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Insecure file upload vulnerabilities are a critical security issue that can lead to severe consequences, including remote code execution and data breaches. This article covers the basics of insecure file uploads, bypass techniques, and how to defend against them.
You Should Know:
1. Basic Bypass Techniques:
- Attackers often bypass file upload restrictions by modifying file extensions or MIME types.
- Example: Changing a `.php` file to `.jpg.php` or altering the `Content-Type` header in Burp Suite.
<h1>Example: Renaming a malicious file</h1> mv shell.php shell.jpg.php
2. Magic Bytes Bypass:
- Magic bytes are the first few bytes of a file that identify its type. Attackers can spoof these to bypass checks.
- Example: Adding GIF magic bytes (
GIF89a) to a PHP file.
<h1>Adding magic bytes to a file</h1> echo -e "\x47\x49\x46\x38\x39\x61" > shell.php
3. Challenge Walkthrough:
- Practice bypassing file upload restrictions in a controlled environment like Hack The Box or TryHackMe.
- Use tools like Burp Suite to intercept and modify upload requests.
<h1>Using Burp Suite to intercept upload requests</h1> burpsuite &
4. Defensive Measures:
- Implement strict file type verification using both extension and magic bytes.
- Store uploaded files in a secure directory with no execution permissions.
<h1>Securing upload directory in Linux</h1> chmod -R 755 /var/www/uploads chown -R www-data:www-data /var/www/uploads
5. Automating Checks:
- Use scripts to validate file uploads on the server side.
<h1>Example: Python script to check magic bytes</h1>
import magic
mime = magic.Magic(mime=True)
file_type = mime.from_file("uploaded_file")
if file_type != "image/jpeg":
print("Invalid file type!")
What Undercode Say:
Insecure file upload vulnerabilities are a common attack vector, but they can be mitigated with proper validation and security practices. Always verify file types, use secure storage, and regularly test your defenses. For further reading, check out OWASP File Upload Cheat Sheet.
<h1>Additional Linux Commands for Security:</h1> <h1>Check for open ports</h1> netstat -tuln <h1>Monitor file changes in upload directory</h1> inotifywait -m /var/www/uploads <h1>Scan for malicious files</h1> clamscan -r /var/www/uploads
<h1>Windows Commands for Security:</h1>
<h1>Check open ports</h1>
netstat -an
<h1>Monitor file changes</h1>
Get-ChildItem -Path C:\uploads -Recurse | ForEach-Object { Write-Output $_.FullName }
Stay vigilant and keep practicing!
References:
Reported By: Todd Mattran – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
