Verify login credentials before spending rate-limit budget (closes #150)

With TRUSTED_PROXIES empty behind the reverse proxy production is
required to run behind, every login POST keyed on the proxy's address
and shared one 5/minute bucket. A stranger sending five POSTs a
minute -- 0.08 requests per second, from anywhere -- kept that bucket
permanently full, and the operator's own correct password was answered
429 indefinitely with no second administrative path.

The login POST no longer has a pre-emptive limiter. The handler
verifies credentials first and spends budget only on a FAILED attempt,
so a correct password is never throttled whatever the counters hold.
Three things follow, and are implemented together because the first is
unsafe without the other two:

- Failures are counted per (client bucket, submitted username), five
  per minute, after which further failures get 429 with a Retry-After.
  A successful login clears the counter, so mistyping and then
  succeeding does not leave the operator throttled.
- Both key sets are capped at 1024 entries. The submitted username is
  attacker-controlled, so past the first cap failures fall back to a
  counter keyed on the client alone, and past both caps a failure is
  answered as throttled without being recorded. Tracked state stays
  under half a megabyte and does not grow with invented usernames.
- Concurrent Argon2id verifications are capped at two, a 128 MB
  ceiling at 64 MB per hash. Every password-hashing endpoint takes a
  slot, including the password-change endpoint, which holds one across
  both its hashes. A request that waits five seconds without a slot is
  answered 503 and no hash runs for it.

An unknown username is verified against a dummy hash instead of
returning early, so a nonexistent account costs the same time as a
real one and the response cannot be used to enumerate usernames.

The password-change limiter is unchanged: RequireAuth runs ahead of
it, so only a request already carrying a valid session reaches its
bucket.

Also adds the missing test for the third bucketKey call site, where
the peer is a trusted proxy but the forwarded chain names no client.
Every existing test of that fallback uses an IPv4 proxy, where
bucketKey is the identity function, so dropping the /64 masking there
left the suite green.

README and the TRUSTED_PROXIES startup warning updated: a shared
bucket now costs precision, not the availability of the admin path.

internal/config/config.go

@@ -430,10 +430,14 @@ func loadFromEnv() (*Config, error) {
 // what is in front of the process, which this code cannot observe:
 // with nothing in front, the peer is the client and the limits are
 // per-client as intended; behind a reverse proxy the peer is the proxy
-// for every request, so all clients share one bucket per limiter. The
-// login limiter's bucket is the dangerous one: any remote client can
-// keep it full, which denies the only administrative login to everyone
-// until the process restarts.
+// for every request, so all clients share one bucket per limiter.
+//
+// The login endpoint no longer spends budget on arrival — it verifies
+// credentials first and charges only failures — so a shared bucket
+// cannot deny the operator a correct password. What it does collapse
+// is the failure counting: one client's wrong passwords throttle
+// everyone else's wrong passwords, and the receiver's limits become
+// service-wide ceilings.
 //
 // The warning is deliberately not gated on WEBHOOKER_ENVIRONMENT. That
 // variable defaults to dev, so gating on it would silence the warning
@@ -454,11 +458,11 @@ func (c *Config) warnSharedRateLimitBucket(log *slog.Logger) {
 			"this process that is the client itself and the limits "+
 			"are per-client as intended. Behind a reverse proxy the "+
 			"peer is the proxy on every request, so all clients "+
-			"share one bucket per limit and any remote client can "+
-			"keep the login limit full, denying the admin login — "+
-			"the only administrative path — until restart. If "+
-			"anything proxies to this process, set TRUSTED_PROXIES "+
-			"to its address.",
+			"share one bucket per limit: the receiver limits become "+
+			"service-wide ceilings, and one client's failed logins "+
+			"throttle every other client's failed logins — a "+
+			"correct password still gets in. If anything proxies to "+
+			"this process, set TRUSTED_PROXIES to its address.",
 		"environment", c.Environment,
 		"trustedProxies", len(c.TrustedProxies),
 	)

internal/config/config_test.go

@@ -629,8 +629,9 @@ func testTrustedProxiesSuccess(
 
 // TestSharedRateLimitBucketWarning covers the startup warning that
 // tells an operator a deployment behind a reverse proxy shares one
-// rate-limit bucket between every client, which makes the admin login
-// remotely deniable. It must fire whenever TRUSTED_PROXIES is empty,
+// rate-limit bucket between every client, which turns the receiver
+// limits into service-wide ceilings and collapses login failure
+// counting. It must fire whenever TRUSTED_PROXIES is empty,
 // in any environment: WEBHOOKER_ENVIRONMENT defaults to dev, so gating
 // on it would silence the warning for exactly the operator who never
 // configured the deployment. It stays quiet once proxies are named.
@@ -707,7 +708,15 @@ func TestSharedRateLimitBucketWarning(t *testing.T) {
 			assert.Contains(t, logged, `"level":"WARN"`)
 			assert.Contains(t, logged, "TRUSTED_PROXIES")
 			assert.Contains(t, logged, "share one bucket")
-			assert.Contains(t, logged, "denying the admin login")
+			assert.Contains(
+				t, logged, "throttle every other client's failed logins",
+			)
+			// The warning must not claim a lockout the login
+			// endpoint no longer permits: credentials are verified
+			// before any budget is spent.
+			assert.Contains(
+				t, logged, "a correct password still gets in",
+			)
 			// The text must stay accurate for a developer with
 			// nothing in front of the process, where an empty
 			// list costs nothing.

internal/database/export_test.go

@@ -65,3 +65,9 @@ func (r *RetentionReaper) ExportWedgeLoop(
 func (r *RetentionReaper) ExportSetInterval(d time.Duration) {
 	r.interval = d
 }
+
+// DummyPasswordHashForTest exposes the encoded hash that unknown
+// usernames are verified against.
+func DummyPasswordHashForTest() string {
+	return dummyPasswordHash()
+}

internal/database/password.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"math/big"
 	"strings"
+	"sync"
 
 	"golang.org/x/crypto/argon2"
 )
@@ -29,6 +30,10 @@ const hashParts = 6
 // triggers per-character-class complexity enforcement.
 const minPasswordComplexityLen = 4
 
+// dummyPasswordLen is the length of the throwaway password behind
+// dummyPasswordHash.
+const dummyPasswordLen = 32
+
 // Sentinel errors returned by decodeHash.
 var (
 	errInvalidHashFormat    = errors.New("invalid hash format")
@@ -122,6 +127,38 @@ func VerifyPassword(
 	return subtle.ConstantTimeCompare(hash, otherHash) == 1, nil
 }
 
+// dummyPasswordHash is an encoded Argon2id hash of a random
+// password, computed once on first use. Nothing can match it: the
+// password it encodes is discarded as soon as it is hashed. It is
+// process-wide because building it per request would add a second
+// 64 MB Argon2id pass to every login for an unknown username.
+//
+//nolint:gochecknoglobals // computed once, see above
+var dummyPasswordHash = sync.OnceValue(func() string {
+	password, err := GenerateRandomPassword(dummyPasswordLen)
+	if err != nil {
+		panic(fmt.Sprintf("generating the dummy password: %v", err))
+	}
+
+	hash, err := HashPassword(password)
+	if err != nil {
+		panic(fmt.Sprintf("hashing the dummy password: %v", err))
+	}
+
+	return hash
+})
+
+// VerifyDummyPassword performs a credential verification that cannot
+// succeed, at the same cost as a real one.
+//
+// Login must charge an unknown username the same work as a known
+// one. Returning early for an account that does not exist answers in
+// microseconds where a real account takes tens of milliseconds, which
+// is a username oracle any client can read off the response time.
+func VerifyDummyPassword(password string) {
+	_, _ = VerifyPassword(password, dummyPasswordHash())
+}
+
 // decodeHash extracts parameters, salt, and hash from an
 // encoded hash string.
 func decodeHash(

internal/database/password_test.go

@@ -191,3 +191,41 @@ func TestHashPasswordUniqueness(t *testing.T) {
 		)
 	}
 }
+
+// TestVerifyDummyPassword_DoesRealWork covers the anti-enumeration
+// path. Login charges an unknown username a verification against a
+// dummy hash so that a nonexistent account is not answered in
+// microseconds where a real one takes tens of milliseconds. That only
+// works if the dummy hash is a real, decodable Argon2id hash: a
+// malformed one would make VerifyPassword fail on the decode and
+// return before hashing anything.
+func TestVerifyDummyPassword_DoesRealWork(t *testing.T) {
+	t.Parallel()
+
+	// Runs the OnceValue that builds the dummy hash, so a panic in
+	// it surfaces here rather than on a live login.
+	database.VerifyDummyPassword("whatever was submitted")
+
+	dummy := database.DummyPasswordHashForTest()
+
+	// A hash the verifier cannot decode would make VerifyPassword
+	// return on the decode error, before hashing anything — the
+	// timing oracle this path exists to close.
+	valid, err := database.VerifyPassword("whatever", dummy)
+	if err != nil {
+		t.Fatalf(
+			"the dummy hash must decode like a real one: %v", err,
+		)
+	}
+
+	if valid {
+		t.Error("nothing may authenticate against the dummy hash")
+	}
+
+	if !strings.HasPrefix(dummy, "$argon2id$") {
+		t.Errorf(
+			"the dummy hash must use the same algorithm as real "+
+				"hashes, got %q", dummy,
+		)
+	}
+}

internal/handlers/auth.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"net/http"
+	"strconv"
 
 	"sneak.berlin/go/webhooker/internal/database"
 )
@@ -93,6 +94,16 @@ func (h *Handlers) renderLoginError(
 
 // authenticateUser looks up and verifies a user's credentials.
 // On failure it writes an HTTP response and returns an error.
+//
+// The credential check runs BEFORE any rate-limit budget is
+// consulted, and only a failed check spends budget. That is what
+// keeps the single administrative path reachable: behind the reverse
+// proxy this deployment requires, with TRUSTED_PROXIES unset, every
+// client shares one bucket, so a limiter spent on arrival lets any
+// stranger deny the operator's own correct password indefinitely.
+//
+// Verifying first means every login POST costs an Argon2id hash, so
+// the work is taken under a bounded number of verification slots.
 func (h *Handlers) authenticateUser(
 	w http.ResponseWriter,
 	r *http.Request,
@@ -100,16 +111,37 @@ func (h *Handlers) authenticateUser(
 ) (database.User, error) {
 	var user database.User
 
+	release, ok := h.mw.BeginPasswordVerification(r.Context())
+	if !ok {
+		h.log.Warn(
+			"password verification capacity exhausted",
+			"path", r.URL.Path,
+		)
+		h.renderLoginError(
+			w, r,
+			"The server is busy verifying credentials. "+
+				"Please try again.",
+			http.StatusServiceUnavailable,
+		)
+
+		return user, errVerificationBusy
+	}
+
+	defer release()
+
 	err := h.db.DB().Where(
 		"username = ?", username,
 	).First(&user).Error
 	if err != nil {
+		// A username that does not exist is charged the same work
+		// as one that does. Skipping the hash here would answer in
+		// microseconds where a real account takes tens of
+		// milliseconds, handing every client a username oracle.
+		h.dummyVerifications.Add(1)
+		database.VerifyDummyPassword(password)
+
 		h.log.Debug("user not found", "username", username)
-		h.renderLoginError(
-			w, r,
-			"Invalid username or password",
-			http.StatusUnauthorized,
-		)
+		h.rejectLogin(w, r, username)
 
 		return user, err
 	}
@@ -127,16 +159,49 @@ func (h *Handlers) authenticateUser(
 
 	if !valid {
 		h.log.Debug("invalid password", "username", username)
+		h.rejectLogin(w, r, username)
+
+		return user, errInvalidPassword
+	}
+
+	// The password was correct, so forgive whatever failures this
+	// client accumulated: an operator who mistypes a few times and
+	// then gets it right must not stay throttled afterwards.
+	h.mw.ForgiveLoginFailures(r, username)
+
+	return user, nil
+}
+
+// rejectLogin counts one failed credential verification and answers
+// it: 401 while this client still has failure budget against the
+// submitted username, 429 with a Retry-After once it is spent.
+//
+// The 429 throttles wrong passwords only. A correct one never
+// reaches here, so no amount of failure — from this client or any
+// other sharing its bucket — can keep the operator out.
+func (h *Handlers) rejectLogin(
+	w http.ResponseWriter,
+	r *http.Request,
+	username string,
+) {
+	if !h.mw.RecordLoginFailure(r, username) {
 		h.renderLoginError(
 			w, r,
 			"Invalid username or password",
 			http.StatusUnauthorized,
 		)
 
-		return user, errInvalidPassword
+		return
 	}
 
-	return user, nil
+	w.Header().Set("Retry-After", strconv.Itoa(int(
+		h.mw.LoginFailureInterval().Seconds(),
+	)))
+	h.renderLoginError(
+		w, r,
+		"Too many failed login attempts. Please try again later.",
+		http.StatusTooManyRequests,
+	)
 }
 
 // createAuthenticatedSession regenerates the session and stores

internal/handlers/auth_test.go

@@ -0,0 +1,455 @@
+package handlers_test
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/handlers"
+	"sneak.berlin/go/webhooker/internal/session"
+)
+
+const (
+	// operatorUser and operatorPassword are the single admin account
+	// these tests defend.
+	operatorUser     = "admin"
+	operatorPassword = "correct horse battery staple"
+
+	// sharedProxyPeer is the whole point of this file. Production is
+	// required to run behind a TLS-terminating reverse proxy, and
+	// TRUSTED_PROXIES defaults to empty, so every client — attacker
+	// and operator alike — reaches the process from the proxy's
+	// address and shares one rate-limit bucket. Both parties in
+	// these tests therefore use the same RemoteAddr.
+	sharedProxyPeer = "10.0.0.1:44444"
+
+	// loginFailureLimit is the failure budget one client has against
+	// one submitted username. Restated here rather than imported
+	// from the middleware package, so that changing the production
+	// limit fails these tests instead of silently moving with them.
+	loginFailureLimit = 5
+)
+
+// seedOperator gives the bootstrapped admin account a password these
+// tests know. The account itself is created at startup with a random
+// password, which is exactly why its username is predictable to an
+// attacker and why keying failures by username alone does not fix
+// this issue.
+func seedOperator(t *testing.T, db *database.Database) {
+	t.Helper()
+
+	hash, err := database.HashPassword(operatorPassword)
+	require.NoError(t, err)
+
+	result := db.DB().Model(&database.User{}).
+		Where("username = ?", operatorUser).
+		Update("password", hash)
+
+	require.NoError(t, result.Error)
+	require.EqualValues(
+		t, 1, result.RowsAffected,
+		"the bootstrap admin account must exist",
+	)
+}
+
+// loginPost builds a login form POST arriving from peer.
+func loginPost(peer, username, password string) *http.Request {
+	form := url.Values{}
+	form.Set("username", username)
+	form.Set("password", password)
+
+	req := httptest.NewRequestWithContext(
+		context.Background(),
+		http.MethodPost,
+		"/pages/login",
+		strings.NewReader(form.Encode()),
+	)
+	req.Header.Set(
+		"Content-Type", "application/x-www-form-urlencoded",
+	)
+	req.RemoteAddr = peer
+
+	return req
+}
+
+// submitLogin drives one login POST through the handler.
+func submitLogin(
+	h *handlers.Handlers, peer, username, password string,
+) *httptest.ResponseRecorder {
+	w := httptest.NewRecorder()
+	h.HandleLoginSubmit().ServeHTTP(w, loginPost(
+		peer, username, password,
+	))
+
+	return w
+}
+
+// floodFailures sends attempts wrong-password logins for username
+// from peer, which is what an attacker does.
+func floodFailures(
+	t *testing.T,
+	h *handlers.Handlers,
+	peer, username string,
+	attempts int,
+) {
+	t.Helper()
+
+	for i := range attempts {
+		w := submitLogin(h, peer, username, fmt.Sprintf("guess-%d", i))
+		require.NotEqual(
+			t, http.StatusSeeOther, w.Code,
+			"attempt %d must not authenticate", i,
+		)
+	}
+}
+
+// TestLogin_StrangersFloodCannotLockOutTheOperator is the
+// done-criterion of https://git.eeqj.de/sneak/webhooker/issues/150.
+//
+// The attacker and the operator share one rate-limit bucket, because
+// behind the mandated reverse proxy with TRUSTED_PROXIES unset every
+// client keys on the proxy's address. The attacker floods the
+// operator's own username — a single-admin product has a predictable
+// one — far past the failure limit. The operator must still be able
+// to log in with the correct password.
+//
+// This fails if credentials stop being verified ahead of the limiter.
+func TestLogin_StrangersFloodCannotLockOutTheOperator(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	// Well past the limit, and from the same bucket the operator
+	// will arrive in.
+	floodFailures(
+		t, h, sharedProxyPeer, operatorUser,
+		loginFailureLimit*2,
+	)
+
+	w := submitLogin(
+		h, sharedProxyPeer, operatorUser, operatorPassword,
+	)
+
+	assert.Equal(
+		t, http.StatusSeeOther, w.Code,
+		"a correct password must never be throttled: the operator "+
+			"has no second administrative path",
+	)
+	assert.Equal(t, "/", w.Header().Get("Location"))
+}
+
+// TestLogin_StrangersFloodCannotDenyAnotherAccount is the
+// cross-account half: flooding one username must not spend another
+// account's budget, even from the same shared bucket.
+func TestLogin_StrangersFloodCannotDenyAnotherAccount(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	floodFailures(
+		t, h, sharedProxyPeer, "someone-else",
+		loginFailureLimit*2,
+	)
+
+	w := submitLogin(h, sharedProxyPeer, operatorUser, "wrong")
+
+	assert.Equal(
+		t, http.StatusUnauthorized, w.Code,
+		"a flood against one username must not spend another "+
+			"account's failure budget",
+	)
+}
+
+// TestLogin_RepeatedWrongPasswordsAreThrottled is the brute-force
+// half. Verifying before counting must not remove the throttle:
+// repeated wrong passwords for one username from one client key run
+// out of budget and are answered 429 with a Retry-After.
+func TestLogin_RepeatedWrongPasswordsAreThrottled(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	for i := range loginFailureLimit - 1 {
+		w := submitLogin(
+			h, sharedProxyPeer, operatorUser,
+			fmt.Sprintf("guess-%d", i),
+		)
+		assert.Equal(
+			t, http.StatusUnauthorized, w.Code,
+			"attempt %d is still inside the budget", i,
+		)
+	}
+
+	w := submitLogin(h, sharedProxyPeer, operatorUser, "guess-last")
+	assert.Equal(
+		t, http.StatusTooManyRequests, w.Code,
+		"wrong passwords must still run out of budget",
+	)
+	assert.NotEmpty(
+		t, w.Header().Get("Retry-After"),
+		"a throttled login must say when to come back",
+	)
+}
+
+// TestLogin_SuccessForgivesEarlierMistakes covers the operator who
+// mistypes several times and then gets it right: the successful
+// attempt clears the counter, so the next mistake is answered 401
+// rather than 429.
+func TestLogin_SuccessForgivesEarlierMistakes(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	floodFailures(
+		t, h, sharedProxyPeer, operatorUser,
+		loginFailureLimit,
+	)
+
+	require.Equal(
+		t, http.StatusSeeOther,
+		submitLogin(
+			h, sharedProxyPeer, operatorUser, operatorPassword,
+		).Code,
+	)
+
+	w := submitLogin(h, sharedProxyPeer, operatorUser, "typo")
+	assert.Equal(
+		t, http.StatusUnauthorized, w.Code,
+		"a success must forgive the failures before it",
+	)
+}
+
+// TestLogin_UnknownUsernameCostsTheSameVerification is the
+// username-enumeration guard. Verifying credentials before the
+// limiter means response time is observable per attempt, so an
+// unknown username must be charged an equivalent-cost verification
+// against a dummy hash rather than returning early.
+//
+// The assertion is on the code path, not on wall-clock time: timing
+// assertions are flaky, and what actually has to hold is that the
+// hash is computed.
+func TestLogin_UnknownUsernameCostsTheSameVerification(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	require.Zero(t, h.DummyVerificationsForTest())
+
+	// A username that exists, with the wrong password: a real
+	// Argon2id verification runs, and no dummy is needed.
+	require.Equal(
+		t, http.StatusUnauthorized,
+		submitLogin(h, sharedProxyPeer, operatorUser, "wrong").Code,
+	)
+	assert.Zero(
+		t, h.DummyVerificationsForTest(),
+		"a known username verifies against its own hash",
+	)
+
+	// A username that does not exist: indistinguishable response,
+	// and the equivalent-cost verification must have run.
+	require.Equal(
+		t, http.StatusUnauthorized,
+		submitLogin(h, sharedProxyPeer, "nosuchuser", "wrong").Code,
+	)
+	assert.Equal(
+		t, uint64(1), h.DummyVerificationsForTest(),
+		"an unknown username must still pay for a hash, or the "+
+			"response time says whether the account exists",
+	)
+}
+
+// TestLogin_ConcurrentLoginsAreAllAnswered covers the login path
+// under the verification bound. The bound itself is pinned in the
+// middleware package; what matters here is that funnelling every
+// login through two slots does not lose or wedge a request — each one
+// is answered, whether it got a slot or was shed with 503.
+func TestLogin_ConcurrentLoginsAreAllAnswered(t *testing.T) {
+	t.Parallel()
+
+	const workers = 4
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	var (
+		wg      sync.WaitGroup
+		mu      sync.Mutex
+		answers = map[int]int{}
+	)
+
+	for i := range workers {
+		wg.Go(func() {
+			w := submitLogin(
+				h, fmt.Sprintf("203.0.113.%d:5000", i),
+				operatorUser, fmt.Sprintf("guess-%d", i),
+			)
+
+			mu.Lock()
+			answers[w.Code]++
+			mu.Unlock()
+		})
+	}
+
+	wg.Wait()
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	assert.Zero(
+		t, answers[http.StatusInternalServerError],
+		"concurrent logins must not error",
+	)
+	assert.Equal(
+		t, workers,
+		answers[http.StatusUnauthorized]+
+			answers[http.StatusTooManyRequests]+
+			answers[http.StatusServiceUnavailable],
+		"every concurrent login must be answered, whether it got "+
+			"a verification slot or was shed with 503",
+	)
+}
+
+// TestLogin_MissingCredentialsRejectedBeforeAnyHash pins that the
+// empty-field check still runs ahead of the verification slot, so a
+// client sending nothing cannot occupy one.
+func TestLogin_MissingCredentialsRejectedBeforeAnyHash(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	w := submitLogin(h, sharedProxyPeer, "", "")
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+	assert.Zero(
+		t, h.DummyVerificationsForTest(),
+		"an empty submission must not cost a hash",
+	)
+}
+
+// TestLogin_SuccessCreatesSession is the control for the tests above:
+// the success path they assert on really does authenticate.
+func TestLogin_SuccessCreatesSession(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h    *handlers.Handlers
+		db   *database.Database
+		sess *session.Session
+	)
+
+	app := newTestApp(t, &h, &db, &sess)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	seedOperator(t, db)
+
+	w := submitLogin(
+		h, sharedProxyPeer, operatorUser, operatorPassword,
+	)
+
+	require.Equal(t, http.StatusSeeOther, w.Code)
+	require.NotEmpty(
+		t, w.Result().Cookies(), "a session cookie must be issued",
+	)
+
+	next := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/", nil,
+	)
+
+	// Login regenerates the session, so the response carries two
+	// Set-Cookie headers under the same name: one expiring the
+	// pre-login cookie and one issuing the new one. A browser keeps
+	// only the second, so replay only the one that is not an
+	// expiry.
+	for _, c := range w.Result().Cookies() {
+		if c.MaxAge >= 0 {
+			next.AddCookie(c)
+		}
+	}
+
+	s, err := sess.Get(next)
+	require.NoError(t, err)
+	assert.True(
+		t, sess.IsAuthenticated(s),
+		"the issued cookie must carry an authenticated session",
+	)
+}

internal/handlers/export_test.go

@@ -11,6 +11,14 @@ import (
 // to the handlers_test package.
 const MaxRenderedBodyBytesForTest = maxRenderedBodyBytes
 
+// DummyVerificationsForTest reports how many equivalent-cost
+// verifications were charged for usernames that do not exist. It
+// lets a test prove the anti-enumeration path ran without timing
+// anything.
+func (s *Handlers) DummyVerificationsForTest() uint64 {
+	return s.dummyVerifications.Load()
+}
+
 // TrimPartialRuneForTest exposes trimPartialRune for use in the
 // handlers_test package.
 func TrimPartialRuneForTest(b []byte) []byte {

internal/handlers/handlers.go

@@ -10,6 +10,7 @@ import (
 	"html/template"
 	"log/slog"
 	"net/http"
+	"sync/atomic"
 
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/database"
@@ -39,6 +40,12 @@ const (
 // errInvalidPassword is returned when a password does not match.
 var errInvalidPassword = errors.New("invalid password")
 
+// errVerificationBusy is returned when no password-verification slot
+// became free before the wait elapsed, so no password was verified.
+var errVerificationBusy = errors.New(
+	"password verification capacity exhausted",
+)
+
 //nolint:revive // HandlersParams is a standard fx naming convention.
 type HandlersParams struct {
 	fx.In
@@ -49,6 +56,7 @@ type HandlersParams struct {
 	WebhookDBMgr *database.WebhookDBManager
 	Healthcheck  *healthcheck.Healthcheck
 	Session      *session.Session
+	Middleware   *middleware.Middleware
 	Notifier     delivery.Notifier
 	Evictor      delivery.WebhookEvictor
 }
@@ -62,9 +70,15 @@ type Handlers struct {
 	db        *database.Database
 	dbMgr     *database.WebhookDBManager
 	session   *session.Session
+	mw        *middleware.Middleware
 	notifier  delivery.Notifier
 	evictor   delivery.WebhookEvictor
 	templates map[string]*template.Template
+
+	// dummyVerifications counts the equivalent-cost verifications
+	// charged for usernames that do not exist. It exists so a test
+	// can prove that path runs without measuring wall-clock time.
+	dummyVerifications atomic.Uint64
 }
 
 // parsePageTemplate parses a page-specific template set from the
@@ -97,6 +111,7 @@ func New(
 	s.db = params.Database
 	s.dbMgr = params.WebhookDBMgr
 	s.session = params.Session
+	s.mw = params.Middleware
 	s.notifier = params.Notifier
 	s.evictor = params.Evictor
 

internal/handlers/handlers_test.go

@@ -20,6 +20,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/handlers"
 	"sneak.berlin/go/webhooker/internal/healthcheck"
 	"sneak.berlin/go/webhooker/internal/logger"
+	"sneak.berlin/go/webhooker/internal/middleware"
 	"sneak.berlin/go/webhooker/internal/session"
 )
 
@@ -82,6 +83,7 @@ func newTestApp(
 			func(r *recordingEvictor) delivery.WebhookEvictor {
 				return r
 			},
+			middleware.New,
 			handlers.New,
 		),
 		fx.Populate(targets...),

internal/handlers/profile.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"context"
 	"net/http"
 
 	"github.com/go-chi/chi"
@@ -42,6 +43,7 @@ func (h *Handlers) HandlePasswordChange() http.HandlerFunc {
 		}
 
 		successMessage, errorMessage, handled := h.applyPasswordChange(
+			r.Context(),
 			w,
 			sessionUsername,
 			r.FormValue("current_password"),
@@ -66,9 +68,30 @@ func (h *Handlers) HandlePasswordChange() http.HandlerFunc {
 // 500 response itself and returns handled=false, signalling the caller
 // to stop without re-rendering the page.
 func (h *Handlers) applyPasswordChange(
+	ctx context.Context,
 	w http.ResponseWriter,
 	username, currentPassword, newPassword, confirmPassword string,
 ) (string, string, bool) {
+	// This endpoint verifies one password and hashes another, at
+	// 64 MB each, so it takes a slot from the same bound the login
+	// endpoint uses. The bound is per hash, not per endpoint: leaving
+	// this path outside it would leave a hole in it. The slot is held
+	// across both hashes.
+	release, ok := h.mw.BeginPasswordVerification(ctx)
+	if !ok {
+		h.log.Warn("password verification capacity exhausted")
+		http.Error(
+			w,
+			"The server is busy verifying credentials. "+
+				"Please try again.",
+			http.StatusServiceUnavailable,
+		)
+
+		return "", "", false
+	}
+
+	defer release()
+
 	// Load the user row so we can verify the current password and
 	// persist the new hash.
 	var user database.User

internal/middleware/export_test.go

@@ -1,7 +1,9 @@
 package middleware
 
 import (
+	"context"
 	"net/http"
+	"time"
 )
 
 // NewLoggingResponseWriterForTest wraps newLoggingResponseWriter
@@ -35,9 +37,69 @@ func IsClientTLS(r *http.Request) bool {
 	return isClientTLS(r)
 }
 
-// LoginRateLimitConst exposes the loginRateLimit constant.
+// LoginRateLimitConst exposes the loginRateLimit constant: the
+// number of FAILED login attempts one client may make against one
+// submitted username per interval.
 const LoginRateLimitConst = loginRateLimit
 
+// LoginFailureMaxKeysConst exposes the cap on each of the login
+// guard's key sets.
+const LoginFailureMaxKeysConst = loginFailureMaxKeys
+
+// PasswordVerifyConcurrencyConst exposes the bound on concurrent
+// Argon2id verifications.
+const PasswordVerifyConcurrencyConst = passwordVerifyConcurrency
+
+// LoginGuard is the login failure counter and verification
+// semaphore, exposed for direct testing.
+type LoginGuard = loginGuard
+
+// NewLoginGuardForTest builds a guard with test-sized parameters.
+func NewLoginGuardForTest(
+	limit int,
+	interval time.Duration,
+	maxKeys, concurrency int,
+	wait time.Duration,
+) *LoginGuard {
+	return newLoginGuard(
+		limit, interval, maxKeys, concurrency, wait,
+	)
+}
+
+// SetNowForTest replaces the guard's clock.
+func (g *LoginGuard) SetNowForTest(now func() time.Time) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	g.now = now
+}
+
+// FailForTest exposes fail.
+func (g *LoginGuard) FailForTest(clientKey, username string) bool {
+	return g.fail(clientKey, username)
+}
+
+// SucceedForTest exposes succeed.
+func (g *LoginGuard) SucceedForTest(clientKey, username string) {
+	g.succeed(clientKey, username)
+}
+
+// AcquireForTest exposes acquire.
+func (g *LoginGuard) AcquireForTest(
+	ctx context.Context,
+) (func(), bool) {
+	return g.acquire(ctx)
+}
+
+// TrackedKeysForTest reports how many failure counters the guard
+// holds, per-username and per-address respectively.
+func (g *LoginGuard) TrackedKeysForTest() (int, int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	return len(g.byUser), len(g.byAddr)
+}
+
 // PasswordChangeRateLimitConst exposes the
 // passwordChangeRateLimit constant.
 const PasswordChangeRateLimitConst = passwordChangeRateLimit

internal/middleware/loginguard.go

@@ -0,0 +1,299 @@
+package middleware
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/http"
+	"sync"
+	"time"
+)
+
+const (
+	// loginFailureMaxKeys bounds how many distinct failure counters
+	// each of the guard's two key sets holds. The submitted username
+	// is part of a key, so the key set is attacker-influenced and
+	// needs a hard cap or the limiter becomes the memory
+	// amplification surface it exists to protect.
+	//
+	// A single-admin deployment has a handful of legitimate (client,
+	// username) pairs, so 1024 is three orders of magnitude of
+	// headroom before a real operator can be pushed onto the
+	// fallback. It costs little: a counter is a ~64-byte key string,
+	// a 32-byte window and map overhead, call it 170 bytes, so both
+	// key sets full is 2 * 1024 * 170 bytes, under 0.4 MB.
+	loginFailureMaxKeys = 1024
+
+	// passwordVerifyConcurrency bounds how many Argon2id
+	// verifications may run at once across every password-verifying
+	// endpoint. Because credentials are now verified before any
+	// limiter budget is spent, an attacker can force one hash per
+	// request, and each hash allocates argon2Memory — 64 MB. Two
+	// slots commit at most 128 MB to password hashing, which fits
+	// inside the smallest container this service is realistically
+	// given alongside its own working set; four would commit 256 MB
+	// and crowd it. A single-admin product needs no concurrent
+	// logins at all, so the second slot exists only so that one
+	// stalled request does not serialise the endpoint.
+	passwordVerifyConcurrency = 2
+
+	// passwordVerifyWait is how long a request waits for a
+	// verification slot before it is answered 503. Slots are handed
+	// out in arrival order, so a legitimate request queues behind
+	// the requests already waiting rather than behind the flood as a
+	// whole. The wait is well inside the 60s request timeout.
+	passwordVerifyWait = 5 * time.Second
+
+	// failureKeyHashBytes is how much of the username digest goes
+	// into a failure key. 64 bits over at most loginFailureMaxKeys
+	// live keys makes a collision negligible, and a collision would
+	// only merge two usernames' failure counters, which throttles
+	// sooner rather than later.
+	failureKeyHashBytes = 8
+)
+
+// failureWindow counts failed credential verifications for one
+// bucket, and records when that count lapses.
+type failureWindow struct {
+	count   int
+	resetAt time.Time
+}
+
+// loginGuard is what replaced the pre-emptive rate limiter on the
+// login POST.
+//
+// A limiter that spends budget on arrival cannot protect a
+// single-admin product: behind the reverse proxy the deployment
+// requires, with TRUSTED_PROXIES unset, every client keys on the
+// proxy, so a stranger trickling five POSTs a minute keeps the one
+// bucket full and the operator's own correct password is answered 429
+// forever. There is no second administrative path.
+//
+// So budget is spent only by a FAILED verification. A correct
+// password is never throttled, whatever the counters say, which is
+// the only shape that guarantees the operator can get in. Two
+// consequences follow and are handled here:
+//
+//   - Every login request now costs an Argon2id hash, so the number
+//     running concurrently is bounded by slots. Without that bound
+//     this trades an admin lockout for memory exhaustion, which is
+//     strictly worse.
+//   - Counting per (client, username) makes the key set
+//     attacker-influenced, so both key sets are capped. Beyond the
+//     per-username cap, failures fall back to a counter keyed on the
+//     client alone; beyond that cap too, a failure is answered as
+//     throttled without being recorded, since refusing to answer a
+//     wrong password costs the operator nothing.
+type loginGuard struct {
+	mu     sync.Mutex
+	byUser map[string]*failureWindow
+	byAddr map[string]*failureWindow
+
+	slots chan struct{}
+
+	limit    int
+	interval time.Duration
+	maxKeys  int
+	wait     time.Duration
+
+	// now is time.Now outside tests.
+	now func() time.Time
+}
+
+// newLoginGuard builds a guard with the given failure limit per
+// interval, key-set cap, verification concurrency and slot wait.
+func newLoginGuard(
+	limit int,
+	interval time.Duration,
+	maxKeys, concurrency int,
+	wait time.Duration,
+) *loginGuard {
+	return &loginGuard{
+		byUser:   make(map[string]*failureWindow),
+		byAddr:   make(map[string]*failureWindow),
+		slots:    make(chan struct{}, concurrency),
+		limit:    limit,
+		interval: interval,
+		maxKeys:  maxKeys,
+		wait:     wait,
+		now:      time.Now,
+	}
+}
+
+// acquire reserves a verification slot, waiting up to the guard's
+// wait for one. It reports false when none became available or the
+// request was cancelled first; the caller must then answer 503
+// without verifying anything. The returned function releases the
+// slot and must be called exactly once.
+func (g *loginGuard) acquire(ctx context.Context) (func(), bool) {
+	timer := time.NewTimer(g.wait)
+	defer timer.Stop()
+
+	select {
+	case g.slots <- struct{}{}:
+		return func() { <-g.slots }, true
+	case <-timer.C:
+		return nil, false
+	case <-ctx.Done():
+		return nil, false
+	}
+}
+
+// fail records one failed credential verification by clientKey
+// against username, and reports whether this client has now spent
+// its failure budget and should be answered 429.
+func (g *loginGuard) fail(clientKey, username string) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	now := g.now()
+
+	window := g.window(
+		g.byUser, userFailureKey(clientKey, username), now,
+	)
+	if window == nil {
+		window = g.window(g.byAddr, clientKey, now)
+	}
+
+	if window == nil {
+		// Both key sets are full and neither already tracks this
+		// client, so nothing can be counted without unbounded
+		// growth. Answering the failure as throttled is the safe
+		// direction: it never touches a correct password.
+		return true
+	}
+
+	window.count++
+
+	return window.count >= g.limit
+}
+
+// succeed forgives clientKey's failures against username. A correct
+// password clears the counters, so an operator who mistypes several
+// times and then gets it right is not throttled afterwards.
+func (g *loginGuard) succeed(clientKey, username string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	delete(g.byUser, userFailureKey(clientKey, username))
+	delete(g.byAddr, clientKey)
+}
+
+// window returns the live counter for key in set, resetting a lapsed
+// one and creating a missing one when the cap allows. It returns nil
+// only when key is absent and set is full even after lapsed entries
+// are swept.
+func (g *loginGuard) window(
+	set map[string]*failureWindow,
+	key string,
+	now time.Time,
+) *failureWindow {
+	window, ok := set[key]
+	if ok {
+		if !now.Before(window.resetAt) {
+			window.count = 0
+			window.resetAt = now.Add(g.interval)
+		}
+
+		return window
+	}
+
+	if len(set) >= g.maxKeys {
+		sweepLapsed(set, now)
+	}
+
+	if len(set) >= g.maxKeys {
+		return nil
+	}
+
+	window = &failureWindow{resetAt: now.Add(g.interval)}
+	set[key] = window
+
+	return window
+}
+
+// sweepLapsed drops counters whose interval has elapsed.
+func sweepLapsed(set map[string]*failureWindow, now time.Time) {
+	for key, window := range set {
+		if !now.Before(window.resetAt) {
+			delete(set, key)
+		}
+	}
+}
+
+// userFailureKey identifies one (client, submitted username) pair.
+// The username is hashed rather than embedded: a submitted username
+// is attacker-controlled text of attacker-chosen length, and hashing
+// makes every key the same size whatever was sent.
+func userFailureKey(clientKey, username string) string {
+	sum := sha256.Sum256([]byte(username))
+
+	return clientKey + "|" +
+		hex.EncodeToString(sum[:failureKeyHashBytes])
+}
+
+// guard returns the middleware's login guard, building it on first
+// use so that every construction path — fx and the test constructor
+// alike — gets one.
+func (m *Middleware) guard() *loginGuard {
+	m.loginGuardOnce.Do(func() {
+		m.loginGuard = newLoginGuard(
+			loginRateLimit,
+			loginRateInterval,
+			loginFailureMaxKeys,
+			passwordVerifyConcurrency,
+			passwordVerifyWait,
+		)
+	})
+
+	return m.loginGuard
+}
+
+// BeginPasswordVerification reserves one of the bounded Argon2id
+// verification slots. It reports false when none became free within
+// passwordVerifyWait, in which case the caller must answer 503 and
+// must not verify a password. The returned function releases the
+// slot and must be called exactly once.
+//
+// Every endpoint that hashes a password on request must go through
+// this, or the bound has a hole: the memory is committed per hash,
+// not per endpoint.
+func (m *Middleware) BeginPasswordVerification(
+	ctx context.Context,
+) (func(), bool) {
+	return m.guard().acquire(ctx)
+}
+
+// RecordLoginFailure counts a failed credential verification for the
+// request's client against the submitted username, and reports
+// whether the response should be 429 rather than 401.
+func (m *Middleware) RecordLoginFailure(
+	r *http.Request,
+	username string,
+) bool {
+	throttled := m.guard().fail(m.clientKey(r), username)
+	if throttled {
+		m.log.Warn(
+			"login failure limit exceeded", "path", r.URL.Path,
+		)
+	}
+
+	return throttled
+}
+
+// ForgiveLoginFailures clears the failure counters for the request's
+// client and the submitted username after a successful
+// authentication.
+func (m *Middleware) ForgiveLoginFailures(
+	r *http.Request,
+	username string,
+) {
+	m.guard().succeed(m.clientKey(r), username)
+}
+
+// LoginFailureInterval is how long a spent login failure budget
+// takes to refill, which is what a throttled login answers as
+// Retry-After.
+func (m *Middleware) LoginFailureInterval() time.Duration {
+	return m.guard().interval
+}

internal/middleware/loginguard_test.go

@@ -0,0 +1,352 @@
+package middleware_test
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/middleware"
+)
+
+const (
+	// guardInterval is the failure window these tests use. It is
+	// long enough that nothing lapses mid-test on its own; tests
+	// that need a lapse drive the clock instead.
+	guardInterval = time.Minute
+
+	// guardWait is the slot wait for tests that expect to get a
+	// slot. Tests that expect to be refused set their own.
+	guardWait = 2 * time.Second
+
+	guardClient = "198.51.100.7"
+	guardUser   = "admin"
+)
+
+// newGuard builds a guard with production-shaped defaults and the
+// given key-set cap and verification concurrency.
+func newGuard(maxKeys, concurrency int) *middleware.LoginGuard {
+	return middleware.NewLoginGuardForTest(
+		middleware.LoginRateLimitConst,
+		guardInterval,
+		maxKeys,
+		concurrency,
+		guardWait,
+	)
+}
+
+// TestLoginGuard_ThrottlesRepeatedFailures is the brute-force half:
+// wrong passwords for one username from one client key still run out
+// of budget and are answered 429.
+func TestLoginGuard_ThrottlesRepeatedFailures(t *testing.T) {
+	t.Parallel()
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, 1)
+
+	for i := range middleware.LoginRateLimitConst - 1 {
+		assert.False(
+			t, g.FailForTest(guardClient, guardUser),
+			"failure %d is still inside the budget", i,
+		)
+	}
+
+	assert.True(
+		t, g.FailForTest(guardClient, guardUser),
+		"the last failure of the budget must throttle",
+	)
+	assert.True(
+		t, g.FailForTest(guardClient, guardUser),
+		"failures past the budget must stay throttled",
+	)
+}
+
+// TestLoginGuard_SuccessForgivesFailures pins the forgiveness rule:
+// an operator who mistypes several times and then gets it right must
+// not be left throttled.
+func TestLoginGuard_SuccessForgivesFailures(t *testing.T) {
+	t.Parallel()
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, 1)
+
+	for range middleware.LoginRateLimitConst {
+		g.FailForTest(guardClient, guardUser)
+	}
+
+	g.SucceedForTest(guardClient, guardUser)
+
+	assert.False(
+		t, g.FailForTest(guardClient, guardUser),
+		"a success must reset the counter, so the next mistake "+
+			"starts a fresh budget",
+	)
+}
+
+// TestLoginGuard_FailuresAreKeyedPerUsername proves the second half
+// of the keying: one username's spent budget does not throttle
+// another's from the same client.
+func TestLoginGuard_FailuresAreKeyedPerUsername(t *testing.T) {
+	t.Parallel()
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, 1)
+
+	for range middleware.LoginRateLimitConst {
+		g.FailForTest(guardClient, guardUser)
+	}
+
+	assert.True(t, g.FailForTest(guardClient, guardUser))
+	assert.False(
+		t, g.FailForTest(guardClient, "someone-else"),
+		"a different submitted username must have its own budget",
+	)
+}
+
+// TestLoginGuard_WindowLapses covers the interval: a counter that has
+// gone quiet for the whole window starts again from zero.
+func TestLoginGuard_WindowLapses(t *testing.T) {
+	t.Parallel()
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, 1)
+
+	var now atomic.Int64
+
+	now.Store(time.Now().UnixNano())
+	g.SetNowForTest(func() time.Time {
+		return time.Unix(0, now.Load())
+	})
+
+	for range middleware.LoginRateLimitConst {
+		g.FailForTest(guardClient, guardUser)
+	}
+
+	assert.True(t, g.FailForTest(guardClient, guardUser))
+
+	now.Add(int64(guardInterval) + 1)
+
+	assert.False(
+		t, g.FailForTest(guardClient, guardUser),
+		"a lapsed window must start a fresh budget",
+	)
+}
+
+// TestLoginGuard_UsernameKeySetIsBounded is the memory bound. The
+// submitted username is attacker-controlled, so an attacker rotating
+// usernames must not be able to grow the guard without limit: past
+// the cap, tracking falls back to a counter keyed on the client
+// address alone.
+func TestLoginGuard_UsernameKeySetIsBounded(t *testing.T) {
+	t.Parallel()
+
+	const (
+		maxKeys  = 8
+		attempts = 500
+	)
+
+	g := newGuard(maxKeys, 1)
+
+	for i := range attempts {
+		g.FailForTest(guardClient, fmt.Sprintf("user-%d", i))
+	}
+
+	byUser, byAddr := g.TrackedKeysForTest()
+
+	assert.LessOrEqual(
+		t, byUser, maxKeys,
+		"the per-username key set must not grow past its cap",
+	)
+	assert.LessOrEqual(
+		t, byAddr, maxKeys,
+		"the fallback key set must not grow past its cap either",
+	)
+	assert.Positive(
+		t, byAddr,
+		"past the cap, failures must fall back to the address "+
+			"bucket rather than being dropped",
+	)
+	assert.Less(
+		t, byUser+byAddr, attempts,
+		"memory must not grow with the number of distinct "+
+			"usernames submitted",
+	)
+}
+
+// TestLoginGuard_BeyondBothCapsStaysThrottled covers the hard stop.
+// When both key sets are full of live counters and the client is in
+// neither, there is nothing to count without unbounded growth, so the
+// failure is answered as throttled. That costs the operator nothing:
+// a correct password never reaches this path.
+func TestLoginGuard_BeyondBothCapsStaysThrottled(t *testing.T) {
+	t.Parallel()
+
+	const maxKeys = 4
+
+	g := newGuard(maxKeys, 1)
+
+	// Fill the per-username set from one client, then fill the
+	// address set from distinct clients.
+	for i := range maxKeys {
+		g.FailForTest(guardClient, fmt.Sprintf("user-%d", i))
+	}
+
+	for i := range maxKeys {
+		g.FailForTest(fmt.Sprintf("203.0.113.%d", i), "whoever")
+	}
+
+	assert.True(
+		t, g.FailForTest("203.0.113.200", "brand-new"),
+		"a client that fits in neither full key set must be "+
+			"answered as throttled rather than tracked",
+	)
+
+	byUser, byAddr := g.TrackedKeysForTest()
+	assert.LessOrEqual(t, byUser, maxKeys)
+	assert.LessOrEqual(t, byAddr, maxKeys)
+}
+
+// TestLoginGuard_SemaphoreBoundsConcurrentVerifications is the memory
+// bound on the hashing itself. Verifying credentials before spending
+// limiter budget means an attacker can force one Argon2id hash per
+// request, and each allocates 64 MB; without this bound the fix for
+// an admin lockout would be a memory-exhaustion DoS instead.
+func TestLoginGuard_SemaphoreBoundsConcurrentVerifications(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	const (
+		concurrency = 2
+		workers     = 12
+	)
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, concurrency)
+
+	var (
+		mu      sync.Mutex
+		inside  int
+		highest int
+		wg      sync.WaitGroup
+	)
+
+	for range workers {
+		wg.Go(func() {
+			release, ok := g.AcquireForTest(context.Background())
+			if !ok {
+				return
+			}
+
+			defer release()
+
+			mu.Lock()
+
+			inside++
+			if inside > highest {
+				highest = inside
+			}
+
+			mu.Unlock()
+
+			// Hold the slot long enough that the other workers are
+			// certainly contending for it.
+			time.Sleep(10 * time.Millisecond)
+
+			mu.Lock()
+			inside--
+			mu.Unlock()
+		})
+	}
+
+	wg.Wait()
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	assert.Equal(
+		t, concurrency, highest,
+		"no more than %d verifications may run at once", concurrency,
+	)
+}
+
+// TestLoginGuard_SaturatedSemaphoreRefusesRatherThanQueueing pins
+// what happens when every slot is taken for longer than the wait: the
+// request is refused, so the caller answers 503 without allocating
+// another 64 MB hash.
+func TestLoginGuard_SaturatedSemaphoreRefusesRatherThanQueueing(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	g := middleware.NewLoginGuardForTest(
+		middleware.LoginRateLimitConst,
+		guardInterval,
+		middleware.LoginFailureMaxKeysConst,
+		1,
+		10*time.Millisecond,
+	)
+
+	release, ok := g.AcquireForTest(context.Background())
+	require.True(t, ok, "the first acquire must get the only slot")
+
+	_, ok = g.AcquireForTest(context.Background())
+	assert.False(
+		t, ok,
+		"with the only slot held, a second request must be refused "+
+			"rather than wait indefinitely",
+	)
+
+	release()
+
+	release, ok = g.AcquireForTest(context.Background())
+	assert.True(
+		t, ok, "the slot must be reusable once released",
+	)
+
+	release()
+}
+
+// TestLoginGuard_AcquireHonoursCancellation proves a client that
+// disconnects while queued frees its place immediately instead of
+// holding it for the full wait.
+func TestLoginGuard_AcquireHonoursCancellation(t *testing.T) {
+	t.Parallel()
+
+	g := newGuard(middleware.LoginFailureMaxKeysConst, 1)
+
+	release, ok := g.AcquireForTest(context.Background())
+	require.True(t, ok)
+
+	defer release()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, ok = g.AcquireForTest(ctx)
+	assert.False(
+		t, ok, "a cancelled request must not wait for a slot",
+	)
+}
+
+// TestPasswordVerifyConcurrency_MatchesMemoryBudget pins the
+// concurrency constant to the arithmetic behind it: Argon2id here is
+// 64 MB per hash, so the number of slots is the number of 64 MB
+// allocations the process is willing to commit to password hashing.
+// Raising it raises peak resident memory by 64 MB a slot.
+func TestPasswordVerifyConcurrency_MatchesMemoryBudget(t *testing.T) {
+	t.Parallel()
+
+	const (
+		argon2MemoryMB = 64
+		budgetMB       = 128
+	)
+
+	assert.Equal(
+		t,
+		budgetMB/argon2MemoryMB,
+		middleware.PasswordVerifyConcurrencyConst,
+		"the verification concurrency is %d MB of Argon2id memory "+
+			"divided by %d MB per hash",
+		budgetMB, argon2MemoryMB,
+	)
+}

internal/middleware/middleware.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"net"
 	"net/http"
+	"sync"
 	"time"
 
 	basicauth "github.com/99designs/basicauth-go"
@@ -43,6 +44,12 @@ type Middleware struct {
 	log     *slog.Logger
 	params  *MiddlewareParams
 	session *session.Session
+
+	// loginGuard counts failed credential verifications and bounds
+	// concurrent password hashing. It is built on first use so that
+	// every construction path gets one; see guard().
+	loginGuardOnce sync.Once
+	loginGuard     *loginGuard
 }
 
 // New creates a Middleware from the provided fx parameters.

internal/middleware/ratelimit.go

@@ -12,11 +12,15 @@ import (
 )
 
 const (
-	// loginRateLimit is the maximum number of login attempts
-	// per interval.
+	// loginRateLimit is the maximum number of FAILED login attempts
+	// one client may make against one submitted username per
+	// interval before further failures are answered 429. Successful
+	// attempts are never counted and never throttled — see
+	// loginGuard.
 	loginRateLimit = 5
 
-	// loginRateInterval is the time window for the rate limit.
+	// loginRateInterval is the time window for the login failure
+	// limit.
 	loginRateInterval = 1 * time.Minute
 
 	// passwordChangeRateLimit is the maximum number of password
@@ -216,7 +220,7 @@ func (m *Middleware) clientKey(r *http.Request) string {
 	return bucketKey(peer)
 }
 
-// tooManyRequests returns the 429 handler used by the login,
+// tooManyRequests returns the 429 handler used by the
 // password-change and per-entrypoint receiver limiters: it logs the
 // rejection with logMessage and answers with responseMessage.
 // httprate adds the Retry-After header (RFC 6585). The aggregate
@@ -255,26 +259,15 @@ func (m *Middleware) floodTooManyRequests(
 	}
 }
 
-// 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. Clients are
-// identified by rateLimitKey.
-func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
-	return m.postRateLimit(
-		loginRateLimit,
-		loginRateInterval,
-		"login rate limit exceeded",
-		"Too many login attempts. Please try again later.",
-	)
-}
-
 // PasswordChangeRateLimit returns middleware that enforces
 // per-IP rate limiting on password change attempts. The change
 // endpoint verifies the current password, so without a limit a
-// stolen session could be used to brute-force it; the limit
-// matches the login endpoint's.
+// stolen session could be used to brute-force it.
+//
+// Unlike the login POST this limit is still spent on arrival, which
+// is safe here: RequireAuth runs ahead of it, so only a request
+// already carrying a valid session can reach the bucket, and an
+// operator locked out of changing a password can still log in.
 func (m *Middleware) PasswordChangeRateLimit() func(http.Handler) http.Handler {
 	return m.postRateLimit(
 		passwordChangeRateLimit,

internal/middleware/ratelimit_test.go

@@ -20,14 +20,14 @@ import (
 	"sneak.berlin/go/webhooker/internal/middleware"
 )
 
-func TestLoginRateLimit_AllowsGET(t *testing.T) {
+func TestPostRateLimit_AllowsGET(t *testing.T) {
 	t.Parallel()
 
 	m, _ := testMiddleware(t, config.EnvironmentDev)
 
 	var callCount int
 
-	handler := m.LoginRateLimit()(http.HandlerFunc(
+	handler := m.PasswordChangeRateLimit()(http.HandlerFunc(
 		func(w http.ResponseWriter, _ *http.Request) {
 			callCount++
 
@@ -39,7 +39,7 @@ func TestLoginRateLimit_AllowsGET(t *testing.T) {
 	for i := range 20 {
 		req := httptest.NewRequestWithContext(
 			context.Background(),
-			http.MethodGet, "/pages/login", nil,
+			http.MethodGet, "/user/admin/password", nil,
 		)
 		req.RemoteAddr = "192.168.1.1:12345"
 
@@ -110,20 +110,6 @@ func runPostLimitTest(
 	assert.Equal(t, limit, callCount)
 }
 
-func TestLoginRateLimit_LimitsPOST(t *testing.T) {
-	t.Parallel()
-
-	m, _ := testMiddleware(t, config.EnvironmentDev)
-
-	runPostLimitTest(
-		t,
-		m.LoginRateLimit(),
-		middleware.LoginRateLimitConst,
-		"/pages/login",
-		"10.0.0.1:12345",
-	)
-}
-
 func TestPasswordChangeRateLimit_LimitsPOST(t *testing.T) {
 	t.Parallel()
 
@@ -138,19 +124,19 @@ func TestPasswordChangeRateLimit_LimitsPOST(t *testing.T) {
 	)
 }
 
-func TestLoginRateLimit_IndependentPerIP(t *testing.T) {
+func TestPostRateLimit_IndependentPerIP(t *testing.T) {
 	t.Parallel()
 
 	m, _ := testMiddleware(t, config.EnvironmentDev)
 
-	handler := m.LoginRateLimit()(http.HandlerFunc(
+	handler := m.PasswordChangeRateLimit()(http.HandlerFunc(
 		func(w http.ResponseWriter, _ *http.Request) {
 			w.WriteHeader(http.StatusOK)
 		},
 	))
 
 	// Exhaust limit for IP1
-	for range middleware.LoginRateLimitConst {
+	for range middleware.PasswordChangeRateLimitConst {
 		req := httptest.NewRequestWithContext(
 			context.Background(),
 			http.MethodPost, "/pages/login", nil,
@@ -367,7 +353,14 @@ func TestReceiverRateLimit_CountsEveryMethod(t *testing.T) {
 }
 
 const (
-	loginPath  = "/pages/login"
+	// limitedPath is the endpoint these tests drive the shared POST
+	// rate limiter through. It is the password-change path: since
+	// the login POST verifies credentials before spending any
+	// budget, the password-change limiter is the only pre-emptive
+	// POST limiter left, and it is what pins the shared key
+	// function's behaviour here.
+	limitedPath = "/user/admin/password"
+
 	headerXFF  = "X-Forwarded-For"
 	headerReal = "X-Real-IP"
 	headerTrue = "True-Client-IP"
@@ -415,10 +408,10 @@ func assertSharedBucket(
 	m := rateLimitMiddleware(
 		t, &config.Config{TrustedProxies: proxies},
 	)
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for i := range middleware.LoginRateLimitConst {
-		w := postWithHeaders(handler, peer, loginPath, headers(i))
+	for i := range middleware.PasswordChangeRateLimitConst {
+		w := postWithHeaders(handler, peer, limitedPath, headers(i))
 		assert.Equal(
 			t, http.StatusOK, w.Code,
 			"request %d should pass", i,
@@ -426,8 +419,8 @@ func assertSharedBucket(
 	}
 
 	w := postWithHeaders(
-		handler, peer, loginPath,
-		headers(middleware.LoginRateLimitConst),
+		handler, peer, limitedPath,
+		headers(middleware.PasswordChangeRateLimitConst),
 	)
 	assert.Equal(t, http.StatusTooManyRequests, w.Code, msg)
 }
@@ -549,24 +542,24 @@ func TestRateLimitKey_ForwardedHonouredFromTrustedPeer(
 	m := rateLimitMiddleware(t, &config.Config{
 		TrustedProxies: trustedProxies(trustedProxyCIDR),
 	})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
 	const peer = trustedPeer
 
 	first := map[string]string{headerXFF: clientIPv4}
 
-	for range middleware.LoginRateLimitConst {
-		postWithHeaders(handler, peer, loginPath, first)
+	for range middleware.PasswordChangeRateLimitConst {
+		postWithHeaders(handler, peer, limitedPath, first)
 	}
 
-	w := postWithHeaders(handler, peer, loginPath, first)
+	w := postWithHeaders(handler, peer, limitedPath, first)
 	assert.Equal(
 		t, http.StatusTooManyRequests, w.Code,
 		"the forwarded client's own bucket must fill up",
 	)
 
 	w = postWithHeaders(
-		handler, peer, loginPath,
+		handler, peer, limitedPath,
 		map[string]string{headerXFF: clientIPv4Alt},
 	)
 	assert.Equal(
@@ -662,7 +655,7 @@ func TestRateLimitKey_LongChainAllocationIsBounded(t *testing.T) {
 	})
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = trustedPeer
 	req.Header.Set(
@@ -869,7 +862,7 @@ func clientKeyFor(
 	t.Helper()
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = remoteAddr
 
@@ -1019,23 +1012,23 @@ func TestRateLimitKey_UnparseablePeerKeepsDistinctBuckets(
 	)
 }
 
-// TestLoginRateLimit_IPv6SharesBucketWithinSlash64 is the behavioural
+// TestPostRateLimit_IPv6SharesBucketWithinSlash64 is the behavioural
 // half, and the regression test for the bypass itself: a client that
 // rotates source addresses inside its own routed /64 must stay in one
 // bucket. Reverting the masking makes this test fail, because each
 // rotated address would mint a fresh bucket and nothing would be
 // rejected.
-func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
+func TestPostRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for i := range middleware.LoginRateLimitConst {
+	for i := range middleware.PasswordChangeRateLimitConst {
 		w := postWithHeaders(
 			handler,
 			fmt.Sprintf("[2001:db8:1:2::%d]:44444", i+1),
-			loginPath, nil,
+			limitedPath, nil,
 		)
 		assert.Equal(
 			t, http.StatusOK, w.Code, "request %d should pass", i,
@@ -1043,7 +1036,7 @@ func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	}
 
 	w := postWithHeaders(
-		handler, "[2001:db8:1:2::ffff]:44444", loginPath, nil,
+		handler, "[2001:db8:1:2::ffff]:44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusTooManyRequests, w.Code,
@@ -1052,23 +1045,23 @@ func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	)
 }
 
-// TestLoginRateLimit_IPv6IndependentAcrossSlash64 is the other side
+// TestPostRateLimit_IPv6IndependentAcrossSlash64 is the other side
 // of the trade: bucketing by /64 must not merge separate allocations,
 // so a client in a different /64 keeps its own limit.
-func TestLoginRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
+func TestPostRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for range middleware.LoginRateLimitConst + 1 {
+	for range middleware.PasswordChangeRateLimitConst + 1 {
 		postWithHeaders(
-			handler, "[2001:db8:1:2::1]:44444", loginPath, nil,
+			handler, "[2001:db8:1:2::1]:44444", limitedPath, nil,
 		)
 	}
 
 	w := postWithHeaders(
-		handler, "[2001:db8:1:3::1]:44444", loginPath, nil,
+		handler, "[2001:db8:1:3::1]:44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusOK, w.Code,
@@ -1076,23 +1069,23 @@ func TestLoginRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
 	)
 }
 
-// TestLoginRateLimit_IPv4IndependentPerAddress guards against the
+// TestPostRateLimit_IPv4IndependentPerAddress guards against the
 // masking leaking into IPv4: two addresses one apart must still hold
 // separate buckets.
-func TestLoginRateLimit_IPv4IndependentPerAddress(t *testing.T) {
+func TestPostRateLimit_IPv4IndependentPerAddress(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for range middleware.LoginRateLimitConst + 1 {
+	for range middleware.PasswordChangeRateLimitConst + 1 {
 		postWithHeaders(
-			handler, clientIPv4+":44444", loginPath, nil,
+			handler, clientIPv4+":44444", limitedPath, nil,
 		)
 	}
 
 	w := postWithHeaders(
-		handler, clientIPv4Alt+":44444", loginPath, nil,
+		handler, clientIPv4Alt+":44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusOK, w.Code,
@@ -1112,7 +1105,7 @@ func forwardedKeyFor(
 	t.Helper()
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = trustedPeer
 	req.Header.Set(headerXFF, forwarded)
@@ -1177,11 +1170,77 @@ func TestRateLimitKey_ForwardedIPv6BucketsByPrefix(t *testing.T) {
 	}
 }
 
-// TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64 is the
+// TestRateLimitKey_TrustedPeerUnusableForwardedMasksPeer covers the
+// third bucketKey call site: the peer IS a trusted proxy, but the
+// forwarded chain cannot name a client, so the key falls back to the
+// peer address — and that fallback owes the same /64 masking every
+// other key gets.
+//
+// Every existing test of this fallback uses an IPv4 proxy, where
+// bucketKey is the identity function, so replacing the call with
+// peer.String() leaves the whole suite green. Only operator-listed
+// addresses reach this line and the fallback is fail-closed, so this
+// pins behaviour rather than fixing a defect.
+func TestRateLimitKey_TrustedPeerUnusableForwardedMasksPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	const (
+		proxyCIDR = "2001:db8:ffff::/48"
+		proxyPeer = "[2001:db8:ffff:1::5]:44444"
+		wantKey   = "2001:db8:ffff:1::/64"
+	)
+
+	m := rateLimitMiddleware(t, &config.Config{
+		TrustedProxies: trustedProxies(proxyCIDR),
+	})
+
+	for _, tc := range []struct {
+		name      string
+		forwarded string
+		about     string
+	}{{
+		name:  "absent",
+		about: "no X-Forwarded-For at all falls back to the peer",
+	}, {
+		name:      "unreadable-hop",
+		forwarded: "unknown",
+		about: "a hop that is not a bare address ends the walk " +
+			"and falls back to the peer",
+	}, {
+		name:      "all-hops-trusted",
+		forwarded: "2001:db8:ffff:2::9",
+		about: "a chain naming only trusted proxies names no " +
+			"client, so the peer is used",
+	}} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				context.Background(),
+				http.MethodPost, limitedPath, nil,
+			)
+			req.RemoteAddr = proxyPeer
+
+			if tc.forwarded != "" {
+				req.Header.Set(headerXFF, tc.forwarded)
+			}
+
+			assert.Equal(
+				t, wantKey,
+				middleware.ClientKeyForTest(m, req),
+				"%s, masked to its /64", tc.about,
+			)
+		})
+	}
+}
+
+// TestPostRateLimit_ForwardedIPv6SharesBucketWithinSlash64 is the
 // behavioural half on the production path: behind a trusted proxy, a
 // client rotating source addresses inside its own routed /64 must
 // stay in one bucket.
-func TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
+func TestPostRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
 	t *testing.T,
 ) {
 	t.Parallel()
@@ -1198,10 +1257,10 @@ func TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
 	)
 }
 
-// TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64 is the
+// TestPostRateLimit_ForwardedIPv6IndependentAcrossSlash64 is the
 // other side of that trade on the same path: bucketing by /64 must
 // not merge two allocations reaching the proxy.
-func TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64(
+func TestPostRateLimit_ForwardedIPv6IndependentAcrossSlash64(
 	t *testing.T,
 ) {
 	t.Parallel()
@@ -1209,15 +1268,15 @@ func TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64(
 	m := rateLimitMiddleware(t, &config.Config{
 		TrustedProxies: trustedProxies(trustedProxyCIDR),
 	})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
 	spent := map[string]string{headerXFF: clientIPv6}
-	for range middleware.LoginRateLimitConst + 1 {
-		postWithHeaders(handler, trustedPeer, loginPath, spent)
+	for range middleware.PasswordChangeRateLimitConst + 1 {
+		postWithHeaders(handler, trustedPeer, limitedPath, spent)
 	}
 
 	w := postWithHeaders(
-		handler, trustedPeer, loginPath,
+		handler, trustedPeer, limitedPath,
 		map[string]string{headerXFF: clientIPv6Other},
 	)
 	assert.Equal(

internal/server/routes.go

@@ -96,11 +96,14 @@ func (s *Server) setupPageRoutes() {
 		r.Use(s.mw.CSRF())
 		r.Use(s.mw.NoCache())
 
-		r.Group(func(r chi.Router) {
-			r.Use(s.mw.LoginRateLimit())
-			r.Get("/login", s.h.HandleLoginPage())
-			r.Post("/login", s.h.HandleLoginSubmit())
-		})
+		// The login POST carries no pre-emptive rate limiter. Behind
+		// the reverse proxy production requires, with TRUSTED_PROXIES
+		// unset, every client shares one bucket, so a limiter spent
+		// on arrival lets any stranger deny the operator the only
+		// administrative path. The handler verifies credentials first
+		// and charges only failures; see Handlers.authenticateUser.
+		r.Get("/login", s.h.HandleLoginPage())
+		r.Post("/login", s.h.HandleLoginSubmit())
 
 		r.Post("/logout", s.h.HandleLogout())
 	})