Rate-limit the public webhook receiver endpoint (closes #64)

The public receiver /webhook/{uuid} had no rate limiting: anyone
who learns an entrypoint UUID can flood it, inflating the
per-webhook database and the delivery queue.

Add a dedicated limit scoped to the receiver route, keyed per
client IP per request path (the path contains the entrypoint
UUID), so one misbehaving sender is throttled without affecting
other senders of the same entrypoint or other entrypoints.
Requests over the limit get a 429; httprate adds the Retry-After
header per RFC 6585. IP extraction honours X-Forwarded-For,
X-Real-IP, and True-Client-IP for reverse-proxy deployments.

The limit is RECEIVER_RATE_LIMIT requests per minute, default
120, parsed with the existing envPositiveInt strict parser: a
set-but-unparseable or non-positive value aborts startup rather
than silently falling back to the default.

Also update the README env table and Rate Limiting design
section.

internal/config/config.go

@@ -35,6 +35,13 @@ const (
 	// authenticated activity before it expires.
 	defaultSessionIdleTimeout = 24 * time.Hour
 
+	// defaultReceiverRateLimit is the default number of requests
+	// per minute each client IP may send to a single webhook
+	// receiver entrypoint. Generous for legitimate webhook
+	// senders while bounding abuse of the one unauthenticated,
+	// internet-exposed endpoint.
+	defaultReceiverRateLimit = 120
+
 	// maxPort is the highest valid TCP port number. The lower
 	// bound (at least 1) is enforced by envPositiveInt.
 	maxPort = 65535
@@ -79,6 +86,10 @@ type Config struct {
 	// which a session expires. Non-positive disables idle expiry.
 	SessionIdleTimeout time.Duration
 
+	// ReceiverRateLimit is the number of requests per minute each
+	// client IP may send to a single webhook receiver entrypoint.
+	ReceiverRateLimit int
+
 	params *ConfigParams
 	log    *slog.Logger
 }
@@ -263,6 +274,14 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
+	receiverRateLimit, err := envPositiveInt(
+		"RECEIVER_RATE_LIMIT",
+		defaultReceiverRateLimit,
+	)
+	if err != nil {
+		return nil, err
+	}
+
 	return &Config{
 		DataDir:                envString("DATA_DIR"),
 		Debug:                  debug,
@@ -274,6 +293,7 @@ func loadFromEnv() (*Config, error) {
 		SentryDSN:              envString("SENTRY_DSN"),
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
+		ReceiverRateLimit:      receiverRateLimit,
 	}, nil
 }
 
@@ -314,6 +334,7 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"maintenanceMode", s.MaintenanceMode,
 		"dataDir", s.DataDir,
 		"retentionSweepInterval", s.RetentionSweepInterval.String(),
+		"receiverRateLimit", s.ReceiverRateLimit,
 		"hasSentryDSN", s.SentryDSN != "",
 		"hasMetricsAuth",
 		s.MetricsUsername != "" && s.MetricsPassword != "",

internal/config/config_test.go

@@ -14,6 +14,14 @@ import (
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
+// Shared subtest names for the env-parsing tables below, which all
+// exercise the same three cases against different variables.
+const (
+	caseUnsetUsesDefault = "unset uses default"
+	caseValidValueParsed = "valid value is parsed"
+	caseUnparseableFails = "unparseable value fails startup"
+)
+
 func TestEnvironmentConfig(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -130,18 +138,18 @@ func TestRetentionSweepInterval(t *testing.T) {
 		expected    time.Duration
 	}{
 		{
-			name:     "unset uses default",
+			name:     caseUnsetUsesDefault,
 			set:      false,
 			expected: time.Hour,
 		},
 		{
-			name:     "valid value is parsed",
+			name:     caseValidValueParsed,
 			set:      true,
 			value:    "15m",
 			expected: 15 * time.Minute,
 		},
 		{
-			name:        "unparseable value fails startup",
+			name:        caseUnparseableFails,
 			set:         true,
 			value:       "not-a-duration",
 			expectError: true,
@@ -172,7 +180,7 @@ func TestRetentionSweepInterval(t *testing.T) {
 }
 
 // expectStartupError asserts that fx refuses to build the app,
-// which is what a set-but-unparseable duration must cause.
+// which is what a set-but-invalid environment value must cause.
 func expectStartupError(t *testing.T) {
 	t.Helper()
 
@@ -226,18 +234,18 @@ func TestSessionIdleTimeout(t *testing.T) {
 		expected    time.Duration
 	}{
 		{
-			name:     "unset uses default",
+			name:     caseUnsetUsesDefault,
 			set:      false,
 			expected: 24 * time.Hour,
 		},
 		{
-			name:     "valid value is parsed",
+			name:     caseValidValueParsed,
 			set:      true,
 			value:    "30m",
 			expected: 30 * time.Minute,
 		},
 		{
-			name:        "unparseable value fails startup",
+			name:        caseUnparseableFails,
 			set:         true,
 			value:       "not-a-duration",
 			expectError: true,
@@ -336,3 +344,91 @@ func TestDefaultDataDir(t *testing.T) {
 		})
 	}
 }
+
+func TestReceiverRateLimit(t *testing.T) {
+	tests := []struct {
+		name        string
+		set         bool
+		value       string
+		expectError bool
+		expected    int
+	}{
+		{
+			name:     caseUnsetUsesDefault,
+			set:      false,
+			expected: 120,
+		},
+		{
+			name:     caseValidValueParsed,
+			set:      true,
+			value:    "30",
+			expected: 30,
+		},
+		{
+			name:        caseUnparseableFails,
+			set:         true,
+			value:       "not-a-number",
+			expectError: true,
+		},
+		{
+			name:        "zero fails startup",
+			set:         true,
+			value:       "0",
+			expectError: true,
+		},
+		{
+			name:        "negative fails startup",
+			set:         true,
+			value:       "-5",
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
+
+			if tt.set {
+				t.Setenv("RECEIVER_RATE_LIMIT", tt.value)
+			} else {
+				require.NoError(t, os.Unsetenv(
+					"RECEIVER_RATE_LIMIT",
+				))
+			}
+
+			if tt.expectError {
+				expectStartupError(t)
+			} else {
+				testReceiverRateLimitSuccess(t, tt.expected)
+			}
+		})
+	}
+}
+
+func testReceiverRateLimitSuccess(
+	t *testing.T,
+	expected int,
+) {
+	t.Helper()
+
+	var cfg *config.Config
+
+	app := fxtest.New(
+		t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+		),
+		fx.Populate(&cfg),
+	)
+	require.NoError(t, app.Err())
+
+	app.RequireStart()
+
+	defer app.RequireStop()
+
+	assert.Equal(t, expected, cfg.ReceiverRateLimit)
+}

internal/middleware/ratelimit.go

@@ -24,6 +24,11 @@ const (
 	// passwordChangeRateInterval is the time window for the
 	// password change rate limit.
 	passwordChangeRateInterval = 1 * time.Minute
+
+	// receiverRateInterval is the time window for the webhook
+	// receiver rate limit. The configured limit is expressed in
+	// requests per minute.
+	receiverRateInterval = 1 * time.Minute
 )
 
 // LoginRateLimit returns middleware that enforces per-IP rate
@@ -105,3 +110,37 @@ func (m *Middleware) postRateLimit(
 		})
 	}
 }
+
+// ReceiverRateLimit returns middleware that rate-limits the
+// public webhook receiver endpoint per client IP per request
+// path (the path contains the entrypoint UUID, so each sender
+// is limited per entrypoint without affecting other senders or
+// other entrypoints). The limit is Config.ReceiverRateLimit
+// requests per minute. Requests over the limit receive a 429;
+// httprate adds the Retry-After header (RFC 6585). IP
+// extraction honours X-Forwarded-For, X-Real-IP, and
+// True-Client-IP headers for reverse-proxy setups.
+func (m *Middleware) ReceiverRateLimit() func(http.Handler) http.Handler {
+	return httprate.Limit(
+		m.params.Config.ReceiverRateLimit,
+		receiverRateInterval,
+		httprate.WithKeyFuncs(
+			httprate.KeyByRealIP,
+			httprate.KeyByEndpoint,
+		),
+		httprate.WithLimitHandler(http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				m.log.Warn(
+					"webhook receiver rate limit exceeded",
+					"path", r.URL.Path,
+				)
+				http.Error(
+					w,
+					"Too many requests. "+
+						"Please slow down.",
+					http.StatusTooManyRequests,
+				)
+			},
+		)),
+	)
+}

internal/middleware/ratelimit_test.go

@@ -2,8 +2,10 @@ package middleware_test
 
 import (
 	"context"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -179,3 +181,94 @@ func TestLoginRateLimit_IndependentPerIP(t *testing.T) {
 		"different IP should not be affected",
 	)
 }
+
+// receiverLimitedHandler builds a ReceiverRateLimit-wrapped
+// handler with the given per-minute limit.
+func receiverLimitedHandler(
+	t *testing.T, limit int,
+) http.Handler {
+	t.Helper()
+
+	log := slog.New(slog.NewTextHandler(
+		os.Stderr,
+		&slog.HandlerOptions{Level: slog.LevelDebug},
+	))
+
+	m := middleware.NewForTest(
+		log,
+		&config.Config{ReceiverRateLimit: limit},
+		nil,
+	)
+
+	return m.ReceiverRateLimit()(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		},
+	))
+}
+
+// receiverPost sends one POST to the handler from the given IP
+// and path and returns the recorder.
+func receiverPost(
+	handler http.Handler, ip, path string,
+) *httptest.ResponseRecorder {
+	req := httptest.NewRequestWithContext(
+		context.Background(),
+		http.MethodPost, path, nil,
+	)
+	req.RemoteAddr = ip
+
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, req)
+
+	return w
+}
+
+func TestReceiverRateLimit_LimitsPerIPAndPath(t *testing.T) {
+	t.Parallel()
+
+	const limit = 3
+
+	handler := receiverLimitedHandler(t, limit)
+
+	// The first limit requests from one IP to one entrypoint
+	// pass.
+	for i := range limit {
+		w := receiverPost(
+			handler, "9.9.9.9:1234", "/webhook/uuid-a",
+		)
+		assert.Equal(
+			t, http.StatusOK, w.Code,
+			"request %d should pass", i,
+		)
+	}
+
+	// The next request over the limit is rejected with a 429
+	// carrying a Retry-After header.
+	w := receiverPost(
+		handler, "9.9.9.9:1234", "/webhook/uuid-a",
+	)
+	assert.Equal(t, http.StatusTooManyRequests, w.Code)
+	assert.NotEmpty(
+		t, w.Header().Get("Retry-After"),
+		"429 must carry a Retry-After header",
+	)
+
+	// The same IP is not limited on a different entrypoint.
+	w = receiverPost(
+		handler, "9.9.9.9:1234", "/webhook/uuid-b",
+	)
+	assert.Equal(
+		t, http.StatusOK, w.Code,
+		"a different entrypoint must not be affected",
+	)
+
+	// A different IP is not limited on the same entrypoint.
+	w = receiverPost(
+		handler, "8.8.8.8:1234", "/webhook/uuid-a",
+	)
+	assert.Equal(
+		t, http.StatusOK, w.Code,
+		"a different client IP must not be affected",
+	)
+}

internal/server/routes.go

@@ -162,7 +162,7 @@ func (s *Server) setupSourceRoutes() {
 }
 
 func (s *Server) setupWebhookRoutes() {
-	s.router.HandleFunc(
+	s.router.With(s.mw.ReceiverRateLimit()).HandleFunc(
 		"/webhook/{uuid}",
 		s.h.HandleWebhook(),
 	)