feat(auth): lock logins after repeated failures, trust proxies explicitly (FLAW-09)

Failed logins are counted per account+address (3), per account (20) and
per address (30) within 15 minutes; a full bucket answers 429 with
Retry-After for 15 minutes. Locking the account+address bucket first
means an attacker cannot lock the owner out from elsewhere. Unknown
names count like accounts. Success clears the account buckets, a
password reset (mail or admin) lifts the lock. LOGIN_PROTECTION=false
switches it off.

chi's RealIP believed X-Forwarded-For from anyone. The client address
now comes from the headers only when the connection is from a trusted
proxy: TRUSTED_PROXIES=private (default: loopback, Docker, LAN), all
(old behaviour), none, or CIDRs. A proxy that sends no client address
is logged once and per-address limits are skipped for it.

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

internal/api/loginguard.go

@@ -0,0 +1,147 @@
+package api
+
+import (
+	"fmt"
+	"math"
+	"net/http"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Brute-force protection for the login (FLAW-09). Failed attempts are counted in memory in three
+// buckets; when one is full, logins for it are refused with 429 for the lock time:
+//   account + IP   3 failures   the attacker's address is locked out of this account, the real owner
+//                               logging in from elsewhere is not
+//   account       20 failures   stops attacks spread over many addresses
+//   IP            30 failures   stops one address trying many accounts
+// A bucket counts failures within the lock window and starts over afterwards. A successful login
+// clears the account buckets; a password reset clears the account completely. LOGIN_PROTECTION=false
+// switches all of it off. Counters are lost on restart, which is fine for this purpose.
+
+const (
+	loginLockWindow       = 15 * time.Minute
+	loginLimitAccountIP   = 3
+	loginLimitAccount     = 20
+	loginLimitIP          = 30
+	loginGuardCleanupEach = 5 * time.Minute
+)
+
+type loginBucket struct {
+	count       int
+	windowStart time.Time
+	lockedUntil time.Time
+}
+
+type loginGuard struct {
+	enabled bool
+	now     func() time.Time
+
+	mu          sync.Mutex
+	buckets     map[string]*loginBucket
+	lastCleanup time.Time
+}
+
+func newLoginGuard(enabled bool) *loginGuard {
+	return &loginGuard{enabled: enabled, now: time.Now, buckets: map[string]*loginBucket{}}
+}
+
+type loginKey struct {
+	key   string
+	limit int
+}
+
+// loginKeys: account is the user ID (or the typed login for unknown names, so both look the same).
+// The IP buckets are left out when the IP is not known (proxy without client header).
+func loginKeys(account string, ip clientIPInfo) []loginKey {
+	keys := []loginKey{{"acct:" + account, loginLimitAccount}}
+	if !ip.viaProxyWithoutHeader && ip.ip != "" {
+		keys = append(keys, loginKey{"acctip:" + account + "|" + ip.ip, loginLimitAccountIP}, loginKey{"ip:" + ip.ip, loginLimitIP})
+	}
+	return keys
+}
+
+// lockedFor returns how long the login is still refused (0 = allowed).
+func (g *loginGuard) lockedFor(account string, ip clientIPInfo) time.Duration {
+	if !g.enabled {
+		return 0
+	}
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	now := g.now()
+	var wait time.Duration
+	for _, k := range loginKeys(account, ip) {
+		if b := g.buckets[k.key]; b != nil && b.lockedUntil.After(now) && b.lockedUntil.Sub(now) > wait {
+			wait = b.lockedUntil.Sub(now)
+		}
+	}
+	return wait
+}
+
+func (g *loginGuard) recordFailure(account string, ip clientIPInfo) {
+	if !g.enabled {
+		return
+	}
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	now := g.now()
+	g.cleanupLocked(now)
+	for _, k := range loginKeys(account, ip) {
+		b := g.buckets[k.key]
+		if b == nil || now.Sub(b.windowStart) > loginLockWindow {
+			b = &loginBucket{windowStart: now}
+			g.buckets[k.key] = b
+		}
+		b.count++
+		if b.count >= k.limit {
+			b.lockedUntil = now.Add(loginLockWindow)
+		}
+	}
+}
+
+// recordSuccess clears the counters of this account (the IP bucket stays, so one valid login cannot
+// be used to keep trying other accounts).
+func (g *loginGuard) recordSuccess(account string, ip clientIPInfo) {
+	if !g.enabled {
+		return
+	}
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	delete(g.buckets, "acct:"+account)
+	delete(g.buckets, "acctip:"+account+"|"+ip.ip)
+}
+
+// unlockAccount lifts every lock of the account, e.g. after a password reset by mail or by an admin.
+func (g *loginGuard) unlockAccount(account string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for k := range g.buckets {
+		if k == "acct:"+account || strings.HasPrefix(k, "acctip:"+account+"|") {
+			delete(g.buckets, k)
+		}
+	}
+}
+
+func (g *loginGuard) cleanupLocked(now time.Time) {
+	if now.Sub(g.lastCleanup) < loginGuardCleanupEach {
+		return
+	}
+	g.lastCleanup = now
+	for k, b := range g.buckets {
+		if now.Sub(b.windowStart) > loginLockWindow && !b.lockedUntil.After(now) {
+			delete(g.buckets, k)
+		}
+	}
+}
+
+func writeLoginLocked(w http.ResponseWriter, wait time.Duration) {
+	minutes := int(math.Ceil(wait.Minutes()))
+	w.Header().Set("Retry-After", strconv.Itoa(int(math.Ceil(wait.Seconds()))))
+	unit := "Minuten"
+	if minutes == 1 {
+		unit = "Minute"
+	}
+	writeError(w, http.StatusTooManyRequests,
+		fmt.Sprintf("Zu viele fehlgeschlagene Anmeldeversuche. Die Anmeldung ist für noch %d %s gesperrt.", minutes, unit))
+}