The Hidden Risks in Multipart/Form-Data Requests

— -

```markdown
# Multipart Parser Vulnerabilities in File Upload APIs: Advanced Bypass Techniques and Defense Strategies

— -

### **Introduction: The Hidden Risks in Multipart/Form-Data**
File upload functionality is a cornerstone of modern web applications, but it’s also a prime attack vector. At the heart of this vulnerability lies `multipart/form-data`, a format designed to handle complex data but plagued by inconsistent parsing across frameworks, servers, and security tools. While Web Application Firewalls (WAFs) claim to inspect filenames and content, parser discrepancies often render these defenses ineffective. In this blog, we dissect advanced techniques to exploit multipart parser weaknesses, bypass validation, and compromise systems — along with actionable strategies to secure your APIs.

— -

### **What Are Multipart Parsers?**
Multipart parsers process HTTP requests with `multipart/form-data` content, splitting the body into parts separated by boundaries. Each part includes headers (`Content-Disposition`, `Content-Type`) and a body. However, **inconsistencies in parsing** between web frameworks (PHP, Node.js), servers, and WAFs create exploitable gaps. For example:
- **PHP** tolerates malformed boundaries.
- **Node.js (Busboy)** prioritizes `filename*` over `filename`.
- **WAFs** like ModSecurity may enforce stricter RFC compliance than backend systems.