https://bugs.kde.org/show_bug.cgi?id=464142

--- Comment #3 from Pedro V <voidpointertonull+bugskde...@gmail.com> ---
I'm just somewhat guessing, especially trying to guess what could have been
your issue, not agreeing with how it currently works, and that's also why I'm
not really experienced with it because I don't really view it as secure either,
I just see the logic how it makes sense that bidirectional sharing results in
chained propagation, and users preferring convenience over security are likely
okay with it.

Generally KDE Connect doesn't really seem to be too security focused (yet),
pairing establishes an overly trusting configuration by default.

Still, what you likely experienced isn't really what I'd conclude as A and C
trusting each-other, they can't do anything without B proxying, so the issue is
with the lack of finer grained control/permissions instead of a security bug
existing.
What could have happened:
- A notices clipboard change, sends new data to B
- B gets new data, writes to clipboard
- B notices clipboard change, sends new data to C
- C gets new data, writes to clipboard

Technically there could be an option to explicitly break chaining by making
sure the clipboard changes coming from KDE Connect itself are not propagated,
but I still believe that the earlier proposed solutions are the right way.
The issue isn't really any different than what would you have with other P2P
solutions. For example if Syncthing is used to share a directory from A to B,
and B would share it with C, then you'd still have the same issue, and the
solutions there are pretty much similar to what I proposed:
- There's no "automatic" data sending in the sense that you can choose to use
another directory first, and only copy the result to the shared one when you
are ready to share. Clipboard is quite different, so an optional explicit send
operation would be more sensible there as you can't choose to use an
alternative clipboard if you don't want to send, you just want to copy around
information locally.
- Directories can be set as "Send & Receive", "Send Only", or "Receive Only".
You can't actually have a receive only directory shared with 2 completely
unrelated senders due to the resulting conflicts, but in this case the
clipboard is simpler, you could set up B to only receive from A and C without
sending to them, so the other hosts would be able to send new data, but they
would never receive any. Also, alternatively just like manual sending, manual
receiving could be possible, it may as well be an optional automatic prompt
popping up allowing new clipboard content to be accepted if the user wishes to
do so.

Still, these are just proposals how could it be better, but many users might be
okay with the current complete trust approach already even if it's not
appealing for us.
Look at this "A <-> B <-> malware"-kind of example on a completely different
platform: https://www.youtube.com/watch?v=xZBDmI9fdbs
Of course others also having the same problem doesn't make it right, just
pointing out that it's not considered a bug, it's a permissiveness issue
arising from the coarse grained control of just either propagating widely, or
not using the functionality at all.

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