auth: sign-in limits per address and name; successes free (audit A1, A8, A9)

Security audit 2026-10-17, findings A1 (medium), A8, A9 (low), stage 3.
Under the shipped Apache-on-host deployment with TRUSTED_PROXY_CIDR empty,
every visitor has the Docker gateway's address, and every public limit was
keyed on the address alone: ten POSTs to /login per 15 minutes (any body,
successes counted) locked out every administrator; likewise /login/reset,
the link pages and /setup/. IPv6 was keyed per /128.

Now (internal/web/auth/limitkeys.go):
- sign-in counts per address and username (10 / 15 min) and per address
  alone at 100; a right password refunds both (rateLimiter.Refund);
- a reset request counts per address and username (5) and per address
  alone (50); a link's page per link, by token hash (30), and per address
  alone (300); setup per address as before;
- an IPv6 address is keyed by its /64 (limitAddr);
- proxyWatch: when the first 20 requests after a start all come from one
  private or loopback peer with X-Forwarded-For and no proxy is trusted,
  the log says so once with the TRUSTED_PROXY_CIDR value to set;
- a refused sign-in or a lockout is logged, five lines per address in 15
  minutes, the typed name printable and cut (refusalLog, logName).
The shipped compose cannot know the gateway, so the guide's install step
prints it (docker network inspect) and says to set it.

Tests: TestLoginBehindOneUntrustedProxyKeepsOthersIn (the reader's case),
TestLoginSuccessesDoNotCount, TestIPv6IsLimitedPerSlash64,
TestProxyWatchAndRefusalsAreLogged; the existing lockout tests pass.

Docs: security.md § First-run setup, § Authentication; guide § Full
deployment; CHANGELOG; plan. Also the I1 plan line's Done note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/auth/auth.go

@@ -31,16 +31,28 @@ type Module struct {
 	sessions *sessionStore
 	setup    *setupManager
 
-	loginLimiter    *rateLimiter
-	setupLimiter    *rateLimiter
-	passwordLimiter *rateLimiter
-	// resetLimiter counts the requests for a reset link per client address,
-	// linkLimiter the requests a link's page gets per client address, and
-	// mailLimiter the links mailed to one user's address (links.go).
-	resetLimiter *rateLimiter
-	linkLimiter  *rateLimiter
-	mailLimiter  *rateLimiter
-	mailer       Mailer
+	// loginLimiter counts failed sign-ins per address and username,
+	// loginAddrLimiter per address alone at ten times that; setupLimiter
+	// counts setup requests per address. Neither address bucket is one a
+	// stranger behind the same proxy fills for everyone at the per-user size
+	// (limitkeys.go).
+	loginLimiter     *rateLimiter
+	loginAddrLimiter *rateLimiter
+	setupLimiter     *rateLimiter
+	passwordLimiter  *rateLimiter
+	// resetLimiter counts the requests for a reset link per address and
+	// username, resetAddrLimiter per address alone; linkLimiter the requests
+	// a link's page gets per link, linkAddrLimiter per address at a high
+	// ceiling; mailLimiter the links mailed to one user's address (links.go).
+	resetLimiter     *rateLimiter
+	resetAddrLimiter *rateLimiter
+	linkLimiter      *rateLimiter
+	linkAddrLimiter  *rateLimiter
+	mailLimiter      *rateLimiter
+
+	proxyWatch proxyWatch
+	refusals   *refusalLog
+	mailer     Mailer
 	// mailing counts the links being mailed after their answer, so a stop
 	// waits for them (Wait) before the database closes.
 	mailing sync.WaitGroup
@@ -56,19 +68,23 @@ func New(st *store.Store, cfg Config, v *view.Engine, setupTokenPath string) *Mo
 		idleDays = 7
 	}
 	m := &Module{
-		store:        st,
-		cfg:          cfg,
-		view:         v,
-		sessions:     newSessionStore(st, time.Duration(idleDays)*24*time.Hour),
-		setupLimiter: newRateLimiter(10, time.Minute),
-		loginLimiter: newRateLimiter(10, 15*time.Minute),
+		store:            st,
+		cfg:              cfg,
+		view:             v,
+		sessions:         newSessionStore(st, time.Duration(idleDays)*24*time.Hour),
+		setupLimiter:     newRateLimiter(10, time.Minute),
+		loginLimiter:     newRateLimiter(10, 15*time.Minute),
+		loginAddrLimiter: newRateLimiter(100, 15*time.Minute),
 		// The Account form's current-password check has a bucket of its
 		// own, per user: ten wrong guesses there used to lock the address
 		// out of signing in (audit 2026-10-12, L15), and the one guessing
 		// is whoever holds that user's session, wherever from.
-		passwordLimiter: newRateLimiter(10, 15*time.Minute),
-		resetLimiter:    newRateLimiter(5, 15*time.Minute),
-		linkLimiter:     newRateLimiter(30, 15*time.Minute),
+		passwordLimiter:  newRateLimiter(10, 15*time.Minute),
+		resetLimiter:     newRateLimiter(5, 15*time.Minute),
+		resetAddrLimiter: newRateLimiter(50, 15*time.Minute),
+		linkLimiter:      newRateLimiter(30, 15*time.Minute),
+		linkAddrLimiter:  newRateLimiter(300, 15*time.Minute),
+		refusals:         newRefusalLog(),
 		// Five links an hour to one address, of either kind: three resets
 		// asked for and a couple of confirmations is ordinary use, a mailbox
 		// full of them is not.
@@ -77,6 +93,10 @@ func New(st *store.Store, cfg Config, v *view.Engine, setupTokenPath string) *Mo
 	}
 	m.setupLimiter.startSweeper()
 	m.loginLimiter.startSweeper()
+	m.loginAddrLimiter.startSweeper()
+	m.resetAddrLimiter.startSweeper()
+	m.linkAddrLimiter.startSweeper()
+	m.refusals.limiter.startSweeper()
 	m.passwordLimiter.startSweeper()
 	m.resetLimiter.startSweeper()
 	m.linkLimiter.startSweeper()

internal/web/auth/handlers.go

@@ -101,8 +101,17 @@ func (m *Module) renderLogin(w http.ResponseWriter, status int, formErr string)
 	m.view.Render(w, status, "login", view.NewLogin(m.cfg.Hostname, formErr))
 }
 
+// submitLogin checks a sign-in. Its limits are per address and username,
+// and per address alone at ten times that, and only a failure counts: one
+// stranger behind the proxy everyone shares no longer locks every
+// administrator out, nor does a day of signing in (security audit
+// 2026-10-17, A1). A refusal and a lockout are logged, a few lines per
+// address in a window (A8).
 func (m *Module) submitLogin(w http.ResponseWriter, r *http.Request) {
-	if !m.loginLimiter.Allow(clientIP(r, m.trustedProxies)) {
+	m.proxyWatch.see(r, m.trustedProxies)
+	addr := limitAddr(r, m.trustedProxies)
+	if !m.loginAddrLimiter.Allow(addr) {
+		m.refusals.printf(addr, "panel: sign-in from %s refused: too many attempts from the address", addr)
 		m.renderLogin(w, http.StatusTooManyRequests, "Too many attempts. Please wait and try again.")
 		return
 	}
@@ -112,6 +121,17 @@ func (m *Module) submitLogin(w http.ResponseWriter, r *http.Request) {
 	}
 	username := strings.TrimSpace(r.PostFormValue("username"))
 	password := r.PostFormValue("password")
+	key := userKey(addr, username)
+	if !m.loginLimiter.Allow(key) {
+		m.loginAddrLimiter.Refund(addr) // counted once, against the name
+		m.refusals.printf(addr, "panel: sign-in as %q from %s refused: too many attempts", logName(username), addr)
+		m.renderLogin(w, http.StatusTooManyRequests, "Too many attempts. Please wait and try again.")
+		return
+	}
+	refused := func() {
+		m.refusals.printf(addr, "panel: sign-in as %q from %s refused: wrong username or password", logName(username), addr)
+		m.renderLogin(w, http.StatusUnauthorized, "Invalid username or password.")
+	}
 
 	user, err := m.store.GetUserByUsername(username)
 	if err != nil {
@@ -122,14 +142,17 @@ func (m *Module) submitLogin(w http.ResponseWriter, r *http.Request) {
 		// a wrong password is: the answer's delay told which names exist
 		// (audit 2026-10-12, M8).
 		_ = bcrypt.CompareHashAndPassword(noSuchUserHash(), []byte(password))
-		m.renderLogin(w, http.StatusUnauthorized, "Invalid username or password.")
+		refused()
 		return
 	}
 
 	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(password)); err != nil {
-		m.renderLogin(w, http.StatusUnauthorized, "Invalid username or password.")
+		refused()
 		return
 	}
+	// A right password is no guess: its attempts are given back.
+	m.loginLimiter.Refund(key)
+	m.loginAddrLimiter.Refund(addr)
 
 	// The session the browser presents, if any, ends here: a sign-in issues
 	// a new token and leaves no other behind it - one planted before the
@@ -195,7 +218,8 @@ func (m *Module) HandleLogout(w http.ResponseWriter, r *http.Request) {
 // HandleSetup serves the one-time administrator creation flow at
 // /setup/<token> (security.md).
 func (m *Module) HandleSetup(w http.ResponseWriter, r *http.Request) {
-	if !m.setupLimiter.Allow(clientIP(r, m.trustedProxies)) {
+	m.proxyWatch.see(r, m.trustedProxies)
+	if !m.setupLimiter.Allow(limitAddr(r, m.trustedProxies)) {
 		http.Error(w, "too many requests", http.StatusTooManyRequests)
 		return
 	}

internal/web/auth/limitkeys.go

@@ -0,0 +1,123 @@
+package auth
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"net"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+	"unicode"
+)
+
+// The keys the public routes are limited by (security audit 2026-10-17, A1,
+// A8, A9).
+//
+// A client's address is the one clientIP reads, an IPv6 address cut to its
+// /64: a client with a /64 to itself rotates addresses inside it at will
+// and never met a per-address limit (A9). Behind a reverse proxy the panel
+// does not trust, every visitor has the proxy's address; so no limit that a
+// stranger can fill is keyed on the address alone at a size that stops the
+// others: sign-in counts per address and username, the address alone only
+// at a ceiling ten times higher; a link page counts per link; and a right
+// password gives its attempt back (A1). proxyWatch says in the log when the
+// shared-address case is what the panel sees.
+
+// limitAddr is the client's address as the limiters key it.
+func limitAddr(r *http.Request, trusted []*net.IPNet) string {
+	host := clientIP(r, trusted)
+	ip := net.ParseIP(host)
+	if ip == nil || ip.To4() != nil {
+		return host
+	}
+	return (&net.IPNet{IP: ip.Mask(net.CIDRMask(64, 128)), Mask: net.CIDRMask(64, 128)}).String()
+}
+
+// userKey is a limiter key for one username tried from one address. The
+// name is folded as the store folds it (usernames are matched without
+// regard to case).
+func userKey(addr, username string) string {
+	return addr + "|" + strings.ToLower(strings.TrimSpace(username))
+}
+
+// tokenKey is a limiter key for one link: its hash, so a limiter's map holds
+// no token.
+func tokenKey(token string) string {
+	sum := sha256.Sum256([]byte(token))
+	return "link:" + hex.EncodeToString(sum[:8])
+}
+
+// logName is a typed username as a log line may hold it: printable ASCII,
+// cut. It is the stranger's text.
+func logName(s string) string {
+	s = strings.Map(func(r rune) rune {
+		if r > unicode.MaxASCII || !unicode.IsPrint(r) {
+			return '?'
+		}
+		return r
+	}, s)
+	if len(s) > 64 {
+		s = s[:64] + "…"
+	}
+	return s
+}
+
+// proxyWatchRequests is how many requests in a row must come from one
+// private peer with X-Forwarded-For, while no proxy is trusted, before the
+// panel says so.
+const proxyWatchRequests = 20
+
+// proxyWatch looks at the first requests after the start. When they all
+// come from one private (or loopback) peer that carries X-Forwarded-For and
+// TRUSTED_PROXY_CIDR is empty, every visitor shares that peer's sign-in
+// limits: it says so once, with the value to set. Any other request - no
+// header, another peer, a public one - ends the watch silently.
+type proxyWatch struct {
+	mu   sync.Mutex
+	peer string
+	n    int
+	done bool
+}
+
+func (w *proxyWatch) see(r *http.Request, trusted []*net.IPNet) {
+	if len(trusted) > 0 {
+		return
+	}
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.done {
+		return
+	}
+	peer, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		peer = r.RemoteAddr
+	}
+	ip := net.ParseIP(peer)
+	if ip == nil || !(ip.IsPrivate() || ip.IsLoopback()) || r.Header.Get("X-Forwarded-For") == "" ||
+		(w.peer != "" && w.peer != peer) {
+		w.done = true
+		return
+	}
+	w.peer = peer
+	if w.n++; w.n >= proxyWatchRequests {
+		w.done = true
+		logf("panel: the last %d requests all came from %s with X-Forwarded-For, which is ignored while TRUSTED_PROXY_CIDR is empty: "+
+			"every visitor shares one sign-in limit. If %s is your reverse proxy, set TRUSTED_PROXY_CIDR=%s (guide § Environment variables)",
+			w.n, peer, peer, peer)
+	}
+}
+
+// refusalLog lets a refused sign-in or a lockout be logged a few times per
+// address in a window, so the log shows a brute force without becoming one.
+type refusalLog struct{ limiter *rateLimiter }
+
+func newRefusalLog() *refusalLog {
+	return &refusalLog{limiter: newRateLimiter(5, 15*time.Minute)}
+}
+
+func (l *refusalLog) printf(addr, format string, args ...any) {
+	if l.limiter.Allow(addr) {
+		logf(format, args...)
+	}
+}

internal/web/auth/limitkeys_test.go

@@ -0,0 +1,116 @@
+package auth
+
+import (
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// Behind a reverse proxy the panel does not trust, every visitor has the
+// proxy's address. A stranger there who guesses one name, or sprays many,
+// must not lock out an administrator signing in from behind the same proxy
+// (security audit 2026-10-17, A1: the reader's test, two clients behind one
+// untrusted peer).
+func TestLoginBehindOneUntrustedProxyKeepsOthersIn(t *testing.T) {
+	m := moduleWithAdmin(t)
+	const proxy = "172.18.0.1:40000"
+	for i := 0; i < 15; i++ {
+		postLogin(m, proxy, "admin2", "guess")
+	}
+	if rec := postLogin(m, proxy, "admin", testPassword); rec.Code != http.StatusSeeOther {
+		t.Fatalf("the administrator behind the same proxy = %d after a stranger's guesses at another name", rec.Code)
+	}
+	// Guessing the administrator's own name locks that name from there,
+	// not the others.
+	for i := 0; i < 10; i++ {
+		postLogin(m, proxy, "admin", "guess")
+	}
+	if rec := postLogin(m, proxy, "admin", testPassword); rec.Code != http.StatusTooManyRequests {
+		t.Errorf("the guessed name after ten failures = %d, want 429", rec.Code)
+	}
+	// Spraying names: the address alone is stopped at a hundred failures.
+	m = moduleWithAdmin(t)
+	for i := 0; i < 100; i++ {
+		postLogin(m, "203.0.113.50:1", "spray"+strings.Repeat("x", i%40)+string(rune('a'+i%26)), "guess")
+	}
+	if rec := postLogin(m, "203.0.113.50:1", "admin", testPassword); rec.Code != http.StatusTooManyRequests {
+		t.Errorf("after a hundred failures from one address = %d, want 429", rec.Code)
+	}
+}
+
+// A right password is no guess: a day of signing in uses up no limit
+// (security audit 2026-10-17, A1 (c)).
+func TestLoginSuccessesDoNotCount(t *testing.T) {
+	m := moduleWithAdmin(t)
+	for i := 0; i < 25; i++ {
+		if rec := postLogin(m, "203.0.113.7:1", "admin", testPassword); rec.Code != http.StatusSeeOther {
+			t.Fatalf("sign-in %d = %d", i+1, rec.Code)
+		}
+	}
+}
+
+// An IPv6 client is one client per /64: rotating addresses inside it
+// gives no fresh limit (security audit 2026-10-17, A9).
+func TestIPv6IsLimitedPerSlash64(t *testing.T) {
+	m := moduleWithAdmin(t)
+	for i := 0; i < 10; i++ {
+		postLogin(m, "[2001:db8:1:2::"+strconv.Itoa(100+i)+"]:443", "admin", "guess")
+	}
+	if rec := postLogin(m, "[2001:db8:1:2:ffff::1]:443", "admin", testPassword); rec.Code != http.StatusTooManyRequests {
+		t.Errorf("another address in the same /64 = %d, want 429", rec.Code)
+	}
+	if rec := postLogin(m, "[2001:db8:1:3::1]:443", "admin", testPassword); rec.Code != http.StatusSeeOther {
+		t.Errorf("another /64 = %d, want a sign-in", rec.Code)
+	}
+	if got := limitAddr(httptest.NewRequest(http.MethodGet, "/", nil), nil); got != "192.0.2.1" {
+		t.Errorf("an IPv4 address is keyed as %q", got)
+	}
+}
+
+// When every request comes from one private peer with X-Forwarded-For and no
+// proxy is trusted, the log says so once, with the value to set; a request
+// without the header ends the watch (security audit 2026-10-17, A1 (a)).
+// Refusals are logged, a few per address (A8).
+func TestProxyWatchAndRefusalsAreLogged(t *testing.T) {
+	var buf strings.Builder
+	log.SetOutput(&buf)
+	t.Cleanup(func() { log.SetOutput(os.Stderr) })
+
+	var w proxyWatch
+	req := func(xff bool) *http.Request {
+		r := httptest.NewRequest(http.MethodGet, "/login", nil)
+		r.RemoteAddr = "172.18.0.1:5000"
+		if xff {
+			r.Header.Set("X-Forwarded-For", "198.51.100.4")
+		}
+		return r
+	}
+	for i := 0; i < proxyWatchRequests+5; i++ {
+		w.see(req(true), nil)
+	}
+	if n := strings.Count(buf.String(), "set TRUSTED_PROXY_CIDR=172.18.0.1"); n != 1 {
+		t.Errorf("the warning was logged %d times:\n%s", n, buf.String())
+	}
+	buf.Reset()
+	var quiet proxyWatch
+	quiet.see(req(false), nil)
+	for i := 0; i < proxyWatchRequests+5; i++ {
+		quiet.see(req(true), nil)
+	}
+	if buf.Len() != 0 {
+		t.Errorf("a watch ended by a request without the header logged:\n%s", buf.String())
+	}
+
+	m := moduleWithAdmin(t)
+	for i := 0; i < 8; i++ {
+		postLogin(m, "203.0.113.9:1", "ad\x1bmin", "guess")
+	}
+	got := buf.String()
+	if n := strings.Count(got, "refused: wrong username or password"); n != 5 || !strings.Contains(got, `sign-in as "ad?min" from 203.0.113.9`) {
+		t.Errorf("refusals logged %d times (want 5), or the name unescaped:\n%s", n, got)
+	}
+}

internal/web/auth/links.go

@@ -132,8 +132,16 @@ func (m *Module) HandleReset(w http.ResponseWriter, r *http.Request) {
 	case http.MethodGet:
 		m.view.Render(w, http.StatusOK, "login-reset", view.NewReset(m.cfg.Hostname, ""))
 	case http.MethodPost:
-		if !m.resetLimiter.Allow(clientIP(r, m.trustedProxies)) {
+		// Per address and username, and per address alone at ten times
+		// that: one stranger behind a shared proxy cannot stop everyone
+		// asking for a link (security audit 2026-10-17, A1).
+		m.proxyWatch.see(r, m.trustedProxies)
+		addr := limitAddr(r, m.trustedProxies)
+		tooMany := func() {
 			m.view.Render(w, http.StatusTooManyRequests, "login-reset", view.NewReset(m.cfg.Hostname, "Too many attempts. Please wait and try again."))
+		}
+		if !m.resetAddrLimiter.Allow(addr) {
+			tooMany()
 			return
 		}
 		if err := r.ParseForm(); err != nil {
@@ -141,6 +149,10 @@ func (m *Module) HandleReset(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		username := strings.TrimSpace(r.PostFormValue("username"))
+		if !m.resetLimiter.Allow(userKey(addr, username)) {
+			tooMany()
+			return
+		}
 		m.later(func() { m.sendReset(username) })
 		http.Redirect(w, r, "/login?reset=sent", http.StatusSeeOther)
 	default:
@@ -201,15 +213,26 @@ func linkToken(path, prefix string) string {
 
 const deadLink = "This link has expired or was already used."
 
+// allowLink is the limit of a link's page: per link, so that nobody can use up
+// another's, and per address at a ceiling high enough for everyone behind one
+// proxy (security audit 2026-10-17, A1).
+func (m *Module) allowLink(r *http.Request, token string) bool {
+	m.proxyWatch.see(r, m.trustedProxies)
+	if !m.linkAddrLimiter.Allow(limitAddr(r, m.trustedProxies)) {
+		return false
+	}
+	return token == "" || m.linkLimiter.Allow(tokenKey(token))
+}
+
 // HandleResetLink serves /login/reset/<token>: the new password's form (GET)
 // and setting it (POST). Setting it takes the link, ends every session of the
 // account and every other reset link, and leaves the person at sign-in.
 func (m *Module) HandleResetLink(w http.ResponseWriter, r *http.Request) {
-	if !m.linkLimiter.Allow(clientIP(r, m.trustedProxies)) {
+	token := linkToken(r.URL.Path, "/login/reset/")
+	if !m.allowLink(r, token) {
 		http.Error(w, "too many requests", http.StatusTooManyRequests)
 		return
 	}
-	token := linkToken(r.URL.Path, "/login/reset/")
 	if token == "" {
 		http.NotFound(w, r)
 		return
@@ -305,11 +328,11 @@ func (m *Module) setPasswordFromLink(w http.ResponseWriter, r *http.Request, tok
 // a mistyped address would hand the account to whoever reads it, by a reset
 // link (security review, 2026-10-17).
 func (m *Module) HandleConfirm(w http.ResponseWriter, r *http.Request) {
-	if !m.linkLimiter.Allow(clientIP(r, m.trustedProxies)) {
+	token := linkToken(r.URL.Path, "/confirm/")
+	if !m.allowLink(r, token) {
 		http.Error(w, "too many requests", http.StatusTooManyRequests)
 		return
 	}
-	token := linkToken(r.URL.Path, "/confirm/")
 	if token == "" {
 		http.NotFound(w, r)
 		return