Courses
1. Introduction to Blockchain and Solana
2. Setting Up Your Development Environment
3. Solana Architecture and Key Concepts
4. Creating and Managing Wallets
5. Understanding Solana Programs
6. Building Your First Solana Program
7. Deploying Programs on the Solana Network
8. Interacting with Solana Programs
9. Using Solana CLI and SDKs
10. Developing Decentralized Applications (DApps)
11. Smart Contract Security Best Practices
12. Testing and Debugging Solana Programs
13. Optimizing Performance on Solana
14. Exploring Solana Ecosystem Tools
1. Introduction to Blockchain and Ethereum
2. Setting Up Your Development Environment
3. Ethereum Architecture and Key Concepts
4. Creating and Managing Wallets
5. Understanding Smart Contracts
6. Building Your First Smart Contract
7. Deploying Smart Contracts on the Ethereum Network
8. Interacting with Smart Contracts
9. Using Ethereum CLI and SDKs
10. Developing Decentralized Applications (DApps)
11. Smart Contract Security Best Practices
12. Testing and Debugging Smart Contracts
13. Optimizing Performance on Ethereum
14. Exploring Ethereum Ecosystem Tools
1. Introduction to Blockchain and Polygon
2. Setting Up Your Development Environment
3. Polygon Architecture and Key Concepts
4. Creating and Managing Wallets
5. Understanding Smart Contracts on Polygon
6. Building Your First Smart Contract on Polygon
7. Deploying Smart Contracts on the Polygon Network
8. Interacting with Smart Contracts on Polygon
9. Using Polygon CLI and SDKs
10. Developing Decentralized Applications (DApps) on Polygon
11. Smart Contract Security Best Practices for Polygon
12. Testing and Debugging Smart Contracts on Polygon
13. Optimizing Performance on Polygon
14. Exploring Polygon Ecosystem Tools
1. Introduction to Cybersecurity
2. Report Writing for Penetration Testers
3. Information Gathering
4. Vulnerability Scanning
5. Introduction to Web Applications
6. Common Web Application Attacks
7. SQL Injection Attacks
8. Client-Side Attacks
9. Locating Public Exploits
10. Fixing Exploits
11. Antivirus Evasion
12. Password Attacks
13. Windows Privilege Escalation
14. Linux Privilege Escalation
15. Advanced Tunneling
16. The Metasploit Framework
17. Active Directory: Introduction and Enumeration
18. Attacking Active Directory Authentication
19. Lateral Movement in Active Directory
Bypassing File Upload Restrictions and File Extension Filters
OSWE · Chapter 11
Bypassing File Upload Restrictions and File Extension Filters
File upload functionality is a frequent path to remote code execution when an attacker can upload a server-side executable script (e.g. a `.php` webshell) and then have the server execute it — bypassing weak or purely client-side extension checks.
Key concepts
- Never trust a client-supplied `Content-Type` header — it is trivially spoofed and must not be the only validation performed.
- Extension allow-lists (not deny-lists) plus validating actual file content/signature (magic bytes) are the strongest combination of checks.
- Classic bypasses: double extensions (`shell.php.jpg`), null-byte truncation (`shell.php%00.jpg`), and case variation on filesystems that treat extensions case-insensitively.
- Even with strict validation, store uploads outside the webroot (or in a location with no script execution permission) so that even a successfully uploaded malicious file cannot be directly executed.
Example
# Classic double-extension bypass attempt against a weak filter (lab only)
curl -F "file=@shell.php.jpg" https://target/uploadTry it yourself
On a lab file-upload challenge, try bypassing an extension filter using a double extension or a null-byte trick, then confirm whether the server actually executes the uploaded file when you request it directly.