diff --git a/LibGUI/GClipboard.cpp b/LibGUI/GClipboard.cpp
index d6698c8a35..01e198b1ff 100644
--- a/LibGUI/GClipboard.cpp
+++ b/LibGUI/GClipboard.cpp
@@ -44,6 +44,7 @@ void GClipboard::set_data(const String& data)
         return;
     }
     memcpy(shared_buffer->data(), data.characters(), data.length() + 1);
+    shared_buffer->seal();
     request.clipboard.shared_buffer_id = shared_buffer->shared_buffer_id();
     request.clipboard.contents_size = data.length();
     auto response = GEventLoop::main().sync_request(request, WSAPI_ServerMessage::Type::DidSetClipboardContents);
diff --git a/WindowServer/WSClientConnection.cpp b/WindowServer/WSClientConnection.cpp
index 64327e80ba..08a07bf685 100644
--- a/WindowServer/WSClientConnection.cpp
+++ b/WindowServer/WSClientConnection.cpp
@@ -336,6 +336,7 @@ void WSClientConnection::handle_request(WSAPIGetClipboardContentsRequest&)
         RetainPtr<SharedBuffer> shared_buffer = SharedBuffer::create(m_pid, WSClipboard::the().size());
         ASSERT(shared_buffer);
         memcpy(shared_buffer->data(), WSClipboard::the().data(), WSClipboard::the().size());
+        shared_buffer->seal();
         response.clipboard.shared_buffer_id = shared_buffer->shared_buffer_id();
         response.clipboard.contents_size = WSClipboard::the().size();