Ian Latter, of the Through Glass Transfer project (TGXf), presented a paper at the Kiwicon conference describing a technique for clientless data transfer through pixels on a standard display monitor.
The TGXf project provides a suite of tools intended to debunk the myth that data transfer requires some form of network communication that is detectable by monitoring network traffic. Some security standards, such as the US HIPAA medical records standard, make a distinction between records that can be transmitted over the network and records that should only be used on an isolated system. The TGXf project considers this distinction irrelevant, and they have developed tools to prove it.
The TGXf protocol transfers data by outputting QR codes to the screen. A simple HDMI recorder, or even a smartphone, can capture the screen out put and analyze it later to recover the file. Of course, the system must be compromised to produce the QR-coded screen output, but the important thing is, no trace of the data transfer is left behind, and no server/client communication reveals that the data transfer is taking place.
The TGXf project offers other alternatives for this client-less data transfer, including ThruKeyboardXfer (TKXf), which captures keyboard output through an Arduino board attached to a USB port.
The TGXf techniques do require physical access to the system, but they circumvent all common security measures. According to the TGXf website, the problems revealed through the TGXf project do not even have a Common Vulnerabilities and Exposures ID, because they aren't actually a vulnerabilities but represent, instead, "a flaw in the end-to-end architecture."