Bandit Level 22 → Level 23

A program is running automatically at regular intervals from cron, the time-based job scheduler. Look in /etc/cron.d/ for the configuration and see what command is being executed.

NOTE: Looking at shell scripts written by other people is a very useful skill. The script for this level is intentionally made easy to read. If you are having problems understanding what it does, try executing it to see the debug information it prints.

解密

bandit22@bandit:~$ ls -l /etc/cron.d/
total 12
-rw-r--r-- 1 root root 120 Oct 16  2018 cronjob_bandit22
-rw-r--r-- 1 root root 122 Oct 16  2018 cronjob_bandit23
-rw-r--r-- 1 root root 120 Oct 16  2018 cronjob_bandit24
bandit22@bandit:~$ cat /etc/cron.d/cronjob_bandit23
@reboot bandit23 /usr/bin/cronjob_bandit23.sh  &> /dev/null
* * * * * bandit23 /usr/bin/cronjob_bandit23.sh  &> /dev/null
bandit22@bandit:~$ cat /usr/bin/cronjob_bandit23.sh
#!/bin/bash

myname=$(whoami)
mytarget=$(echo I am user $myname | md5sum | cut -d ' ' -f 1)

echo "Copying passwordfile /etc/bandit_pass/$myname to /tmp/$mytarget"

cat /etc/bandit_pass/$myname > /tmp/$mytarget
bandit22@bandit:~$ echo 'I am user bandit23' | md5sum | cut -d ' ' -f 1
8ca319486bfbbc3663ea0fbe81326349
bandit22@bandit:~$ cat /tmp/8ca319486bfbbc3663ea0fbe81326349
jc1udXuA1tiHqjIsL8yaapX5XIAI6i0n

知识点

  1. whoami这个命令，会打印出当前执行人是谁，此处是bandit23；
  2. md5sum是对输入进行md5哈希；
  3. cut是分割空格，-d表明风格字符，-f取分割后的第几个字符串；