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Absolutely agree! It's even more true for closed source software like Skype - to be able to prevent it from accessing to data it doesn't have to. I know there

by reirob 12y ago

Absolutely agree! It's even more true for closed source software like Skype - to be able to prevent it from accessing to data it doesn't have to. I know there are many alternatives to Skype, but there are people that have to use it from time to time for professional reasons. I started doing this for Skype on Ubuntu 14.04 - but have to admit that I quickly ran out of steam. Need to pick up where I ended.

pflanze 12y ago
Isn't it impossible to secure Skype because of it's use of X11? I've found it to be impossible to run Skype in untrusted X11 (so that it doesn't get full control over your X display), and running it in a nested X server is probably too slow for video. (And running it in a VM didn't work either for me because of audio and video performance. I really decided I couldn't use Skype except when running it on a real machine dedicated to running Skype.)
- marcoamorales 12y ago
  You can run Skype inside a docker container: https://wiki.archlinux.org/index.php/skype#Docker https://wiki.archlinux.org/index.php/skype#Docker
  - pflanze 12y ago
    "It is possible to use Skype in a safe Docker container. The program is then run through an SSH tunnel with X11 forwarding and sound is heard through PulseAudio's Network Server." Well... 1. how is video speed through the ssh tunnel, then? Unsolved, I guess. 2. as I said, Skype won't do untrusted X11 (at least the version I tried didn't). This means that you need to tunnel it with trusted X11 access. Which means, no security (it will log your passwords and open and use X terminals if it wants to). Which makes the whole exercise moot. Other suggestions on this page are suffering from the same flaw. It seems all the writers of that page have never heard that X11 doesn't provide security. Also, they advise to use xhost. In the end you're worse off when following the recipes on this page than you were before.
- shetter 12y ago
  If you have a smartphone you could use that as your Skype-machine. I have a cheap Chinese Android phone bought 1 year ago and it can take video calls easily. The point being that in most cases you don't have much choice giving up security on your phone anyway so you may as well put all the spyware in one place.
- reirob 12y ago
  I agree that it is impossible to completely secure Skype because of X11. And I need as well for some calls the screen sharing feature. So for me this is a wanted feature. But I would appreciate if I knew that Skype is _not_ able to access my confidential files through the file system. As a bonus it would be great if I could as well forbid Skype to access the local network and just allow it to talk through the default gateway. I then would start Skype on the laptop only when I have to speak with people. During these teleconferences I anyway have to clean my screen. After the call I can stop the skype process but keep being reachable through skype on phone. Would this scenario be possible or is there still a security risk for my data?
  - pflanze 12y ago
    It is my understanding that once an app has access to full X11, it can do everything you can do with your keyboard and mouse[1]. Like open terminals, cat files, it won't even have to parse the result from the screen if xsel or a text editor is installed since it will be able to copy their content from the clipboard. Or it could run wget or netcat or similar to download a binary that will connect to a server to upload your files. I suppose that normally you're able to see these activities. But the app could observe periods of inactivity so as to wait until it thinks you're away, then quickly open a terminal, enter the command to wget then start the binary in the background and close the terminal again; that might be like 1/10 second of flickering. Or it might even be possible for it to act similar to a window manager and open the window where it wants, like off-screen or behind other windows, so you wouldn't see anything. Since it directs the window manager or panel program to open the terminal instead of starting the binary directly, any SELinux properties that say that commands exec'ed by Skype don't have the permission to do stuff will not trigger. You can't stop the panel from opening programs of course unless you're fine with preventing yourself from doing the same. (I don't think SELinux is able to track which X connection leads a panel app to start a binary; that would require it to be able to track code execution and understanding the logic of the program, which seems too complex to be feasible. Cooperation between panel and SELinux might solve this. But then this seems more like a cat&mouse game, will you not have missed other ways in which a program can be started? What about terminals that you opened, they would have to cooperate as well. In the end what you need is a replacement for X11.) [1] it is possible to inject keyboard and mouse events into the X11 event delivery system; although it might be possible for programs to differentiate between those and real keyboard/mouse events (I know that the events are tagged as articial or some such, but can the programs ignore them without ill effects, and are they doing it?) Here my knowledge is getting spotty, so perhaps it is still possible to get a (somewhat?) more secure situation than what I've depicted above. You'll have to ask someone who knows better. In any case it seems to me that entities that are willing and able to use Skype to break into people's computers will also have way more funding than you to find any obscure ways in which X11 can be used to break out. X11 (at least its normal, trusted mode of operation (and as I said, Skype won't run in untrusted mode)) isn't designed to provide security.

by reirob 12y ago

pflanze 12y ago

Isn't it impossible to secure Skype because of it's use of X11? I've found it to be impossible to run Skype in untrusted X11 (so that it doesn't get full control over your X display), and running it in a nested X server is probably too slow for video. (And running it in a VM didn't work either for me because of audio and video performance. I really decided I couldn't use Skype except when running it on a real machine dedicated to running Skype.)

marcoamorales 12y ago
You can run Skype inside a docker container: https://wiki.archlinux.org/index.php/skype#Docker https://wiki.archlinux.org/index.php/skype#Docker
- pflanze 12y ago
  "It is possible to use Skype in a safe Docker container. The program is then run through an SSH tunnel with X11 forwarding and sound is heard through PulseAudio's Network Server." Well... 1. how is video speed through the ssh tunnel, then? Unsolved, I guess. 2. as I said, Skype won't do untrusted X11 (at least the version I tried didn't). This means that you need to tunnel it with trusted X11 access. Which means, no security (it will log your passwords and open and use X terminals if it wants to). Which makes the whole exercise moot. Other suggestions on this page are suffering from the same flaw. It seems all the writers of that page have never heard that X11 doesn't provide security. Also, they advise to use xhost. In the end you're worse off when following the recipes on this page than you were before.
shetter 12y ago
If you have a smartphone you could use that as your Skype-machine. I have a cheap Chinese Android phone bought 1 year ago and it can take video calls easily. The point being that in most cases you don't have much choice giving up security on your phone anyway so you may as well put all the spyware in one place.
reirob 12y ago
I agree that it is impossible to completely secure Skype because of X11. And I need as well for some calls the screen sharing feature. So for me this is a wanted feature. But I would appreciate if I knew that Skype is _not_ able to access my confidential files through the file system. As a bonus it would be great if I could as well forbid Skype to access the local network and just allow it to talk through the default gateway. I then would start Skype on the laptop only when I have to speak with people. During these teleconferences I anyway have to clean my screen. After the call I can stop the skype process but keep being reachable through skype on phone. Would this scenario be possible or is there still a security risk for my data?
- pflanze 12y ago
  It is my understanding that once an app has access to full X11, it can do everything you can do with your keyboard and mouse[1]. Like open terminals, cat files, it won't even have to parse the result from the screen if xsel or a text editor is installed since it will be able to copy their content from the clipboard. Or it could run wget or netcat or similar to download a binary that will connect to a server to upload your files. I suppose that normally you're able to see these activities. But the app could observe periods of inactivity so as to wait until it thinks you're away, then quickly open a terminal, enter the command to wget then start the binary in the background and close the terminal again; that might be like 1/10 second of flickering. Or it might even be possible for it to act similar to a window manager and open the window where it wants, like off-screen or behind other windows, so you wouldn't see anything. Since it directs the window manager or panel program to open the terminal instead of starting the binary directly, any SELinux properties that say that commands exec'ed by Skype don't have the permission to do stuff will not trigger. You can't stop the panel from opening programs of course unless you're fine with preventing yourself from doing the same. (I don't think SELinux is able to track which X connection leads a panel app to start a binary; that would require it to be able to track code execution and understanding the logic of the program, which seems too complex to be feasible. Cooperation between panel and SELinux might solve this. But then this seems more like a cat&mouse game, will you not have missed other ways in which a program can be started? What about terminals that you opened, they would have to cooperate as well. In the end what you need is a replacement for X11.) [1] it is possible to inject keyboard and mouse events into the X11 event delivery system; although it might be possible for programs to differentiate between those and real keyboard/mouse events (I know that the events are tagged as articial or some such, but can the programs ignore them without ill effects, and are they doing it?) Here my knowledge is getting spotty, so perhaps it is still possible to get a (somewhat?) more secure situation than what I've depicted above. You'll have to ask someone who knows better. In any case it seems to me that entities that are willing and able to use Skype to break into people's computers will also have way more funding than you to find any obscure ways in which X11 can be used to break out. X11 (at least its normal, trusted mode of operation (and as I said, Skype won't run in untrusted mode)) isn't designed to provide security.