Some embodiments of cloud-based gateway security scanning have been presented. In one embodiment, some data packets are received sequentially at a gateway device. The data packets constitute at least a part of a file being addressed to a client machine coupled to the gateway device. The gateway device forwards an identification of the file to a remote datacenter in parallel with forwarding the data packets to the client machine. The datacenter performs signature matching on the identification and returns a result of the signature matching to the gateway device. The gateway device determining whether to block the file from the client machine based on the result of the signature matching from the datacenter.