"Don't Copy-Paste commands from webpages — you can get hacked"
>"We always recommended to open a plain ascii editor, paste it there and see if there is no other code there. It is an old and sane advise"
The best advise is to not use a desktop terminal at all, only use the gettys started by init.
The clipboard has often been a source of data leaks, going back to beginning of GUIs.
I looked up this specific exploit, and it's called "pastejacking". It was first discovered about six years ago by Dylan Ayrey and apparently recently rediscovered independently by Gabriel Friedlander.
In essence, the clipboard is a user-level IPC, and those are always attractive security targets.
GUI = graphical user interface
IPC = interprocess communication
man getty
man init