					"9.9.9.%d, 198.51.100.7, 10.0.0.2", i+1,
+				),
+			}
+		},
+		"a client-prepended X-Forwarded-For entry must not "+
+			"mint a fresh bucket",
+	)
+}
+
+// TestReceiverRateLimit_IgnoresForwardedFromUntrustedPeer proves
+// the receiver limiter uses the same gated key function as the
+// POST limiters.
+func TestReceiverRateLimit_IgnoresForwardedFromUntrustedPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	const (
+		limit = 3
+		peer  = "203.0.113.10:44444"
+		path  = "/webhook/uuid-d"
+	)
+
+	handler := receiverLimitedHandler(t, limit)
+
+	for i := range limit {
+		w := postWithHeaders(
+			handler, peer, path,
+			map[string]string{
+				headerXFF: fmt.Sprintf(
+					"198.51.100.%d", i+1,
+				),
+			},
+		)
+		assert.Equal(
+			t, http.StatusOK, w.Code,
+			"request %d should pass", i,
+		)
+	}
+
+	w := postWithHeaders(
+		handler, peer, path,
+		map[string]string{headerXFF: "198.51.100.200"},
+	)
+	assert.Equal(
+		t, http.StatusTooManyRequests, w.Code,
+		"a spoofed X-Forwarded-For from an untrusted peer must "+
+			"not mint a fresh receiver bucket",
+	)
+}