diff --git a/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor b/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
index 422f4fc1..17740bf1 100644
--- a/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
+++ b/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
@@ -435,6 +435,13 @@ const int MaxThumbnailWidth = 400;
             return;
         }
 
+        // Check authentication
+        var hasAccess = await AuthService.CheckEnvelopeAccessAsync(EnvelopeKey);
+        if (!hasAccess) {
+            Navigation.NavigateTo($"/login/{Uri.EscapeDataString(EnvelopeKey)}");
+            return;
+        }
+
         try {
             var (pdfBytes, statusCode) = await DocumentService.GetDocumentAsync(EnvelopeKey);