Something running with your privileges could similarly use your existing sudo ticket, or manipulate the memory of your terminal emulator, or modify your shell to grab your credentials the next time you authenticate (and pass those to sudo), etc.
This isn't tmux's fault; this is fundamentally the sort of thing that's possible under the security model of modern Linux desktops.
This isn't tmux's fault; this is fundamentally the sort of thing that's possible under the security model of modern Linux desktops.