diff --git a/packages/core-service/src/core/integrations/integrations.controller.ts b/packages/core-service/src/core/integrations/integrations.controller.ts
index 9ca7753..2e82b4b 100644
--- a/packages/core-service/src/core/integrations/integrations.controller.ts
+++ b/packages/core-service/src/core/integrations/integrations.controller.ts
@@ -5,7 +5,6 @@ import {
   Query,
   Res,
   Logger,
-  UseGuards,
   Param,
 } from '@nestjs/common';
 import { ApiTags, ApiOperation } from '@nestjs/swagger';
@@ -53,15 +52,15 @@ export class IntegrationsController {
 
   /**
    * GET /api/v1/auth/integrations/microsoft-365
-   * OAuth2-Flow fuer Microsoft 365 starten.
-   * User muss eingeloggt sein — userId wird im State gespeichert.
+   * OAuth2-Flow fuer Microsoft 365 vorbereiten.
+   * Gibt die Microsoft-Authorisierungs-URL als JSON zurueck.
+   * Das Frontend leitet den Browser dann client-seitig weiter (axios + window.location).
    */
   @Get('auth/integrations/microsoft-365')
   @ApiOperation({ summary: 'Microsoft 365 Integration starten' })
   async initM365Integration(
     @CurrentUser() user: JwtUser,
-    @Res() res: Response,
-  ): Promise<void> {
+  ): Promise<{ success: boolean; data: { url: string } }> {
     // State = UUID, in Redis mit userId hinterlegen (5 Min TTL)
     const state = uuidv4();
     await this.redis.set(
@@ -74,7 +73,7 @@ export class IntegrationsController {
     this.logger.log(
       `M365-Integration OAuth-Flow gestartet fuer User ${user.sub}`,
     );
-    res.redirect(authUrl);
+    return { success: true, data: { url: authUrl } };
   }
 
   /**
diff --git a/packages/frontend/src/crm/api.ts b/packages/frontend/src/crm/api.ts
index 96e4ba1..2e2feb7 100644
--- a/packages/frontend/src/crm/api.ts
+++ b/packages/frontend/src/crm/api.ts
@@ -771,8 +771,15 @@ export const integrationsApi = {
       )
       .then((r) => r.data),
 
-  /** Gibt die URL zurück, zu der der Browser weitergeleitet werden soll */
-  getM365ConnectUrl: (): string => '/api/v1/auth/integrations/microsoft-365',
+  /** Ruft die Microsoft OAuth-URL via API ab (JWT-geschützt) und leitet den Browser weiter */
+  connectM365: () =>
+    api
+      .get<{ success: boolean; data: { url: string } }>(
+        '/auth/integrations/microsoft-365',
+      )
+      .then((r) => {
+        window.location.href = r.data.data.url;
+      }),
 };
 
 // --- Microsoft Graph Proxy (CRM) ---
diff --git a/packages/frontend/src/crm/contacts/CalendarTab.tsx b/packages/frontend/src/crm/contacts/CalendarTab.tsx
index 1a7e0d2..8f2d506 100644
--- a/packages/frontend/src/crm/contacts/CalendarTab.tsx
+++ b/packages/frontend/src/crm/contacts/CalendarTab.tsx
@@ -32,8 +32,9 @@ export function CalendarTab({ contactId }: Props) {
         <p style={{ color: 'var(--color-text-muted)', fontSize: '0.9375rem', marginBottom: '1rem' }}>
           Verbinden Sie Microsoft 365, um Kalendertermine zu diesem Kontakt zu sehen.
         </p>
-        <a
-          href={integrationsApi.getM365ConnectUrl()}
+        <button
+          type="button"
+          onClick={() => integrationsApi.connectM365()}
           style={{
             display: 'inline-flex',
             alignItems: 'center',
@@ -41,14 +42,15 @@ export function CalendarTab({ contactId }: Props) {
             padding: '0.4375rem 1rem',
             background: 'var(--color-primary)',
             color: 'white',
+            border: 'none',
             borderRadius: 'var(--radius-sm)',
             fontSize: '0.875rem',
             fontWeight: 600,
-            textDecoration: 'none',
+            cursor: 'pointer',
           }}
         >
           Microsoft 365 verbinden
-        </a>
+        </button>
       </div>
     );
   }
diff --git a/packages/frontend/src/crm/contacts/EmailsTab.tsx b/packages/frontend/src/crm/contacts/EmailsTab.tsx
index 4834f48..dd30893 100644
--- a/packages/frontend/src/crm/contacts/EmailsTab.tsx
+++ b/packages/frontend/src/crm/contacts/EmailsTab.tsx
@@ -31,8 +31,9 @@ export function EmailsTab({ contactId }: Props) {
         <p style={{ color: 'var(--color-text-muted)', fontSize: '0.9375rem', marginBottom: '1rem' }}>
           Verbinden Sie Microsoft 365, um E-Mails zu diesem Kontakt zu sehen.
         </p>
-        <a
-          href={integrationsApi.getM365ConnectUrl()}
+        <button
+          type="button"
+          onClick={() => integrationsApi.connectM365()}
           style={{
             display: 'inline-flex',
             alignItems: 'center',
@@ -40,14 +41,15 @@ export function EmailsTab({ contactId }: Props) {
             padding: '0.4375rem 1rem',
             background: 'var(--color-primary)',
             color: 'white',
+            border: 'none',
             borderRadius: 'var(--radius-sm)',
             fontSize: '0.875rem',
             fontWeight: 600,
-            textDecoration: 'none',
+            cursor: 'pointer',
           }}
         >
           Microsoft 365 verbinden
-        </a>
+        </button>
       </div>
     );
   }
diff --git a/packages/frontend/src/crm/contacts/TasksTab.tsx b/packages/frontend/src/crm/contacts/TasksTab.tsx
index 8af2302..dd24764 100644
--- a/packages/frontend/src/crm/contacts/TasksTab.tsx
+++ b/packages/frontend/src/crm/contacts/TasksTab.tsx
@@ -117,8 +117,9 @@ export function TasksTab({ contactId }: Props) {
         <p style={{ color: 'var(--color-text-muted)', fontSize: '0.9375rem', marginBottom: '1rem' }}>
           Verbinden Sie Microsoft 365, um Aufgaben zu diesem Kontakt zu sehen.
         </p>
-        <a
-          href={integrationsApi.getM365ConnectUrl()}
+        <button
+          type="button"
+          onClick={() => integrationsApi.connectM365()}
           style={{
             display: 'inline-flex',
             alignItems: 'center',
@@ -126,14 +127,15 @@ export function TasksTab({ contactId }: Props) {
             padding: '0.4375rem 1rem',
             background: 'var(--color-primary)',
             color: 'white',
+            border: 'none',
             borderRadius: 'var(--radius-sm)',
             fontSize: '0.875rem',
             fontWeight: 600,
-            textDecoration: 'none',
+            cursor: 'pointer',
           }}
         >
           Microsoft 365 verbinden
-        </a>
+        </button>
       </div>
     );
   }
diff --git a/packages/frontend/src/profile/ProfilePage.tsx b/packages/frontend/src/profile/ProfilePage.tsx
index 18fc857..ebd36cc 100644
--- a/packages/frontend/src/profile/ProfilePage.tsx
+++ b/packages/frontend/src/profile/ProfilePage.tsx
@@ -843,9 +843,9 @@ export function ProfilePage() {
                   Verbinden Sie Ihr Microsoft 365 Konto, um E-Mails, Kalendertermine und Aufgaben
                   direkt in Kontaktprofilen zu sehen.
                 </p>
-                <a
-                  href={integrationsApi.getM365ConnectUrl()}
-                  className={styles.buttonPrimary ?? undefined}
+                <button
+                  type="button"
+                  onClick={() => integrationsApi.connectM365()}
                   style={{
                     display: 'inline-flex',
                     alignItems: 'center',
@@ -853,17 +853,18 @@ export function ProfilePage() {
                     padding: '0.5rem 1.25rem',
                     background: 'var(--color-primary)',
                     color: 'white',
+                    border: 'none',
                     borderRadius: 'var(--radius-sm)',
                     fontSize: '0.9375rem',
                     fontWeight: 600,
-                    textDecoration: 'none',
+                    cursor: 'pointer',
                   }}
                 >
                   <svg width="16" height="16" viewBox="0 0 16 16" fill="currentColor">
                     <path d="M0 0h7.5v7.5H0zm8.5 0H16v7.5H8.5zM0 8.5h7.5V16H0zm8.5 0H16V16H8.5z" />
                   </svg>
                   Microsoft 365 verbinden
-                </a>
+                </button>
               </div>
             )}
           </div>