diff --git a/src/backend/InvenTree/order/api.py b/src/backend/InvenTree/order/api.py
index c98479741b..923aaa841d 100644
--- a/src/backend/InvenTree/order/api.py
+++ b/src/backend/InvenTree/order/api.py
@@ -2520,6 +2520,7 @@ class OrderCalendarExport(ICalFeed):
         https://www.djangosnippets.org/snippets/243/
         """
         import base64
+        import binascii
 
         if request.user.is_authenticated:
             # Authenticated on first try - maybe normal browser call?
@@ -2534,11 +2535,20 @@ class OrderCalendarExport(ICalFeed):
                 # NOTE: We are only support basic authentication for now.
                 #
                 if auth[0].lower() == 'basic':
-                    uname, passwd = base64.b64decode(auth[1]).decode('ascii').split(':')
-                    user = authenticate(username=uname, password=passwd)
-                    if user is not None and user.is_active:
-                        login(request, user)
-                        request.user = user
+                    try:
+                        # Split on the first colon only - passwords may contain ':'
+                        uname, passwd = (
+                            base64.b64decode(auth[1]).decode('utf-8').split(':', 1)
+                        )
+                    except (binascii.Error, UnicodeDecodeError, ValueError):
+                        # Malformed header - fall through to the 401 response
+                        uname = passwd = None
+
+                    if uname is not None:
+                        user = authenticate(username=uname, password=passwd)
+                        if user is not None and user.is_active:
+                            login(request, user)
+                            request.user = user
 
         # Check again
         if request.user.is_authenticated:
diff --git a/src/backend/InvenTree/order/test_api.py b/src/backend/InvenTree/order/test_api.py
index c40f35ef94..396c12d72c 100644
--- a/src/backend/InvenTree/order/test_api.py
+++ b/src/backend/InvenTree/order/test_api.py
@@ -1005,6 +1005,39 @@ class PurchaseOrderTest(OrderTest):
         )
         self.assertEqual(response.status_code, 200)
 
+    def test_po_calendar_auth_password_colon(self):
+        """Test header authorization with a password containing a colon."""
+        password = 'pass:word:123'
+        self.user.set_password(password)
+        self.user.save()
+        self.client.logout()
+
+        base64_token = base64.b64encode(f'{self.username}:{password}'.encode()).decode(
+            'ascii'
+        )
+        response = self.client.get(
+            reverse('api-po-so-calendar', kwargs={'ordertype': 'purchase-order'}),
+            format='json',
+            headers={'authorization': f'basic {base64_token}'},
+        )
+        self.assertEqual(response.status_code, 200)
+
+    def test_po_calendar_auth_malformed(self):
+        """Test that malformed basic authorization headers return 401, not 500."""
+        self.client.logout()
+        url = reverse('api-po-so-calendar', kwargs={'ordertype': 'purchase-order'})
+
+        for token in [
+            'not-base64!!',  # invalid base64
+            base64.b64encode(b'\xff\xfe\xfd').decode('ascii'),  # invalid utf-8
+            base64.b64encode(b'no-colon-here').decode('ascii'),  # no separator
+        ]:
+            response = self.client.get(
+                url, format='json', headers={'authorization': f'basic {token}'}
+            )
+            self.assertEqual(response.status_code, 401)
+            self.assertEqual(response['WWW-Authenticate'], 'Basic realm="api"')
+
     def test_po_calendar_no_permission(self):
         """Test that an authenticated user without purchase_order view permission is denied."""
         self.clearRoles()