[Server] Inform client of login/registration rate limit timeout (#7468)

* [Security] Add per-address rate limiting for auth endpoints

Introduce a thread-safe RateLimiter that tracks attempts per key
(IP address) within a sliding time window, and wire it into the
authentication endpoints:

- Login: failed login attempts from an address are counted; once the
  configured maximum is exceeded within the window, further logins
  from that address are rejected with RespTooManyRequests. A
  successful login clears the failed attempts for that address.
- Registration: implement the previously stubbed
  tooManyRegistrationAttempts, limiting how many accounts can be
  created per address per window.
- Forgot-password: throttle both the email-request and the
  email-challenge paths per address.

New [security] settings with defaults:
  max_login_attempts_per_ip=5 / login_attempt_window_seconds=900
  max_registrations_per_ip=2 / registration_window_seconds=3600
  max_forgot_password_requests_per_ip=3 / forgot_password_window_seconds=3600

Adds unit tests for the RateLimiter (window limit, over-limit
blocking, clearing, per-key independence).

Took 3 minutes

* [Server] Fix rate limiter: disable semantics, draining window, bounded keys, pre-work login gate

- recordAttempt()/isBlocked() return false when maxAttempts <= 0 or windowSeconds <= 0, so the documented 'set to 0 to disable' actually disables instead of blocking every address permanently
- Keys at their limit are rejected without recording, so repeated attempts at the attacker's pace cannot keep sliding the window and holding the lockout open; a block now clears once the recorded attempts age out of windowSeconds
- Attempts map is bounded: an opportunistic prune (every 60s) erases keys whose attempts have fully aged out
- Login is throttled before the work: isLoginRateLimited() (a new virtual on Server, backed by isBlocked()) gates loginUser(), so a locked-out address no longer burns a database round trip and password verification per attempt
- recordAttemptAt()/isBlockedAt() time seams make the sliding-window behaviour deterministically testable; tests rewritten for the real semantics (no reliance on the maxAttempts=0 bug, no sleeps)

* [Server] Inform client of login/registration rate limit timeout

- Add retryAfterSeconds/retryAfterSecondsAt to RateLimiter to compute
  seconds until oldest attempt exits the sliding window
- Expose loginRateLimitRemainingSeconds via Server/Servatrice
- Include denied_end_time (absolute epoch) in RespTooManyRequests for
  login pre-auth and when transitioning to lockout after failed attempt
- Include denied_end_time in registration rate limit response
- Client: show retry time for login RespTooManyRequests
- Client: simplify registration RespTooManyRequests message to show
  retry time when provided
- Add test for retryAfterSecondsAt

* [Client] Restore registration guidance and drop duplicated rate limit reason

The registration RespTooManyRequests case now only shows the retry-after
copy when the server actually sent a denied_end_time; the per-email account
cap path (no end time) falls back to the previous guidance about already
having an account and contacting the server operator.

Drop the canned denied_reason_str from the new login rate limit branches so
the client no longer prints the same sentence twice.

* [Server] Exempt trusted_sources from auth rate limits

Addresses in security/trusted_sources bypass per-address limits, matching
the existing max_users_per_address bypass in initSession. Without this,
Webatrice behind local nginx (all clients as 127.0.0.1) would lock out
every web user after 5 mistyped passwords, and cap the whole server at
2 registrations / 3 forgot-password requests per window.

Adds Servatrice::isRateLimitExempt helper; recordFailedLogin,
isLoginRateLimited, recordRegistrationAttempt and
recordForgotPasswordAttempt all check it.

# Conflicts:
#	servatrice/src/servatrice.cpp

* [Server] Key login rate limit by (address, username)

A successful login cleared the whole per-IP failure history, so 4 bad
guesses plus 1 self-login (or guest UnknownUser login) from the same
address bypassed the limiter. Failures are now bucketed per
(address, lowercase username) and a success clears only its own pair.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/src/ratelimiter.cpp

@@ -0,0 +1,125 @@
+#include "ratelimiter.h"
+
+#include <QDateTime>
+
+bool RateLimiter::recordAttempt(const QString &key, int maxAttempts, int windowSeconds)
+{
+    return recordAttemptAt(key, maxAttempts, windowSeconds, QDateTime::currentSecsSinceEpoch());
+}
+
+bool RateLimiter::recordAttemptAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now)
+{
+    if (maxAttempts <= 0 || windowSeconds <= 0) {
+        return false; // rate limiting disabled for this endpoint
+    }
+
+    QMutexLocker locker(&mutex);
+    pruneLocked(now);
+
+    AttemptHistory &history = attempts[key];
+    history.windowSeconds = windowSeconds;
+    QList<qint64> &timestamps = history.timestamps;
+    while (!timestamps.isEmpty() && timestamps.first() <= now - windowSeconds) {
+        timestamps.removeFirst();
+    }
+    if (timestamps.size() >= maxAttempts) {
+        // Already at the limit: reject without appending so repeated attempts
+        // at the attacker's own pace cannot keep sliding the window forward
+        // and hold the lockout open forever.
+        return true;
+    }
+
+    timestamps.append(now);
+    return false;
+}
+
+bool RateLimiter::isBlocked(const QString &key, int maxAttempts, int windowSeconds) const
+{
+    return isBlockedAt(key, maxAttempts, windowSeconds, QDateTime::currentSecsSinceEpoch());
+}
+
+bool RateLimiter::isBlockedAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now) const
+{
+    if (maxAttempts <= 0 || windowSeconds <= 0) {
+        return false;
+    }
+
+    QMutexLocker locker(&mutex);
+
+    const auto it = attempts.constFind(key);
+    if (it == attempts.constEnd()) {
+        return false;
+    }
+
+    int count = 0;
+    for (const qint64 &timestamp : it.value().timestamps) {
+        if (timestamp > now - windowSeconds) {
+            ++count;
+        }
+    }
+    return count >= maxAttempts;
+}
+
+void RateLimiter::clearAttempts(const QString &key)
+{
+    QMutexLocker locker(&mutex);
+    attempts.remove(key);
+}
+
+int RateLimiter::retryAfterSeconds(const QString &key, int maxAttempts, int windowSeconds) const
+{
+    return retryAfterSecondsAt(key, maxAttempts, windowSeconds, QDateTime::currentSecsSinceEpoch());
+}
+
+int RateLimiter::retryAfterSecondsAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now) const
+{
+    if (maxAttempts <= 0 || windowSeconds <= 0) {
+        return 0;
+    }
+
+    QMutexLocker locker(&mutex);
+
+    const auto it = attempts.constFind(key);
+    if (it == attempts.constEnd()) {
+        return 0;
+    }
+
+    QList<qint64> timestamps = it.value().timestamps;
+    while (!timestamps.isEmpty() && timestamps.first() <= now - windowSeconds) {
+        timestamps.removeFirst();
+    }
+    if (timestamps.size() < maxAttempts) {
+        return 0;
+    }
+    if (timestamps.isEmpty()) {
+        return 0;
+    }
+
+    qint64 oldest = timestamps.first();
+    qint64 retryAt = oldest + windowSeconds;
+    if (retryAt <= now) {
+        return 0;
+    }
+    return static_cast<int>(retryAt - now);
+}
+
+void RateLimiter::pruneLocked(qint64 now)
+{
+    if (lastPruneSecs + PruneIntervalSeconds > now) {
+        return;
+    }
+    lastPruneSecs = now;
+
+    auto it = attempts.begin();
+    while (it != attempts.end()) {
+        QList<qint64> &timestamps = it.value().timestamps;
+        while (!timestamps.isEmpty() && timestamps.first() <= now - it.value().windowSeconds) {
+            timestamps.removeFirst();
+        }
+        if (timestamps.isEmpty()) {
+            it = attempts.erase(it);
+        } else {
+            ++it;
+        }
+    }
+}

servatrice/src/ratelimiter.h

@@ -0,0 +1,93 @@
+#ifndef RATELIMITER_H
+#define RATELIMITER_H
+
+#include <QList>
+#include <QMap>
+#include <QMutex>
+#include <QString>
+
+/** @brief Thread-safe per-key attempt limiter used to throttle abusive requests. */
+class RateLimiter
+{
+public:
+    /**
+     * @brief Record an attempt for @p key and report whether the key is now blocked.
+     *
+     * A key that is already at its limit is not extended: the attempt is
+     * rejected without appending, so the lockout lifts once the recorded
+     * attempts age out of the window. Returns false immediately when
+     * @p maxAttempts <= 0 or @p windowSeconds <= 0 (rate limiting disabled),
+     * recording nothing.
+     *
+     * Delegates to recordAttemptAt() with the current wall-clock time.
+     */
+    bool recordAttempt(const QString &key, int maxAttempts, int windowSeconds);
+
+    /**
+     * @brief Record an attempt as seen at epoch second @p now.
+     *
+     * Exposed for deterministic tests; production callers should use
+     * recordAttempt().
+     */
+    bool recordAttemptAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now);
+
+    /**
+     * @brief True if @p key already has at least @p maxAttempts attempts within @p windowSeconds.
+     *
+     * Reads only; records nothing. Returns false when @p maxAttempts <= 0 or
+     * @p windowSeconds <= 0. Delegates to isBlockedAt().
+     */
+    bool isBlocked(const QString &key, int maxAttempts, int windowSeconds) const;
+
+    /**
+     * @brief Whether @p key is blocked as measured at epoch second @p now.
+     *
+     * Exposed for deterministic tests; production callers should use
+     * isBlocked().
+     */
+    bool isBlockedAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now) const;
+
+    /** @brief Drop all recorded attempts for the key, e.g. after a successful login. */
+    void clearAttempts(const QString &key);
+
+    /**
+     * @brief Get the retry-after window in seconds for @p key.
+     *
+     * Returns the number of seconds from @p now until the oldest attempt
+     * exits @p windowSeconds and the key is no longer blocked. If the key is
+     * not blocked, returns 0. Returns 0 if @p maxAttempts <= 0 or
+     * @p windowSeconds <= 0.
+     *
+     * Delegates to retryAfterSecondsAt().
+     */
+    int retryAfterSeconds(const QString &key, int maxAttempts, int windowSeconds) const;
+
+    /**
+     * @brief Get the retry-after window in seconds for @p key as measured at @p now.
+     *
+     * Exposed for deterministic tests.
+     */
+    int retryAfterSecondsAt(const QString &key, int maxAttempts, int windowSeconds, qint64 now) const;
+
+private:
+    struct AttemptHistory
+    {
+        QList<qint64> timestamps; // epoch seconds, oldest first
+        int windowSeconds;
+    };
+
+    /**
+     * @brief Drops keys whose last attempts have fully aged out of their window.
+     *
+     * Runs at most every PruneIntervalSeconds to stop the map from growing
+     * without bound when attackers rotate source addresses.
+     */
+    void pruneLocked(qint64 now);
+
+    mutable QMutex mutex;
+    QMap<QString, AttemptHistory> attempts; // key -> attempt history
+    qint64 lastPruneSecs = 0;
+    static constexpr int PruneIntervalSeconds = 60;
+};
+
+#endif

servatrice/src/servatrice.cpp

@@ -51,6 +51,7 @@
 #include <QSsl>
 #include <QString>
 #include <QStringList>
+#include <QStringLiteral>
 #include <QThread>
 #include <QTimer>
 #include <QTypeInfo>
@@ -1286,6 +1287,105 @@ int Servatrice::getForgotPasswordTokenLife() const
     return settingsCache->value("forgotpassword/tokenlife", 60).toInt();
 }
 
+/**
+ * @brief Rate limiter key for one login target.
+ *
+ * Keyed by (address, username) so a success only clears that pair's history:
+ * a guest login or a login as the attacker's own account from the same
+ * address must not wipe out another user's failure count. Lowercased so
+ * case variations of the same name share one bucket.
+ */
+static QString loginLimitKey(const QString &ipAddress, const QString &userName)
+{
+    return QStringLiteral("login:") + ipAddress + QStringLiteral(":") + userName.toLower();
+}
+
+bool Servatrice::recordFailedLogin(const QString &ipAddress, const QString &userName)
+{
+    if (isRateLimitExempt(ipAddress)) {
+        return false;
+    }
+    return rateLimiter.recordAttempt(loginLimitKey(ipAddress, userName), getMaxLoginAttemptsPerIp(),
+                                     getLoginAttemptWindowSeconds());
+}
+
+bool Servatrice::isLoginRateLimited(const QString &ipAddress, const QString &userName)
+{
+    if (isRateLimitExempt(ipAddress)) {
+        return false;
+    }
+    return rateLimiter.isBlocked(loginLimitKey(ipAddress, userName), getMaxLoginAttemptsPerIp(),
+                                 getLoginAttemptWindowSeconds());
+}
+
+int Servatrice::loginRateLimitRemainingSeconds(const QString &ipAddress, const QString &userName)
+{
+    return rateLimiter.retryAfterSeconds(loginLimitKey(ipAddress, userName), getMaxLoginAttemptsPerIp(),
+                                         getLoginAttemptWindowSeconds());
+}
+
+bool Servatrice::recordRegistrationAttempt(const QString &ipAddress)
+{
+    if (isRateLimitExempt(ipAddress)) {
+        return false;
+    }
+    return rateLimiter.recordAttempt("register:" + ipAddress, getMaxRegistrationsPerIp(),
+                                     getRegistrationWindowSeconds());
+}
+
+bool Servatrice::recordForgotPasswordAttempt(const QString &ipAddress)
+{
+    if (isRateLimitExempt(ipAddress)) {
+        return false;
+    }
+    return rateLimiter.recordAttempt("forgot:" + ipAddress, getMaxForgotPasswordRequestsPerIp(),
+                                     getForgotPasswordWindowSeconds());
+}
+
+bool Servatrice::isRateLimitExempt(const QString &ipAddress) const
+{
+    // Mirrors the max_users_per_address bypass in initSession: trusted sources
+    // (e.g. 127.0.0.1 for Webatrice behind local nginx, where every websocket
+    // client shares one address) are not subject to per-address limits.
+    const QString trustedSources = settingsCache->value("security/trusted_sources", "127.0.0.1,::1").toString();
+    return trustedSources.contains(ipAddress, Qt::CaseInsensitive);
+}
+
+void Servatrice::clearFailedLogins(const QString &ipAddress, const QString &userName)
+{
+    rateLimiter.clearAttempts(loginLimitKey(ipAddress, userName));
+}
+
+int Servatrice::getMaxLoginAttemptsPerIp() const
+{
+    return settingsCache->value("security/max_login_attempts_per_ip", 5).toInt();
+}
+
+int Servatrice::getLoginAttemptWindowSeconds() const
+{
+    return settingsCache->value("security/login_attempt_window_seconds", 900).toInt();
+}
+
+int Servatrice::getMaxRegistrationsPerIp() const
+{
+    return settingsCache->value("security/max_registrations_per_ip", 2).toInt();
+}
+
+int Servatrice::getRegistrationWindowSeconds() const
+{
+    return settingsCache->value("security/registration_window_seconds", 3600).toInt();
+}
+
+int Servatrice::getMaxForgotPasswordRequestsPerIp() const
+{
+    return settingsCache->value("security/max_forgot_password_requests_per_ip", 3).toInt();
+}
+
+int Servatrice::getForgotPasswordWindowSeconds() const
+{
+    return settingsCache->value("security/forgot_password_window_seconds", 3600).toInt();
+}
+
 bool Servatrice::getEnableForgotPasswordChallenge() const
 {
     return settingsCache->value("forgotpassword/enablechallenge", false).toBool();

servatrice/src/servatrice.h

@@ -21,6 +21,7 @@
 #define SERVATRICE_H
 
 #include "metrics_registry.h"
+#include "ratelimiter.h"
 #include "smtpclient.h"
 
 #include <QByteArray>
@@ -204,6 +205,8 @@ private:
     std::shared_ptr<const Response_ReportStats> reportStatsCache;
     static constexpr int reportStatsCacheTtlSeconds = 60;
 
+    RateLimiter rateLimiter;
+
     mutable QMutex serverListMutex;
     QList<ServerProperties> serverList;
     void updateServerList();
@@ -362,6 +365,25 @@ public:
      */
     void watchWorkerThread(QThread *thread);
 
+    RateLimiter *getRateLimiter()
+    {
+        return &rateLimiter;
+    }
+    bool recordFailedLogin(const QString &ipAddress, const QString &userName) override;
+    bool isLoginRateLimited(const QString &ipAddress, const QString &userName) override;
+    int loginRateLimitRemainingSeconds(const QString &ipAddress, const QString &userName) override;
+    void clearFailedLogins(const QString &ipAddress, const QString &userName) override;
+    bool recordRegistrationAttempt(const QString &ipAddress);
+    bool recordForgotPasswordAttempt(const QString &ipAddress);
+    /// True when the address is in security/trusted_sources and therefore exempt from per-address limits.
+    bool isRateLimitExempt(const QString &ipAddress) const;
+    int getMaxLoginAttemptsPerIp() const;
+    int getLoginAttemptWindowSeconds() const;
+    int getMaxRegistrationsPerIp() const;
+    int getRegistrationWindowSeconds() const;
+    int getMaxForgotPasswordRequestsPerIp() const;
+    int getForgotPasswordWindowSeconds() const;
+
     bool islConnectionExists(int _serverId) const;
     void addIslInterface(int _serverId, IslInterface *interface);
     void removeIslInterface(int _serverId);

servatrice/src/serversocketinterface.cpp

@@ -3263,6 +3263,15 @@ Response::ResponseCode AbstractServerSocketInterface::cmdRegisterAccount(const C
                                          false);
         }
 
+        Response_Register *re = new Response_Register;
+        re->set_denied_reason_str("Too many registration attempts");
+        int remaining = servatrice->getRateLimiter()->retryAfterSeconds("register:" + this->getAddress(),
+                                                                        servatrice->getMaxRegistrationsPerIp(),
+                                                                        servatrice->getRegistrationWindowSeconds());
+        if (remaining > 0) {
+            re->set_denied_end_time(QDateTime::currentDateTime().addSecs(remaining).toSecsSinceEpoch());
+        }
+        rc.setResponseExtension(re);
         return Response::RespTooManyRequests;
     }
 
@@ -3330,9 +3339,7 @@ Response::ResponseCode AbstractServerSocketInterface::cmdRegisterAccount(const C
 
 bool AbstractServerSocketInterface::tooManyRegistrationAttempts(const QString &ipAddress)
 {
-    //! \todo Implement registration attempt limiting.
-    Q_UNUSED(ipAddress);
-    return false;
+    return servatrice->recordRegistrationAttempt(ipAddress);
 }
 
 Response::ResponseCode AbstractServerSocketInterface::cmdActivateAccount(const Command_Activate &cmd,
@@ -3687,6 +3694,15 @@ Response::ResponseCode AbstractServerSocketInterface::cmdForgotPasswordRequest(c
 
     qCDebug(AbstractServerSocketInterfaceLog) << "Received reset password request from user:" << userName;
 
+    if (servatrice->recordForgotPasswordAttempt(this->getAddress())) {
+        if (servatrice->getEnableForgotPasswordAudit()) {
+            sqlInterface->addAuditRecord(userName.simplified(), this->getAddress(), clientId.simplified(),
+                                         "PASSWORD_RESET_REQUEST", "Too many requests from this ip address", false);
+        }
+
+        return Response::RespTooManyRequests;
+    }
+
     if (!servatrice->getEnableForgotPassword()) {
         if (servatrice->getEnableForgotPasswordAudit()) {
             sqlInterface->addAuditRecord(userName.simplified(), this->getAddress(), clientId.simplified(),
@@ -3829,6 +3845,14 @@ AbstractServerSocketInterface::cmdForgotPasswordChallenge(const Command_ForgotPa
 
     qCDebug(AbstractServerSocketInterfaceLog) << "Received reset password challenge from user:" << userName;
 
+    if (servatrice->recordForgotPasswordAttempt(this->getAddress())) {
+        if (servatrice->getEnableForgotPasswordAudit()) {
+            sqlInterface->addAuditRecord(userName.simplified(), this->getAddress(), clientId.simplified(),
+                                         "PASSWORD_RESET_CHALLENGE", "Too many requests from this ip address", false);
+        }
+        return Response::RespTooManyRequests;
+    }
+
     if (!servatrice->getEnableForgotPasswordChallenge()) {
         if (servatrice->getEnableForgotPasswordAudit()) {
             sqlInterface->addAuditRecord(userName.simplified(), this->getAddress(), clientId.simplified(),