fix(auth): lock accounts after failed attempts, one-time TOTP codes, stronger MFA changes

- Per-account lockout: five failed attempts (password, MFA code or a
  step-up confirmation) lock the account for 15 minutes, on top of the
  per-IP throttle - guesses spread over many addresses no longer help.
- A TOTP code is accepted once: the time step is stored and that step or
  any earlier one is refused (also between two concurrent requests).
- Login runs bcrypt on every path and answers unknown email, wrong
  password, deactivated and locked accounts with the same message; failed
  logins for unknown emails no longer write attacker-chosen text into the
  audit log.
- Turning MFA off needs password and a current code; turning it on needs
  the password. Both end all other sessions.
- New audit action ACCOUNT_LOCKED.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/src/auth/auth.service.ts

@@ -7,6 +7,7 @@ import { UsersService } from '../users/users.service';
 import { AuditLogService } from '../audit/audit-log.service';
 import { PermissionResolverService } from './permissions/permission-resolver.service';
 import { MfaService } from './mfa/mfa.service';
+import { AccountGuardService } from './mfa/account-guard.service';
 import { parseDurationToMs } from './util/duration';
 import { TOKEN_AUDIENCE, jwtSignOptions } from './util/jwt-claims';
 
@@ -32,6 +33,19 @@ export interface CurrentUserProfile {
 
 const REFRESH_TOKEN_BYTES = 40;
 
+// Same answer for unknown email, wrong password, deactivated or locked
+// account - nothing in the response tells which one it was.
+const INVALID_CREDENTIALS =
+  'E-Mail oder Passwort falsch. Nach mehreren Fehlversuchen ist die Anmeldung für 15 Minuten gesperrt.';
+
+// Hash of a random value nobody knows, at the same cost as real password
+// hashes; compared against when there is no real hash, so every login
+// costs the same time.
+const DUMMY_PASSWORD_HASH = bcrypt.hashSync(
+  crypto.randomBytes(32).toString('hex'),
+  12,
+);
+
 @Injectable()
 export class AuthService {
   private readonly logger = new Logger(AuthService.name);
@@ -43,6 +57,7 @@ export class AuthService {
     private readonly auditLog: AuditLogService,
     private readonly permissionResolver: PermissionResolverService,
     private readonly mfaService: MfaService,
+    private readonly accountGuard: AccountGuardService,
   ) {}
 
   async getCurrentUserProfile(userId: string): Promise<CurrentUserProfile> {
@@ -91,35 +106,36 @@ export class AuthService {
 
   async login(email: string, password: string): Promise<LoginResult> {
     const user = await this.usersService.findByEmail(email);
+    // bcrypt runs on every path - unknown, deactivated, locked or real - so
+    // the response time does not reveal which accounts exist.
+    const passwordMatches = await bcrypt.compare(
+      password,
+      user?.passwordHash ?? DUMMY_PASSWORD_HASH,
+    );
     if (!user) {
-      this.logger.warn(`Login failed: no user with email ${email}`);
-      await this.auditLog.record({
-        entityType: 'AuthSession',
-        entityId: email,
-        action: 'LOGIN_FAILED',
-      });
-      throw new UnauthorizedException('Invalid credentials');
+      this.logger.warn('Login failed: unknown email');
+      throw new UnauthorizedException(INVALID_CREDENTIALS);
     }
-    if (!user.isActive) {
-      this.logger.warn(`Login failed: user ${user.id} is deactivated`);
+    if (!user.isActive || this.accountGuard.isLocked(user)) {
+      this.logger.warn(`Login refused: user ${user.id} inactive or locked`);
       await this.auditLog.record({
         entityType: 'AuthSession',
         entityId: user.id,
         action: 'LOGIN_FAILED',
         userId: user.id,
       });
-      throw new UnauthorizedException('Invalid credentials');
+      throw new UnauthorizedException(INVALID_CREDENTIALS);
     }
-    const passwordMatches = await bcrypt.compare(password, user.passwordHash);
     if (!passwordMatches) {
       this.logger.warn(`Login failed: wrong password for user ${user.id}`);
+      await this.accountGuard.recordFailure(user.id);
       await this.auditLog.record({
         entityType: 'AuthSession',
         entityId: user.id,
         action: 'LOGIN_FAILED',
         userId: user.id,
       });
-      throw new UnauthorizedException('Invalid credentials');
+      throw new UnauthorizedException(INVALID_CREDENTIALS);
     }
     if (user.mfaEnabled) {
       return {
@@ -133,6 +149,7 @@ export class AuthService {
     };
     const accessToken = this.signAccessToken(authenticatedUser);
     const refreshToken = await this.issueRefreshToken(user.id);
+    await this.accountGuard.recordSuccess(user.id);
     await this.auditLog.record({
       entityType: 'AuthSession',
       entityId: user.id,
@@ -180,15 +197,15 @@ export class AuthService {
     if (!user.isActive || !user.mfaEnabled || !user.mfaSecret) {
       throw new UnauthorizedException('MFA nicht verfügbar');
     }
-    const isValid = await this.mfaService.verifyCode(
-      user.mfaSecret,
-      user.email,
-      code,
-    );
+    if (this.accountGuard.isLocked(user)) {
+      throw new UnauthorizedException(INVALID_CREDENTIALS);
+    }
+    const isValid = await this.accountGuard.consumeTotp(user, code);
     if (!isValid) {
       this.logger.warn(
-        `MFA verification failed: wrong code for user ${user.id}`,
+        `MFA verification failed: wrong or reused code for user ${user.id}`,
       );
+      await this.accountGuard.recordFailure(user.id);
       await this.auditLog.record({
         entityType: 'AuthSession',
         entityId: user.id,
@@ -197,6 +214,7 @@ export class AuthService {
       });
       throw new UnauthorizedException('Code ungültig');
     }
+    await this.accountGuard.recordSuccess(user.id);
     const authenticatedUser = {
       id: user.id,
       email: user.email,