This post documents the complete walkthrough of FourAndSix: 2.01, a boot2root VM created by Fred, and hosted at VulnHub. If you are uncomfortable with spoilers, please stop reading now.

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Background

Although there’s no description for this VM, except for “to become root and read /root/flag.txt“, the name alone is interesting. FourAndSix is the homophone for forensic—expect fun challenges ahead.

Information Gathering

Let’s start with a nmap scan to establish the available services in the host.

# nmap -n -v -Pn -p- -A --reason -oN nmap.txt 192.168.30.129
...
PORT     STATE SERVICE REASON         VERSION
22/tcp   open  ssh     syn-ack ttl 64 OpenSSH 7.9 (protocol 2.0)
| ssh-hostkey:
|   2048 ef:3b:2e:cf:40:19:9e:bb:23:1e:aa:24:a1:09:4e:d1 (RSA)
|   256 c8:5c:8b:0b:e1:64:0c:75:c3:63:d7:b3:80:c9:2f:d2 (ECDSA)
|_  256 61:bc:45:9a:ba:a5:47:20:60:13:25:19:b0:47:cb:ad (ED25519)
111/tcp  open  rpcbind syn-ack ttl 64 2 (RPC #100000)
| rpcinfo:
|   program version   port/proto  service
|   100000  2            111/tcp  rpcbind
|   100000  2            111/udp  rpcbind
|   100003  2,3         2049/tcp  nfs
|   100003  2,3         2049/udp  nfs
|   100005  1,3          809/tcp  mountd
|_  100005  1,3          997/udp  mountd
809/tcp  open  mountd  syn-ack ttl 64 1-3 (RPC #100005)
2049/tcp open  nfs     syn-ack ttl 64 2-3 (RPC #100003)

There’s nothing to explore except for NFS at 2049/tcp. We’ll start with that.

Network File System

As usual, when it comes to NFS we’ll use showmount to view the NFS exports from the VM.

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Let’s mount that.

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It appears a 7z archive file is in the directory. Let’s download the file and extract it.

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It’s a 7z archive file alright, but it’s password-protected.

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John the Ripper

Let’s see if John the Ripper can crack the password.

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Awesome. The password is chocolate.

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Now, what do we have here?

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A RSA key pair for SSH access.

If I had to guess, I would say there’s a /home/user/.ssh/authorized_keys and the content is as follows.

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Low-Privilege Shell

Let’s see if we can log in to the host with the private key.

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Another password to crack?

Long story short, I’ve tried John the Ripper and it’s no good. Let’s write a simple password cracker in bash, with ssh-keygen as the main driver for password verification.

brute.sh
#!/bin/bash

FILE=$1
PASSWORD=$2
COMMENT=[email protected]

die() {
  for pid in $(ps aux \
               | grep -v grep \
               | grep 'parallel' \
               | awk '{ print $2 }'); do
    kill -9 $pid &>/dev/null
  done
}

if ssh-keygen -c -C "$COMMENT" -P "$PASSWORD" -f "$FILE" &>/dev/null; then
  echo "Password is '$PASSWORD'" | tee found.txt
  die
fi

Let’s make use of parallel to split the job among my four vCPUs like so.

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Whoa. It’s faster than I can blink my eye.

Time to log in.

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There you have it.

Privilege Escalation

During enumeration of the user account, I notice the account is in the wheel group. Essentially, this is the superuser group; root is also in this group.

With that in mind, let’s check out /etc/doas.conf, a sudo alternative.

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What do we have here? We can run less as root? I smell “escape to shell”.

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Enter v to escape to vi, and then !sh to escape to shell. It’s that simple.

What’s the Flag?

Getting the flag is trivial when you have a root shell.

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:dancer:

Afterthought

To be honest, Fred reminds me of the FRED Forensic Workstation from Digital Intelligence I used to play with years ago. It’s still nice to dabble in OpenBSD once in a while.