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. Within a trusted request
X-Forwarded-For is walked right to left and the first hop that is
not itself a trusted proxy wins, so client-prepended entries cannot
be selected.

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.

README.md

@@ -95,6 +95,28 @@ TTY detection, and security headers are always applied.
 | `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint | `120` |
+| `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted | `""` (none) |
+
+#### Trusted proxies
+
+`TRUSTED_PROXIES` is a comma-separated list of CIDR blocks (a bare
+address such as `10.0.0.1` is accepted and treated as a single host),
+for example `10.0.0.0/8, 192.168.1.7, 2001:db8::/32`. It decides whose
+`X-Forwarded-For`, `X-Real-IP`, and `True-Client-IP` headers the rate
+limiters believe.
+
+Forwarded headers are honoured **only** when the connecting peer is
+inside one of these blocks; for every other peer the client identity is
+the connection's own address and the headers are ignored. The default
+is the empty list, which trusts nobody — anything else would let any
+client pick its own rate limit bucket, minting a fresh one per request
+or draining someone else's. Set it to the address of your reverse
+proxy, and to nothing wider.
+
+Within a trusted request, `X-Forwarded-For` is read right to left and
+the first hop that is not itself a trusted proxy wins, so entries a
+client prepended before reaching the proxy cannot be selected. A set
+but unparseable value aborts startup.
 
 Sessions are bounded by two independent clocks, and end at whichever
 one runs out first:
@@ -124,8 +146,9 @@ fatal configuration error: webhooker logs the offending variable and
 its value and refuses to start, rather than silently running with a
 substituted default. `PORT=eighty`, `DEBUG=ture`, and
 `RETENTION_SWEEP_INTERVAL=1 hour` all abort startup. `PORT` must
-additionally be a number in the range 1–65535, and
+additionally be a number in the range 1–65535,
-`RECEIVER_RATE_LIMIT` must be at least 1.
+`RECEIVER_RATE_LIMIT` must be at least 1, and every entry in
+`TRUSTED_PROXIES` must be a CIDR block or a bare IP address.
 
 Boolean variables (`DEBUG`, `MAINTENANCE_MODE`) accept exactly the
 spellings Go's `strconv.ParseBool` accepts — `1`, `t`, `T`, `TRUE`,
@@ -802,6 +825,16 @@ legitimate webhook senders). Requests over the limit receive HTTP 429
 with a `Retry-After` header. A set-but-invalid `RECEIVER_RATE_LIMIT`
 value aborts startup rather than silently falling back to the default.
 
+Every limiter here — receiver, login, and password change — identifies
+the client the same way, through one shared key function: the
+connection's own address, unless the peer is listed in
+`TRUSTED_PROXIES`, in which case the forwarded client address is used
+instead. See [Trusted proxies](#trusted-proxies). Deployed without that
+variable set, a client behind a reverse proxy shares one bucket with
+every other client behind the same proxy, which is the safe direction
+to be wrong in: set `TRUSTED_PROXIES` to the proxy's address to get
+per-client limits back.
+
 Finer-grained per-webhook rate limits (configured in the web UI and
 enforced in the webhook handler) can layer on top of this env-level
 abuse limit later; they are tracked as future work.

internal/config/config.go

@@ -5,8 +5,10 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/netip"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	"go.uber.org/fx"
@@ -59,6 +61,11 @@ var ErrNonPositiveValue = errors.New("value must be positive")
 // TCP port number is set above the valid port range.
 var ErrInvalidPort = errors.New("invalid port")
 
+// ErrInvalidCIDR is returned when an environment variable holding a
+// list of CIDR blocks contains an entry that is neither a CIDR block
+// nor a bare IP address.
+var ErrInvalidCIDR = errors.New("invalid CIDR")
+
 //nolint:revive // ConfigParams is a standard fx naming convention.
 type ConfigParams struct {
 	fx.In
@@ -90,6 +97,14 @@ type Config struct {
 	// client IP may send to a single webhook receiver entrypoint.
 	ReceiverRateLimit int
 
+	// TrustedProxies is the set of networks whose members are
+	// allowed to speak for the client with forwarded headers
+	// (X-Forwarded-For, X-Real-IP, True-Client-IP). It is empty
+	// unless TRUSTED_PROXIES is set, and empty means no peer is
+	// trusted: forwarded headers are then ignored entirely and
+	// clients are identified by the connection's own address.
+	TrustedProxies []netip.Prefix
+
 	params *ConfigParams
 	log    *slog.Logger
 }
@@ -212,6 +227,60 @@ func envDuration(
 	return d, nil
 }
 
+// parseCIDR parses one trusted-proxy list entry, which may be a
+// CIDR block ("10.0.0.0/8") or a bare address ("10.0.0.1", treated
+// as a single-host block).
+func parseCIDR(entry string) (netip.Prefix, error) {
+	if strings.Contains(entry, "/") {
+		prefix, err := netip.ParsePrefix(entry)
+		if err != nil {
+			return netip.Prefix{}, err //nolint:wrapcheck // wrapped by caller
+		}
+
+		return prefix.Masked(), nil
+	}
+
+	addr, err := netip.ParseAddr(entry)
+	if err != nil {
+		return netip.Prefix{}, err //nolint:wrapcheck // wrapped by caller
+	}
+
+	return netip.PrefixFrom(addr.Unmap(), addr.Unmap().BitLen()), nil
+}
+
+// envPrefixList returns the value of the named environment variable
+// parsed as a comma-separated list of CIDR blocks (bare addresses
+// allowed). An unset, empty, or blank value yields an empty list. A
+// set value containing an unparseable entry is a hard error naming
+// the key and the bad entry, so startup fails loudly rather than
+// silently running with a list the operator did not intend.
+func envPrefixList(key string) ([]netip.Prefix, error) {
+	v := strings.TrimSpace(os.Getenv(key))
+	if v == "" {
+		return nil, nil
+	}
+
+	var prefixes []netip.Prefix
+
+	for entry := range strings.SplitSeq(v, ",") {
+		entry = strings.TrimSpace(entry)
+		if entry == "" {
+			continue
+		}
+
+		prefix, err := parseCIDR(entry)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"%w: %s: %q: %w", ErrInvalidCIDR, key, entry, err,
+			)
+		}
+
+		prefixes = append(prefixes, prefix)
+	}
+
+	return prefixes, nil
+}
+
 // resolveEnvironment reads WEBHOOKER_ENVIRONMENT, defaulting to
 // dev, and rejects unrecognised values.
 func resolveEnvironment() (string, error) {
@@ -282,6 +351,11 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
+	trustedProxies, err := envPrefixList("TRUSTED_PROXIES")
+	if err != nil {
+		return nil, err
+	}
+
 	return &Config{
 		DataDir:                envString("DATA_DIR"),
 		Debug:                  debug,
@@ -294,6 +368,7 @@ func loadFromEnv() (*Config, error) {
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
 		ReceiverRateLimit:      receiverRateLimit,
+		TrustedProxies:         trustedProxies,
 	}, nil
 }
 
@@ -335,6 +410,7 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"dataDir", s.DataDir,
 		"retentionSweepInterval", s.RetentionSweepInterval.String(),
 		"receiverRateLimit", s.ReceiverRateLimit,
+		"trustedProxies", len(s.TrustedProxies),
 		"hasSentryDSN", s.SentryDSN != "",
 		"hasMetricsAuth",
 		s.MetricsUsername != "" && s.MetricsPassword != "",

internal/config/config_test.go

@@ -179,9 +179,10 @@ func TestRetentionSweepInterval(t *testing.T) {
 	}
 }
 
-// expectStartupError asserts that fx refuses to build the app,
+// startupError builds the app config.New belongs to and returns
-// which is what a set-but-invalid environment value must cause.
+// the error fx reports, which is non-nil whenever an environment
-func expectStartupError(t *testing.T) {
+// value is set but invalid.
+func startupError(t *testing.T) error {
 	t.Helper()
 
 	var cfg *config.Config
@@ -196,7 +197,33 @@ func expectStartupError(t *testing.T) {
 		fx.Populate(&cfg),
 	)
 
-	assert.Error(t, app.Err())
+	return app.Err()
+}
+
+// expectStartupError asserts that fx refuses to build the app,
+// which is what a set-but-invalid environment value must cause.
+func expectStartupError(t *testing.T) {
+	t.Helper()
+
+	assert.Error(t, startupError(t))
+}
+
+// expectStartupErrorFor asserts that startup fails, that the error
+// names the offending variable so an operator can find it, and,
+// when sentinel is non-nil, that it wraps that sentinel.
+func expectStartupErrorFor(
+	t *testing.T,
+	key string,
+	sentinel error,
+) {
+	t.Helper()
+
+	err := startupError(t)
+	require.ErrorContains(t, err, key)
+
+	if sentinel != nil {
+		require.ErrorIs(t, err, sentinel)
+	}
 }
 
 func testRetentionSweepIntervalSuccess(
@@ -351,7 +378,11 @@ func TestReceiverRateLimit(t *testing.T) {
 		set         bool
 		value       string
 		expectError bool
-		expected    int
+		// sentinel, when set, must be wrapped by the startup
+		// error; every error case must additionally name the
+		// variable in its message.
+		sentinel error
+		expected int
 	}{
 		{
 			name:     caseUnsetUsesDefault,
@@ -375,12 +406,14 @@ func TestReceiverRateLimit(t *testing.T) {
 			set:         true,
 			value:       "0",
 			expectError: true,
+			sentinel:    config.ErrNonPositiveValue,
 		},
 		{
 			name:        "negative fails startup",
 			set:         true,
 			value:       "-5",
 			expectError: true,
+			sentinel:    config.ErrNonPositiveValue,
 		},
 	}
 
@@ -399,7 +432,9 @@ func TestReceiverRateLimit(t *testing.T) {
 			}
 
 			if tt.expectError {
-				expectStartupError(t)
+				expectStartupErrorFor(
+					t, "RECEIVER_RATE_LIMIT", tt.sentinel,
+				)
 			} else {
 				testReceiverRateLimitSuccess(t, tt.expected)
 			}
@@ -432,3 +467,107 @@ func testReceiverRateLimitSuccess(
 
 	assert.Equal(t, expected, cfg.ReceiverRateLimit)
 }
+
+func TestTrustedProxies(t *testing.T) {
+	tests := []struct {
+		name        string
+		set         bool
+		value       string
+		expectError bool
+		expected    []string
+	}{
+		{
+			// The default must be "trust nobody": an empty list
+			// means forwarded headers are ignored, never that
+			// every peer may speak for the client.
+			name:     caseUnsetUsesDefault,
+			set:      false,
+			expected: []string{},
+		},
+		{
+			name:     "blank value trusts nothing",
+			set:      true,
+			value:    "  ",
+			expected: []string{},
+		},
+		{
+			name:  caseValidValueParsed,
+			set:   true,
+			value: "10.0.0.0/8, 192.168.1.7 ,2001:db8::/32",
+			expected: []string{
+				"10.0.0.0/8", "192.168.1.7/32", "2001:db8::/32",
+			},
+		},
+		{
+			name:     "host bits are masked off",
+			set:      true,
+			value:    "10.1.2.3/8",
+			expected: []string{"10.0.0.0/8"},
+		},
+		{
+			name:        caseUnparseableFails,
+			set:         true,
+			value:       "10.0.0.0/8,not-an-address",
+			expectError: true,
+		},
+		{
+			name:        "out-of-range prefix length fails startup",
+			set:         true,
+			value:       "10.0.0.0/33",
+			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("TRUSTED_PROXIES", tt.value)
+			} else {
+				require.NoError(t, os.Unsetenv("TRUSTED_PROXIES"))
+			}
+
+			if tt.expectError {
+				expectStartupErrorFor(
+					t, "TRUSTED_PROXIES", config.ErrInvalidCIDR,
+				)
+			} else {
+				testTrustedProxiesSuccess(t, tt.expected)
+			}
+		})
+	}
+}
+
+func testTrustedProxiesSuccess(
+	t *testing.T,
+	expected []string,
+) {
+	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()
+
+	got := make([]string, 0, len(cfg.TrustedProxies))
+	for _, prefix := range cfg.TrustedProxies {
+		got = append(got, prefix.String())
+	}
+
+	assert.Equal(t, expected, got)
+}

internal/middleware/ratelimit.go

@@ -2,6 +2,9 @@ package middleware
 
 import (
 	"net/http"
+	"net/netip"
+	"slices"
+	"strings"
 	"time"
 
 	"github.com/go-chi/httprate"
@@ -31,13 +34,118 @@ const (
 	receiverRateInterval = 1 * time.Minute
 )
 
+// normalizeAddr strips the IPv4-in-IPv6 wrapper and any zone from
+// addr so that comparisons and bucket keys are canonical.
+func normalizeAddr(addr netip.Addr) netip.Addr {
+	return addr.Unmap().WithZone("")
+}
+
+// isTrustedProxy reports whether addr belongs to a network the
+// operator listed in TRUSTED_PROXIES. The list is empty by default,
+// so by default nothing is trusted.
+func (m *Middleware) isTrustedProxy(addr netip.Addr) bool {
+	for _, prefix := range m.params.Config.TrustedProxies {
+		if prefix.Contains(addr) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// forwardedClientAddr returns the client address named by this
+// request's forwarded headers. It is consulted only for requests
+// whose direct peer is a trusted proxy.
+//
+// True-Client-IP and X-Real-IP are single-valued, and a trusted
+// proxy is expected to overwrite whatever the client sent, so they
+// are taken as given. X-Forwarded-For is a chain the client can
+// prepend to, so it is walked right to left and the first hop that
+// is not itself a trusted proxy wins: entries the client inserted
+// sit to the left of the proxies' own appends and cannot be picked
+// while the chain is intact.
+func (m *Middleware) forwardedClientAddr(
+	r *http.Request,
+) (netip.Addr, bool) {
+	for _, header := range []string{"True-Client-IP", "X-Real-IP"} {
+		addr, err := netip.ParseAddr(
+			strings.TrimSpace(r.Header.Get(header)),
+		)
+		if err == nil {
+			return normalizeAddr(addr), true
+		}
+	}
+
+	hops := strings.Split(
+		strings.Join(r.Header.Values("X-Forwarded-For"), ","), ",",
+	)
+
+	for _, hop := range slices.Backward(hops) {
+		addr, err := netip.ParseAddr(strings.TrimSpace(hop))
+		if err != nil {
+			continue
+		}
+
+		if addr = normalizeAddr(addr); !m.isTrustedProxy(addr) {
+			return addr, true
+		}
+	}
+
+	return netip.Addr{}, false
+}
+
+// rateLimitKey is the client identity every rate limiter in this
+// package buckets on. Forwarded headers are honoured only when the
+// direct peer (RemoteAddr) is inside the configured trusted-proxy
+// set; otherwise the peer address itself is the key. Without that
+// gate any client could mint a fresh bucket per request, or starve
+// another client's bucket, by picking an X-Forwarded-For value —
+// which makes every limit here decorative against a deliberate
+// attacker.
+func (m *Middleware) rateLimitKey(r *http.Request) (string, error) {
+	return m.clientKey(r), nil
+}
+
+// clientKey computes the bucket key described on rateLimitKey.
+func (m *Middleware) clientKey(r *http.Request) string {
+	peer, err := netip.ParseAddr(ipFromHostPort(r.RemoteAddr))
+	if err != nil {
+		// Not an address we can reason about; key on the raw
+		// value rather than collapsing such peers into one
+		// shared bucket.
+		return r.RemoteAddr
+	}
+
+	peer = normalizeAddr(peer)
+	if !m.isTrustedProxy(peer) {
+		return peer.String()
+	}
+
+	if addr, ok := m.forwardedClientAddr(r); ok {
+		return addr.String()
+	}
+
+	return peer.String()
+}
+
+// tooManyRequests returns the 429 handler shared by every limiter:
+// it logs the rejection with logMessage and answers with
+// responseMessage. httprate adds the Retry-After header (RFC 6585).
+func (m *Middleware) tooManyRequests(
+	logMessage, responseMessage string,
+) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		m.log.Warn(logMessage, "path", r.URL.Path)
+		http.Error(w, responseMessage, http.StatusTooManyRequests)
+	}
+}
+
 // LoginRateLimit returns middleware that enforces per-IP rate
 // limiting on login attempts using go-chi/httprate. Only POST
 // requests are rate-limited; GET requests (rendering the login
 // form) pass through unaffected. When the rate limit is exceeded,
-// a 429 Too Many Requests response is returned. IP extraction
+// a 429 Too Many Requests response is returned. Clients are
-// honours X-Forwarded-For, X-Real-IP, and True-Client-IP headers
+// identified by rateLimitKey.
-// for reverse-proxy setups.
 func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
 	return m.postRateLimit(
 		loginRateLimit,
@@ -66,9 +174,7 @@ func (m *Middleware) PasswordChangeRateLimit() func(http.Handler) http.Handler {
 // limit on POST requests only; all other methods pass through
 // unaffected. Requests over the limit receive a 429 with the
 // given response message, and each rejection is logged with the
-// given log message. IP extraction honours X-Forwarded-For,
+// given log message. Clients are identified by rateLimitKey.
-// X-Real-IP, and True-Client-IP headers for reverse-proxy
-// setups.
 func (m *Middleware) postRateLimit(
 	limit int,
 	interval time.Duration,
@@ -77,19 +183,10 @@ func (m *Middleware) postRateLimit(
 	limiter := httprate.Limit(
 		limit,
 		interval,
-		httprate.WithKeyFuncs(httprate.KeyByRealIP),
+		httprate.WithKeyFuncs(m.rateLimitKey),
-		httprate.WithLimitHandler(http.HandlerFunc(
+		httprate.WithLimitHandler(
-			func(w http.ResponseWriter, r *http.Request) {
+			m.tooManyRequests(logMessage, responseMessage),
-				m.log.Warn(logMessage,
+		),
-					"path", r.URL.Path,
-				)
-				http.Error(
-					w,
-					responseMessage,
-					http.StatusTooManyRequests,
-				)
-			},
-		)),
 	)
 
 	return func(next http.Handler) http.Handler {
@@ -116,31 +213,19 @@ func (m *Middleware) postRateLimit(
 // 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;
+// requests per minute. Requests over the limit receive a 429.
-// httprate adds the Retry-After header (RFC 6585). IP
+// Clients are identified by rateLimitKey.
-// 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,
+			m.rateLimitKey,
 			httprate.KeyByEndpoint,
 		),
-		httprate.WithLimitHandler(http.HandlerFunc(
+		httprate.WithLimitHandler(m.tooManyRequests(
-			func(w http.ResponseWriter, r *http.Request) {
+			"webhook receiver rate limit exceeded",
-				m.log.Warn(
+			"Too many requests. Please slow down.",
-					"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

@@ -2,9 +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
+// okHandler is the terminal handler the limiter middleware wraps
-// handler with the given per-minute limit.
+// in these tests: it answers 200 to anything that reaches it.
-func receiverLimitedHandler(
+func okHandler() http.Handler {
-	t *testing.T, limit int,
+	return http.HandlerFunc(
-) http.Handler {
+		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(
+	return middleware.NewForTest(log, cfg, nil)
-		log,
+}
-		&config.Config{ReceiverRateLimit: limit},
+
-		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(
+	return m.ReceiverRateLimit()(okHandler())
-		func(w http.ResponseWriter, _ *http.Request) {
-			w.WriteHeader(http.StatusOK)
-		},
-	))
 }
 
 // receiverPost sends one POST to the handler from the given IP
@@ -311,3 +360,182 @@ 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"
+)
+
+// 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()
+
+			m := rateLimitMiddleware(t, &config.Config{})
+			handler := m.LoginRateLimit()(okHandler())
+
+			const peer = "203.0.113.9:44444"
+
+			for i := range middleware.LoginRateLimitConst {
+				w := postWithHeaders(
+					handler, peer, loginPath,
+					map[string]string{
+						header: fmt.Sprintf(
+							"198.51.100.%d", i+1,
+						),
+					},
+				)
+				assert.Equal(
+					t, http.StatusOK, w.Code,
+					"request %d should pass", i,
+				)
+			}
+
+			w := postWithHeaders(
+				handler, peer, loginPath,
+				map[string]string{header: "198.51.100.200"},
+			)
+			assert.Equal(
+				t, http.StatusTooManyRequests, w.Code,
+				"a spoofed %s from an untrusted peer must "+
+					"not mint a fresh bucket", header,
+			)
+		})
+	}
+}
+
+// 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()
+
+	m := rateLimitMiddleware(t, &config.Config{
+		TrustedProxies: trustedProxies("10.0.0.0/8"),
+	})
+	handler := m.LoginRateLimit()(okHandler())
+
+	const peer = "10.0.0.1:44444"
+
+	chain := func(spoof string) map[string]string {
+		return map[string]string{
+			headerXFF: spoof + ", 198.51.100.7, 10.0.0.2",
+		}
+	}
+
+	for i := range middleware.LoginRateLimitConst {
+		w := postWithHeaders(
+			handler, peer, loginPath,
+			chain(fmt.Sprintf("9.9.9.%d", i+1)),
+		)
+		assert.Equal(
+			t, http.StatusOK, w.Code,
+			"request %d should pass", i,
+		)
+	}
+
+	w := postWithHeaders(
+		handler, peer, loginPath, chain("9.9.9.200"),
+	)
+	assert.Equal(
+		t, http.StatusTooManyRequests, w.Code,
+		"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",
+	)
+}