You connect your terminal to a program (pts, which may map to a sandbox or a remote SSH server you don’t trust or just a file you feed to cat). And it contains an escape sequence that causes your terminal to read and process ~/.ssh/id_rsa or /etc/shadow or /dev/sda or /proc/self/something or some other wildly inappropriate object. And your terminal opens and reads the file.

My terminal does not live in a mountain fastness, and it’s not as exposed as a web browser, but it should at least try to make it safe to feed it untrustworthy input.