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Security Technology and Internet

Controlling the access to the clipboard contents

In a previous blog post published earlier this year I explored some security considerations of the well known “clipboard” functionality that most operating systems provide.

Long story short, in my opinion there is a lot more that could be done to protect the users (and their sensitive data) from many attacks that use of clipboard as a vector to trick the user or extract sensitive material.

The proof-of-concept I ended up building to demonstrate some of the ideas worked in X11 but didn’t achieve one of the desired goals:

It seems possible to detect all attempts of accessing the clipboard, but after struggling a bit, it seems that due to the nature of X11 it is not possible to know which running process owns the window that is accessing the clipboard. A shame.

Myself, last blog post on this topic

The good news about the above quote, is that it no longer is true. A kind soul contributed a patch that allows “clipboard-watcher” to fetch the required information about the process accessing the clipboard. Now we have all ingredients to make the tool fulfill its initial intended purpose (and it does).

With this lengthily introduction we are ready to address the real subject of this post, giving the user more control over how the clipboard is used. Notifying the users about an access is just a first step, but restricting the access is what we want.

On this topic, several comments to the previous post mentioned the strategy used by Qubes OS. It relies on having one clipboard specific to each app and a second clipboard that is shared between apps. The later requires user intervention to be used. While I think this is a good approach, it is not easy to replicate in a regular Linux distribution.

However as I mentioned in my initial post, I think we can achieve a similar result by asking the user for permission when an app requests the data currently stored on the clipboard. This was the approach Apple implemented on the recent iOS release.

So in order to check/test how this could work, I tried to adapt my proof-of-concept to ask for permission before sharing any data. Here’s an example:

Working example of clipboard-watcher requesting permission before letting other apps access the clipboard contents.

As we can see, it asks of permission before the requesting app is given the data and it kinda works (ignore the clunky interface and UX). Of course that there are many possible improvements to make its usage bearable, such as whitelisting certain apps, “de-duplicate” the content requests (apps can generate a new one for each available content type, which ends up being spammy), etc.

Overall I’m pleased with the result and in my humble opinion this should be a “must have” security feature for any good clipboard manager on Linux. I say it even taking into account that this approach is not bulletproof, given that a malicious application could continuously fight/race the clipboard manager for the control of the “X selections”.

Anyhow, the new changes for the proof-of-concept are available here, please give it a try and let me know what you think and if you find any other problems.

By Gonçalo Valério

Software developer and owner of this blog. More in the "about" page.

2 replies on “Controlling the access to the clipboard contents”

The correct thing to do would be to make it push rather than pull. Don’t let the application request the clipboard content, let the clipboard send the content to the application. That is, rather than the application receiving ctrl-v, the application should receive a paste event with the data.

Pasting with the mouse would probably require some kind of system level menu bar like MacOS.

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