feat(auth): add periodic clock re-sync to protect against mid-session drift in Citrix/VM environments

- Add startPeriodicSync() and stopPeriodicSync() to TimeSyncService
- Re-sync clock offset every 5 minutes and on document visibility change
- Add security cap (maxAllowedOffsetMs) to reject unreasonably large offsets
- Preserve existing offset on sync failures instead of resetting to 0
- Wire periodic sync start in RedirectAuthService.configureAuth()
- Wire periodic sync stop in RedirectAuthService.logout()
- Add unit tests for periodic sync and offset cap behavior

lib/core/src/lib/auth/oidc/redirect-auth.service.spec.ts

@@ -52,7 +52,13 @@ describe('RedirectAuthService', () => {
 
     beforeEach(() => {
         retryLoginServiceSpy = jasmine.createSpyObj('RetryLoginService', ['tryToLoginTimes']);
-        timeSyncServiceSpy = jasmine.createSpyObj('TimeSyncService', ['checkTimeSync', 'getCorrectedNow', 'syncClockOffset']);
+        timeSyncServiceSpy = jasmine.createSpyObj('TimeSyncService', [
+            'checkTimeSync',
+            'getCorrectedNow',
+            'syncClockOffset',
+            'startPeriodicSync',
+            'stopPeriodicSync'
+        ]);
         oauthLoggerSpy = jasmine.createSpyObj('OAuthLogger', ['error', 'info', 'warn']);
         oauthServiceSpy = jasmine.createSpyObj(
             'OAuthService',

lib/core/src/lib/auth/oidc/redirect-auth.service.ts

@@ -261,6 +261,7 @@ export class RedirectAuthService extends AuthService {
     }
 
     logout() {
+        this._timeSyncService.stopPeriodicSync();
         this.oauthService.logOut();
     }
 
@@ -351,6 +352,7 @@ export class RedirectAuthService extends AuthService {
                 this._isDiscoveryDocumentLoadedSubject$.next(true);
                 this.oauthService.setupAutomaticSilentRefresh();
                 this._timeSyncService.syncClockOffset().subscribe();
+                this._timeSyncService.startPeriodicSync(undefined, this.oauthService.clockSkewInSec * 1000);
                 return void this.allowRefreshTokenAndSilentRefreshOnMultipleTabs();
             })
             .catch(() => {

lib/core/src/lib/auth/services/time-sync.service.spec.ts

@@ -39,6 +39,7 @@ describe('TimeSyncService', () => {
     });
 
     afterEach(() => {
+        service.stopPeriodicSync();
         httpMock.verify();
     });
 
@@ -235,16 +236,62 @@ describe('TimeSyncService', () => {
             httpMock.expectNone('http://fake-server-time-url');
         });
 
-        it('should leave clockOffsetMs at 0 when the server time endpoint fails', () => {
+        it('should leave clockOffsetMs unchanged when the server time endpoint fails', () => {
             appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+            service.clockOffsetMs = 5000;
 
             service.syncClockOffset().subscribe(() => {
-                expect(service.clockOffsetMs).toBe(0);
+                expect(service.clockOffsetMs).toBe(5000);
             });
 
             const req = httpMock.expectOne('http://fake-server-time-url');
             req.error(new ProgressEvent(''));
         });
+
+        it('should not update offset when it exceeds maxAllowedOffsetMs', () => {
+            appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+
+            const timeBeforeRequest = 1728911579000;
+            const timeResponseReceived = 1728911580000;
+
+            // Server is 600 seconds ahead (way beyond our cap)
+            const serverTime = 1728912180000;
+
+            spyOn(Date, 'now').and.returnValues(timeBeforeRequest, timeResponseReceived);
+
+            service.clockOffsetMs = 1000;
+
+            // Cap at 60 seconds (60000 ms)
+            service.syncClockOffset(60000).subscribe(() => {
+                // Offset should remain unchanged because computed offset exceeds cap
+                expect(service.clockOffsetMs).toBe(1000);
+            });
+
+            const req = httpMock.expectOne('http://fake-server-time-url');
+            req.flush(serverTime);
+        });
+
+        it('should update offset when it is within maxAllowedOffsetMs', () => {
+            appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+
+            const timeBeforeRequest = 1728911579000;
+            const timeResponseReceived = 1728911580000;
+
+            // Server is 30 seconds ahead (within our cap)
+            const serverTime = 1728911610000;
+            // adjustedServerTime = 1728911610000 + 1000/2 = 1728911610500
+            // offset = 1728911610500 - 1728911580000 = 30500 ms
+
+            spyOn(Date, 'now').and.returnValues(timeBeforeRequest, timeResponseReceived);
+
+            // Cap at 60 seconds (60000 ms)
+            service.syncClockOffset(60000).subscribe(() => {
+                expect(service.clockOffsetMs).toBe(30500);
+            });
+
+            const req = httpMock.expectOne('http://fake-server-time-url');
+            req.flush(serverTime);
+        });
     });
 
     describe('getCorrectedNow', () => {
@@ -273,4 +320,63 @@ describe('TimeSyncService', () => {
             expect(service.getCorrectedNow()).toBe(fixedNow - 60000);
         });
     });
+
+    describe('startPeriodicSync', () => {
+        it('should re-sync on visibility change', () => {
+            appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+
+            const timeBeforeRequest = 1728911579000;
+            const timeResponseReceived = 1728911580000;
+            const serverTime = 1728911610000;
+
+            spyOn(Date, 'now').and.returnValues(timeBeforeRequest, timeResponseReceived);
+
+            service.startPeriodicSync(60000);
+
+            Object.defineProperty(document, 'visibilityState', { value: 'visible', writable: true, configurable: true });
+            document.dispatchEvent(new Event('visibilitychange'));
+
+            const req = httpMock.expectOne('http://fake-server-time-url');
+            req.flush(serverTime);
+
+            expect(service.clockOffsetMs).toBe(30500);
+        });
+
+        it('should apply maxAllowedOffsetMs cap during visibility re-sync', () => {
+            appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+
+            const timeBeforeRequest = 1728911579000;
+            const timeResponseReceived = 1728911580000;
+            // Server is 600 seconds ahead — exceeds cap
+            const serverTime = 1728912180000;
+
+            spyOn(Date, 'now').and.returnValues(timeBeforeRequest, timeResponseReceived);
+
+            service.clockOffsetMs = 1000;
+            service.startPeriodicSync(60000, 60000);
+
+            Object.defineProperty(document, 'visibilityState', { value: 'visible', writable: true, configurable: true });
+            document.dispatchEvent(new Event('visibilitychange'));
+
+            const req = httpMock.expectOne('http://fake-server-time-url');
+            req.flush(serverTime);
+
+            // Offset should remain unchanged
+            expect(service.clockOffsetMs).toBe(1000);
+        });
+    });
+
+    describe('stopPeriodicSync', () => {
+        it('should remove visibility change listener', () => {
+            appConfigSpy.get.and.returnValue('http://fake-server-time-url');
+
+            service.startPeriodicSync(60000);
+            service.stopPeriodicSync();
+
+            Object.defineProperty(document, 'visibilityState', { value: 'visible', writable: true, configurable: true });
+            document.dispatchEvent(new Event('visibilitychange'));
+
+            httpMock.expectNone('http://fake-server-time-url');
+        });
+    });
 });

lib/core/src/lib/auth/services/time-sync.service.ts

@@ -16,10 +16,10 @@
  */
 
 import { HttpClient } from '@angular/common/http';
-import { Injectable, Injector, inject } from '@angular/core';
+import { Injectable, Injector, NgZone, inject } from '@angular/core';
 import { AppConfigService } from '../../app-config/app-config.service';
-import { from, Observable, of, throwError } from 'rxjs';
-import { catchError, map, timeout } from 'rxjs/operators';
+import { from, interval, Observable, of, Subscription, throwError } from 'rxjs';
+import { catchError, map, switchMap, timeout } from 'rxjs/operators';
 
 export interface TimeSync {
     outOfSync: boolean;
@@ -28,12 +28,16 @@ export interface TimeSync {
     serverDateTimeISO: string;
 }
 
+/** Default interval for periodic clock re-sync (5 minutes). */
+const DEFAULT_PERIODIC_SYNC_INTERVAL_MS = 5 * 60 * 1000;
+
 @Injectable({
     providedIn: 'root'
 })
 export class TimeSyncService {
     private readonly _injector = inject(Injector);
     private readonly _appConfigService = inject(AppConfigService);
+    private readonly _ngZone = inject(NgZone);
 
     private readonly _http: HttpClient;
 
@@ -44,6 +48,9 @@ export class TimeSyncService {
      */
     clockOffsetMs = 0;
 
+    private _periodicSyncSubscription: Subscription | null = null;
+    private _visibilityChangeHandler: (() => void) | null = null;
+
     constructor() {
         this._http = this._injector.get(HttpClient);
     }
@@ -63,11 +70,15 @@ export class TimeSyncService {
      * Fetches the server time once and stores the signed clock offset in `clockOffsetMs`.
      * Call this at application start-up (fire-and-forget) so subsequent calls to
      * `getCorrectedNow` compensate for any VM / Citrix clock drift.
-     * If `serverTimeUrl` is not configured or the request fails, the offset is left at 0.
+     * If `serverTimeUrl` is not configured or the request fails, the offset is left unchanged
+     * (or at 0 if this is the first call).
      *
+     * @param maxAllowedOffsetMs Optional safety cap. If the computed offset exceeds this value,
+     *                           it is ignored to prevent a compromised time endpoint from
+     *                           tricking the client into accepting expired tokens.
      * @returns Observable that completes after the offset has been stored (or silently on error)
      */
-    syncClockOffset(): Observable<void> {
+    syncClockOffset(maxAllowedOffsetMs?: number): Observable<void> {
         try {
             const startTime = Date.now();
             return this.getServerTime().pipe(
@@ -79,15 +90,17 @@ export class TimeSyncService {
                     const serverTimeInMs = isServerTimeResponseInMs ? serverTimeResponse : serverTimeResponse * 1000;
                     const adjustedServerTimeInMs = serverTimeInMs + roundTripTimeInMs / 2;
 
-                    this.clockOffsetMs = adjustedServerTimeInMs - endTime;
+                    const newOffset = adjustedServerTimeInMs - endTime;
+
+                    if (maxAllowedOffsetMs != null && Math.abs(newOffset) > maxAllowedOffsetMs) {
+                        return;
+                    }
+
+                    this.clockOffsetMs = newOffset;
                 }),
-                catchError(() => {
-                    this.clockOffsetMs = 0;
-                    return of(void 0);
-                })
+                catchError(() => of(void 0))
             );
         } catch {
-            this.clockOffsetMs = 0;
             return of(void 0);
         }
     }
@@ -135,6 +148,50 @@ export class TimeSyncService {
         return this.checkTimeSync(maxAllowedClockSkewInSec).pipe(map((sync) => sync.outOfSync));
     }
 
+    /**
+     * Starts periodic re-synchronization of the clock offset to protect against
+     * progressive clock drift during a user session (common in Citrix/VM environments).
+     *
+     * Re-sync is triggered:
+     * - On a regular interval (default: every 5 minutes)
+     * - When the document becomes visible again (e.g., Citrix session resumes after idle)
+     *
+     * @param intervalMs How often to re-sync in milliseconds (default: 5 minutes)
+     * @param maxAllowedOffsetMs Safety cap for the offset. If exceeded, the new offset is ignored.
+     */
+    startPeriodicSync(intervalMs: number = DEFAULT_PERIODIC_SYNC_INTERVAL_MS, maxAllowedOffsetMs?: number): void {
+        this.stopPeriodicSync();
+
+        this._ngZone.runOutsideAngular(() => {
+            this._periodicSyncSubscription = interval(intervalMs)
+                .pipe(switchMap(() => this.syncClockOffset(maxAllowedOffsetMs)))
+                .subscribe();
+
+            this._visibilityChangeHandler = () => {
+                if (typeof document !== 'undefined' && document.visibilityState === 'visible') {
+                    this.syncClockOffset(maxAllowedOffsetMs).subscribe();
+                }
+            };
+
+            if (typeof document !== 'undefined') {
+                document.addEventListener('visibilitychange', this._visibilityChangeHandler);
+            }
+        });
+    }
+
+    /**
+     * Stops the periodic clock re-synchronization and removes the visibility change listener.
+     */
+    stopPeriodicSync(): void {
+        this._periodicSyncSubscription?.unsubscribe();
+        this._periodicSyncSubscription = null;
+
+        if (this._visibilityChangeHandler && typeof document !== 'undefined') {
+            document.removeEventListener('visibilitychange', this._visibilityChangeHandler);
+            this._visibilityChangeHandler = null;
+        }
+    }
+
     private getServerTime(): Observable<number> {
         return from(this._http.get<number>(this.getServerTimeUrl())).pipe(
             timeout(5000),