feat(auth): per-account brute-force lockout with exponential backoff

Existing protection was per-IP only (login_attempts table). On a loopback
deployment everyone hits 127.0.0.1, so the per-IP counter is mostly
ornamental — the real attacker is on the same machine. Adds a second
defense layer that tracks failures per username with an exponential
backoff schedule.

Schema:
  account_lockouts (username TEXT PK, failed_count INT,
                    locked_until DATETIME, last_attempt_at, last_attempt_ip)

Backoff after threshold (4+ failures):
  1, 2, 3 failures → no lockout (grace window for typos)
  4th             → 60 s
  5th             → 5 min
  6th             → 15 min
  7th             → 1 h
  8th             → 6 h
  9th and beyond  → 24 h (capped)

Counter resets to 0 on successful login or reauth. Old non-locked rows
older than 30 days are pruned by CleanupExpired alongside the existing
sessions / audit_log / login_attempts cleanups.

Wiring:
 - HandleLogin / HandleReauth both check RejectIfAccountLocked() before
   touching the users table. Lockout responses are 429 with JSON body
   { error, retry_after } and a Retry-After header.
 - Failed attempts are recorded against the username even when the user
   doesn't exist, preventing account enumeration via differential
   "is this account locked?" probes.
 - PBKDF2 hash comparison was already constant-time (ConstantTimeEquals);
   no change there.

Client (js/app.js):
 - api() now preserves response status + body on Error so callers can
   distinguish 429-lockout from other errors.
 - New showLockoutCountdown(seconds) renders a live "Account locked —
   try again in Xm Ys" message in #authHint, disables #loginBtn until
   the countdown reaches 0, then re-enables it.
 - doLogin / doUnlock both branch on err.status === 429 + retry_after
   to call showLockoutCountdown instead of a generic error toast.

Known limitation: an attacker can DoS-lock arbitrary usernames by
spamming /login with that name. This is intentional — the alternative
(per-(username,IP) tracking) would let attackers enumerate accounts.
DoS-lock is acceptable; auth bypass is not.

delphi-backend/Handlers/PM.Handler.Auth.pas

@@ -171,6 +171,11 @@ begin
     LBody.Free;
   end;
 
+  // Per-username lockout check — runs BEFORE touching the users table, so
+  // attackers can't probe account existence via timing differences between
+  // "locked" and "not found" responses.
+  if RejectIfAccountLocked(AResponse, LUser) then Exit;
+
   DB.Lock;
   try
     LQ := TFDQuery.Create(nil);
@@ -182,7 +187,13 @@ begin
       LQ.Open;
       if LQ.IsEmpty then
       begin
+        // Unknown username — still record the failure against this username
+        // so attackers can't enumerate accounts by observing which usernames
+        // can be locked vs not. TCriticalSection is reentrant for the same
+        // thread, so calling RecordAttempt/RecordFailedAccountAttempt from
+        // inside our DB.Lock block is safe (they re-acquire the same lock).
         RecordAttempt(LIP);
+        RecordFailedAccountAttempt(LUser, LIP);
         TJSONHelper.SendError(AResponse, 401, 'Invalid credentials');
         Exit;
       end;
@@ -208,6 +219,7 @@ begin
   begin
     // Not implemented in Delphi backend yet
     RecordAttempt(LIP);
+    RecordFailedAccountAttempt(LUser, LIP);
     LogAudit(LUserId, 'failed_login_bcrypt', LIP);
     TJSONHelper.SendError(AResponse, 501,
       'This account was created with bcrypt (PHP). The Delphi backend does ' +
@@ -218,12 +230,14 @@ begin
   if not LValid then
   begin
     RecordAttempt(LIP);
+    RecordFailedAccountAttempt(LUser, LIP);
     LogAudit(LUserId, 'failed_login', LIP);
     TJSONHelper.SendError(AResponse, 401, 'Invalid credentials');
     Exit;
   end;
 
   ClearAttempts(LIP);
+  ClearAccountLockout(LUser);
   DeleteAllUserSessions(LUserId);
   EnsureDefaultFolders(LUserId);
   CreateSession(LUserId, LToken, LCSRF);
@@ -264,7 +278,7 @@ procedure HandleReauth(ARequest: TIdHTTPRequestInfo;
 var
   LUserId: Integer;
   LBody: TJSONObject;
-  LPwd, LStoredHash, LSalt, LAlgo, LIP, LComputed: string;
+  LUser, LPwd, LStoredHash, LSalt, LAlgo, LIP, LComputed: string;
   LQ: TFDQuery;
   LValid: Boolean;
 begin
@@ -294,7 +308,9 @@ begin
     LQ := TFDQuery.Create(nil);
     try
       LQ.Connection := DB.Connection;
-      LQ.SQL.Text := 'SELECT password_hash, salt, hash_algo FROM users WHERE id = :uid';
+      // Pull username too — needed for the per-account lockout calls.
+      LQ.SQL.Text :=
+        'SELECT username, password_hash, salt, hash_algo FROM users WHERE id = :uid';
       LQ.ParamByName('uid').AsInteger := LUserId;
       LQ.Open;
       if LQ.IsEmpty then
@@ -303,6 +319,7 @@ begin
         TJSONHelper.SendError(AResponse, 401, 'User not found');
         Exit;
       end;
+      LUser       := LQ.FieldByName('username').AsString;
       LStoredHash := LQ.FieldByName('password_hash').AsString;
       LSalt       := LQ.FieldByName('salt').AsString;
       LAlgo       := LQ.FieldByName('hash_algo').AsString;
@@ -314,6 +331,12 @@ begin
     DB.Unlock;
   end;
 
+  // Check account lockout AFTER we have the username. Even though the user
+  // is already authenticated by their session token, the master-pw re-prompt
+  // is itself brute-forceable (e.g. attacker hijacked a session and now tries
+  // to escalate by guessing the master pw to unlock the JS crypto key).
+  if RejectIfAccountLocked(AResponse, LUser) then Exit;
+
   LValid := False;
   if SameText(LAlgo, 'pbkdf2') then
   begin
@@ -324,12 +347,14 @@ begin
   if not LValid then
   begin
     RecordAttempt(LIP);
+    RecordFailedAccountAttempt(LUser, LIP);
     LogAudit(LUserId, 'failed_reauth', LIP);
     TJSONHelper.SendError(AResponse, 401, 'Invalid password');
     Exit;
   end;
 
   ClearAttempts(LIP);
+  ClearAccountLockout(LUser);
   LogAudit(LUserId, 'reauth', LIP);
   TJSONHelper.SendOK(AResponse, 'OK');
 end;

delphi-backend/Source/PM.Database.pas

@@ -132,6 +132,21 @@ begin
     '  ip TEXT NOT NULL,' +
     '  attempted_at DATETIME DEFAULT CURRENT_TIMESTAMP' +
     ')');
+  // Per-username lockout state, complementing the per-IP login_attempts
+  // counter. On a loopback-only deployment the per-IP counter is mostly
+  // useless (everyone hits 127.0.0.1), so the per-username counter is the
+  // real defense against brute-force.
+  // - failed_count: total failures since the last successful auth
+  // - locked_until: timestamp the account becomes available again (NULL = not locked)
+  // - last_attempt_at / _ip: forensic info for the audit log
+  FConn.ExecSQL(
+    'CREATE TABLE IF NOT EXISTS account_lockouts (' +
+    '  username TEXT PRIMARY KEY,' +
+    '  failed_count INTEGER NOT NULL DEFAULT 0,' +
+    '  locked_until DATETIME,' +
+    '  last_attempt_at DATETIME DEFAULT CURRENT_TIMESTAMP,' +
+    '  last_attempt_ip TEXT' +
+    ')');
   FConn.ExecSQL(
     'CREATE TABLE IF NOT EXISTS audit_log (' +
     '  id INTEGER PRIMARY KEY AUTOINCREMENT,' +
@@ -209,6 +224,13 @@ begin
   FConn.ExecSQL('DELETE FROM sessions WHERE expires_at < datetime(''now'')');
   FConn.ExecSQL('DELETE FROM login_attempts WHERE attempted_at < datetime(''now'', ''-15 minutes'')');
   FConn.ExecSQL('DELETE FROM audit_log WHERE created_at < datetime(''now'', ''-30 days'')');
+  // Account lockout entries: prune rows that are no longer locked AND haven't
+  // been touched in 30 days (the user clearly isn't being attacked anymore).
+  // Active lockouts and recent attempts are preserved.
+  FConn.ExecSQL(
+    'DELETE FROM account_lockouts ' +
+    'WHERE (locked_until IS NULL OR locked_until < datetime(''now'')) ' +
+    'AND last_attempt_at < datetime(''now'', ''-30 days'')');
   FConn.ExecSQL('DELETE FROM passkey_challenges WHERE created_at < datetime(''now'', ''-10 minutes'')');
 end;
 

delphi-backend/Source/PM.RateLimit.pas

@@ -1,21 +1,58 @@
 unit PM.RateLimit;
 
 {
-  Mirrors api.php checkRateLimit / recordAttempt / clearAttempts.
+  Two-layer brute-force protection:
-  15-minute window. Caller decides the threshold (5 for register, 10 for login).
+
+  1) Per-IP rate limit (login_attempts table, 15-minute window).
+     Useful for non-loopback deployments and as a defense-in-depth layer.
+
+  2) Per-username lockout (account_lockouts table, exponential backoff).
+     The real defense for a loopback-only setup where everyone shares
+     127.0.0.1. After 4 consecutive failed attempts for a given username,
+     the account is locked for an increasing duration (1m, 5m, 15m, 1h,
+     6h, 24h-capped). Counter resets on successful authentication.
+
+     IMPORTANT: failed attempts are recorded even when the username doesn't
+     exist in the users table. This prevents attackers from enumerating
+     existing accounts by observing which usernames trigger a lockout.
+     Trade-off: an attacker can DoS-lock arbitrary usernames, but they can
+     never bypass authentication this way.
 }
 
 interface
 
 uses
-  System.SysUtils, FireDAC.Comp.Client, FireDAC.Stan.Param, IdCustomHTTPServer,
+  System.SysUtils, System.JSON,
+  FireDAC.Comp.Client, FireDAC.Stan.Param, IdCustomHTTPServer,
   PM.Database;
 
 function GetClientIP(ARequest: TIdHTTPRequestInfo): string;
+
+// ---- Per-IP rate limit (legacy) ----
 function CheckRateLimit(const AIP: string): Integer;
 procedure RecordAttempt(const AIP: string);
 procedure ClearAttempts(const AIP: string);
 
+// ---- Per-username account lockout ----
+
+// Seconds remaining until the account is unlocked. 0 means not locked.
+function GetAccountLockoutRemaining(const AUsername: string): Integer;
+
+// Records a failed authentication attempt against AUsername (whether or not
+// the user exists), applies exponential backoff, and returns the new
+// failed_count. Caller should pass this back to the audit log.
+function RecordFailedAccountAttempt(const AUsername, AIP: string): Integer;
+
+// Resets the failed counter and clears any lockout for AUsername. Call on
+// successful login / reauth.
+procedure ClearAccountLockout(const AUsername: string);
+
+// One-stop helper for handlers: if the account is currently locked, sends
+// a 429 with a JSON body { error, retry_after } and returns True. Caller
+// should Exit immediately. Returns False if not locked (caller proceeds).
+function RejectIfAccountLocked(AResponse: TIdHTTPResponseInfo;
+  const AUsername: string): Boolean;
+
 implementation
 
 function GetClientIP(ARequest: TIdHTTPRequestInfo): string;
@@ -91,4 +128,180 @@ begin
   end;
 end;
 
+// =============================================================================
+//  Per-username lockout with exponential backoff
+// =============================================================================
+
+// Exponential backoff schedule. Index = failed_count value AFTER this attempt.
+// First 3 failures give no lockout — a real user typing their master pw
+// wrong once or twice shouldn't be punished. From the 4th failure on, the
+// duration ramps up sharply. Capped at 24 h regardless of further failures.
+function ComputeBackoffSeconds(AFailedCount: Integer): Integer;
+begin
+  case AFailedCount of
+    0..3: Result := 0;       // grace window
+    4:    Result := 60;      // 1 minute
+    5:    Result := 300;     // 5 minutes
+    6:    Result := 900;     // 15 minutes
+    7:    Result := 3600;    // 1 hour
+    8:    Result := 21600;   // 6 hours
+  else
+    Result := 86400;         // 24 hours — capped, regardless of count
+  end;
+end;
+
+function GetAccountLockoutRemaining(const AUsername: string): Integer;
+var
+  LQ: TFDQuery;
+begin
+  Result := 0;
+  if AUsername = '' then Exit;
+
+  DB.Lock;
+  try
+    LQ := TFDQuery.Create(nil);
+    try
+      LQ.Connection := DB.Connection;
+      // CAST(strftime(...) - strftime(...) AS INTEGER) gives seconds remaining.
+      // If locked_until is NULL or in the past, the SELECT returns 0.
+      LQ.SQL.Text :=
+        'SELECT MAX(0, CAST(' +
+        '  (strftime(''%s'', locked_until) - strftime(''%s'', ''now''))' +
+        ' AS INTEGER)) AS remaining ' +
+        'FROM account_lockouts WHERE username = :u';
+      LQ.ParamByName('u').AsString := AUsername;
+      LQ.Open;
+      if not LQ.IsEmpty then
+        Result := LQ.FieldByName('remaining').AsInteger;
+    finally
+      LQ.Free;
+    end;
+  finally
+    DB.Unlock;
+  end;
+end;
+
+function RecordFailedAccountAttempt(const AUsername, AIP: string): Integer;
+var
+  LQ: TFDQuery;
+  LNewCount, LBackoff: Integer;
+begin
+  Result := 0;
+  if AUsername = '' then Exit;
+
+  DB.Lock;
+  try
+    // Step 1: upsert + increment in a single statement. SQLite's
+    // ON CONFLICT(...) DO UPDATE handles the "row already exists" case
+    // atomically without a separate SELECT/UPDATE race.
+    LQ := TFDQuery.Create(nil);
+    try
+      LQ.Connection := DB.Connection;
+      LQ.SQL.Text :=
+        'INSERT INTO account_lockouts ' +
+        '  (username, failed_count, last_attempt_at, last_attempt_ip) ' +
+        'VALUES (:u, 1, CURRENT_TIMESTAMP, :ip) ' +
+        'ON CONFLICT(username) DO UPDATE SET ' +
+        '  failed_count    = failed_count + 1, ' +
+        '  last_attempt_at = CURRENT_TIMESTAMP, ' +
+        '  last_attempt_ip = excluded.last_attempt_ip';
+      LQ.ParamByName('u').AsString := AUsername;
+      LQ.ParamByName('ip').AsString := AIP;
+      LQ.ExecSQL;
+    finally
+      LQ.Free;
+    end;
+
+    // Step 2: read the new failed_count and apply backoff if the threshold
+    // is crossed. Done in two queries because SQLite's RETURNING clause
+    // requires 3.35+ and we want to support older versions.
+    LQ := TFDQuery.Create(nil);
+    try
+      LQ.Connection := DB.Connection;
+      LQ.SQL.Text :=
+        'SELECT failed_count FROM account_lockouts WHERE username = :u';
+      LQ.ParamByName('u').AsString := AUsername;
+      LQ.Open;
+      if LQ.IsEmpty then Exit;
+      LNewCount := LQ.FieldByName('failed_count').AsInteger;
+    finally
+      LQ.Free;
+    end;
+    Result := LNewCount;
+
+    LBackoff := ComputeBackoffSeconds(LNewCount);
+    if LBackoff > 0 then
+    begin
+      LQ := TFDQuery.Create(nil);
+      try
+        LQ.Connection := DB.Connection;
+        LQ.SQL.Text :=
+          'UPDATE account_lockouts SET ' +
+          '  locked_until = datetime(''now'', ''+'' || :sec || '' seconds'') ' +
+          'WHERE username = :u';
+        LQ.ParamByName('sec').AsInteger := LBackoff;
+        LQ.ParamByName('u').AsString := AUsername;
+        LQ.ExecSQL;
+      finally
+        LQ.Free;
+      end;
+    end;
+  finally
+    DB.Unlock;
+  end;
+end;
+
+procedure ClearAccountLockout(const AUsername: string);
+var
+  LQ: TFDQuery;
+begin
+  if AUsername = '' then Exit;
+
+  DB.Lock;
+  try
+    LQ := TFDQuery.Create(nil);
+    try
+      LQ.Connection := DB.Connection;
+      // Hard delete on success: keeps the table small, and there's no
+      // value in remembering past failures once the user has proven they
+      // know the password.
+      LQ.SQL.Text := 'DELETE FROM account_lockouts WHERE username = :u';
+      LQ.ParamByName('u').AsString := AUsername;
+      LQ.ExecSQL;
+    finally
+      LQ.Free;
+    end;
+  finally
+    DB.Unlock;
+  end;
+end;
+
+function RejectIfAccountLocked(AResponse: TIdHTTPResponseInfo;
+  const AUsername: string): Boolean;
+var
+  LRemaining: Integer;
+  LBody: TJSONObject;
+begin
+  LRemaining := GetAccountLockoutRemaining(AUsername);
+  Result := LRemaining > 0;
+  if not Result then Exit;
+
+  // 429 Too Many Requests with a structured body so the client can show
+  // a countdown timer ("Try again in X seconds") instead of a generic
+  // error toast. Standard Retry-After header included as a fallback for
+  // HTTP-aware tooling.
+  AResponse.ResponseNo := 429;
+  AResponse.ContentType := 'application/json; charset=utf-8';
+  AResponse.CustomHeaders.Values['Retry-After'] := IntToStr(LRemaining);
+
+  LBody := TJSONObject.Create;
+  try
+    LBody.AddPair('error', 'Account temporarily locked due to repeated failed attempts');
+    LBody.AddPair('retry_after', TJSONNumber.Create(LRemaining));
+    AResponse.ContentText := LBody.ToJSON;
+  finally
+    LBody.Free;
+  end;
+end;
+
 end.

js/app.js

@@ -158,10 +158,50 @@ async function api(path, opts) {
     const r = await fetch(API + path, opts);
     let body = null;
     try { body = await r.json(); } catch (e) { body = {}; }
-    if (!r.ok) throw new Error(body.error || ('HTTP ' + r.status));
+    if (!r.ok) {
+        // Preserve status + body on the Error so callers can distinguish
+        // 429-with-retry_after (account lockout) from a generic auth error.
+        const err = new Error(body.error || ('HTTP ' + r.status));
+        err.status = r.status;
+        err.body   = body || {};
+        throw err;
+    }
     return body;
 }
 
+// ============================================================
+// ACCOUNT LOCKOUT UI
+// ============================================================
+
+let lockoutTimer = null;
+
+// Called when the backend responds with 429 + retry_after on /login or
+// /reauth. Disables the auth form and displays a live countdown in
+// #authHint. When the countdown reaches 0, the form is re-enabled.
+function showLockoutCountdown(seconds) {
+    if (lockoutTimer) { clearInterval(lockoutTimer); lockoutTimer = null; }
+    const hint  = $('#authHint');
+    const btn   = $('#loginBtn');
+    const fmt   = (s) => {
+        if (s >= 3600) return Math.ceil(s / 3600) + ' h';
+        if (s >= 60)   return Math.ceil(s / 60)   + ' min';
+        return s + ' s';
+    };
+    const tick = () => {
+        if (seconds <= 0) {
+            clearInterval(lockoutTimer); lockoutTimer = null;
+            if (hint) hint.textContent = 'You can try again now.';
+            if (btn)  btn.disabled = false;
+            return;
+        }
+        if (hint) hint.textContent = 'Account locked — try again in ' + fmt(seconds);
+        seconds--;
+    };
+    if (btn) btn.disabled = true;
+    tick();                             // show first frame immediately
+    lockoutTimer = setInterval(tick, 1000);
+}
+
 // ============================================================
 // TOAST
 // ============================================================
@@ -242,9 +282,18 @@ async function doLogin(e) {
         toast('Welcome back, ' + u);
         await enterApp();
     } catch (err) {
+        // 429 with retry_after = account lockout. Show countdown in the
+        // auth hint instead of a generic error toast, and keep the login
+        // button disabled until the lockout expires.
+        if (err.status === 429 && err.body && err.body.retry_after) {
+            showLockoutCountdown(err.body.retry_after);
+            return;     // do NOT re-enable the button in finally
+        }
         toast(err.message, 'error');
     } finally {
-        $('#loginBtn').disabled = false;
+        // Only re-enable when not in lockout (showLockoutCountdown manages
+        // the button itself for the lockout case).
+        if (!lockoutTimer) $('#loginBtn').disabled = false;
     }
 }
 
@@ -301,11 +350,25 @@ function lockVault() {
     state.trashed = [];
     state.locked = true;
     showAuth();
-    $('#loginUsername').value = state.username;
+
-    $('#loginUsername').readOnly = true;
+    // Two UI variants for the auth screen:
-    $('#authHint').textContent = 'Vault locked — enter master password to unlock';
+    //   - We know the username (user was logged in before lock) →
+    //     pre-fill it as readonly so the user only types the master pw.
+    //   - We don't know the username (lock fired before any login — e.g.
+    //     tray "Lock vault" clicked on a fresh session, or Win+L right
+    //     after launch) → show a normal fresh login (editable username).
+    if (state.username) {
+        $('#loginUsername').value    = state.username;
+        $('#loginUsername').readOnly = true;
+        $('#authHint').textContent   = 'Vault locked — enter master password to unlock';
+        $('#loginPassword').focus();
+    } else {
+        $('#loginUsername').value    = '';
+        $('#loginUsername').readOnly = false;
+        $('#authHint').textContent   = '';
+        $('#loginUsername').focus();
+    }
     $('#loginPassword').value = '';
-    $('#loginPassword').focus();
 }
 
 // Unlock flow: validate master pw via /reauth (which uses current session),
@@ -326,6 +389,12 @@ async function doUnlock(p) {
         await enterApp();
         return true;
     } catch (err) {
+        // Account lockout (too many wrong master pw attempts): show
+        // countdown in the auth hint, keep the form disabled.
+        if (err.status === 429 && err.body && err.body.retry_after) {
+            showLockoutCountdown(err.body.retry_after);
+            return false;
+        }
         if (err.message === 'Invalid password') {
             toast('Wrong master password', 'error');
         } else {