Evil pixels: Researcher demos data-theft over screen-share protocols

Users see white noise, attackers see whatever they just stole from you

It's the kind of thinking you expect from someone who lives in a volcano lair: exfiltrating data from remote screen pixel values.

The idea comes from Pen Test Partners' Alan Monie, taking a break from sex toy hacks and wondering how to get data over a connection like RDP (remote desktop protocol) when the target had blocked file transfer (or the target is using a VMware console that can't transfer files).

His approach: since the point of a remote desktop is that it's the contents of a victim's user's screen that gets copied over the wire, encode the data you want to steal into the screen data, by flashing up a screen full of what looks like white noise.

Monie's PTP-RAT proof-of-concept code to do this zips down to a mere 13 KB. At that size, a skilled attacker could upload during an RDP session without attracting attention.

The only stumbling block Monie had to deal with to create it is compression: RDP didn't transmit screen colour data completely accurately.

However, restricting the exfiltration encoding rate to three bits per pixel (encoding just one bit on each of the Red, Green and Blue values) took care of the compression issue, allowing him to pull a 3 MB file off the target in “a few seconds”.

Here's how PTP-RAT functions, pretty much in full:

Each screen flash starts with a header. This contains a magic string, “PTP-RAT-CHUNK” followed by a sequence number. When the receiver is activated, it starts taking screenshots at twice the transmission frequency (the Nyquist rate). When it detects a valid header, it decodes the pixel colour information and waits on the next flash. As soon as a valid header is not detected, it reconstructs all the flashes and saves the result to a file.

To transfer a file, you run an instance of the Rat locally on your hacktop, and set that up as a receiver. Another instance is run on the remote server and this acts as a sender. You simply click on send file, and select a file to send. The mouse pointer disappears and the screen begins to flash as the file is transmitted via the pixel colour values. At the end of the transfer, a file-save dialog appears on the receiver, and the file is saved.

As far as Vulture South can tell from Monie's video, the victim won't see the alteration – because that's what's sent over the wire to the attacker, and they're expecting to see the screen flashes. ®

Similar topics

Other stories you might like

  • Graviton 3: AWS attempts to gain silicon advantage with latest custom hardware

    Key to faster, more predictable cloud

    RE:INVENT AWS had a conviction that "modern processors were not well optimized for modern workloads," the cloud corp's senior veep of Infrastructure, Peter DeSantis, claimed at its latest annual Re:invent gathering in Las Vegas.

    DeSantis was speaking last week about AWS's Graviton 3 Arm-based processor, providing a bit more meat around the bones, so to speak – and in his comment the word "modern" is doing a lot of work.

    The computing landscape looks different from the perspective of a hyperscale cloud provider; what counts is not flexibility but intensive optimization and predictable performance.

    Continue reading
  • The Omicron dilemma: Google goes first on delaying office work

    Hurrah, employees can continue to work from home and take calls in pyjamas

    Googlers can continue working from home and will no longer be required to return to campuses on 10 January 2022 as previously expected.

    The decision marks another delay in getting more employees back to their desks. For Big Tech companies, setting a firm return date during the COVID-19 pandemic has been a nightmare. All attempts were pushed back so far due to rising numbers of cases or new variants of the respiratory disease spreading around the world, such as the new Omicron strain.

    Google's VP of global security, Chris Rackow, broke the news to staff in a company-wide email, first reported by CNBC. He said Google would wait until the New Year to figure out when campuses in the US can safely reopen for a mandatory return.

    Continue reading
  • This House believes: A unified, agnostic software environment can be achieved

    How long will we keep reinventing software wheels?

    Register Debate Welcome to the latest Register Debate in which writers discuss technology topics, and you the reader choose the winning argument. The format is simple: we propose a motion, the arguments for the motion will run this Monday and Wednesday, and the arguments against on Tuesday and Thursday. During the week you can cast your vote on which side you support using the poll embedded below, choosing whether you're in favour or against the motion. The final score will be announced on Friday, revealing whether the for or against argument was most popular.

    This week's motion is: A unified, agnostic software environment can be achieved. We debate the question: can the industry ever have a truly open, unified, agnostic software environment in HPC and AI that can span multiple kinds of compute engines?

    Our first contributor arguing FOR the motion is Nicole Hemsoth, co-editor of The Next Platform.

    Continue reading

Biting the hand that feeds IT © 1998–2021