What You Will Learn

• Why file upload is risky.
• What validations to apply.
• How to prevent path traversal.
• Where to store uploaded files.
• Production scanning and limits.

Introduction

File upload is one of the riskiest features in web applications.

Attackers may upload:

• Malware.
• Oversized files.
• Scripts.
• Fake content types.
• Path traversal filenames.

Secure Upload Flow

flowchart TD
    A["Upload request"] --> B["Authentication"]
    B --> C["Size check"]
    C --> D["Extension and content check"]
    D --> E["Generate safe filename"]
    E --> F["Store outside web root"]
    F --> G["Scan and process"]

Basic Checks

• Maximum file size.
• Allowed file extensions.
• Allowed MIME types.
• Magic number or content inspection.
• Generated server-side filename.
• Virus scan where required.

Filename Safety

Never trust the original filename for storage paths.

Bad:

../../app/config.yml

Good:

generated-uuid.pdf

Spring Boot Size Limits

spring:
  servlet:
    multipart:
      max-file-size: 10MB
      max-request-size: 10MB

Storage Practices

• Store outside the application web root.
• Use object storage for large files.
• Apply access checks before download.
• Do not execute uploaded files.
• Scan files before processing.

Summary

Secure file upload requires validation, safe naming, storage isolation, size limits, access control, and malware scanning for high-risk systems.

Learning Path Navigation

• Series home: Spring Security Learning Path
• Previous: Input Validation and SQL Injection Prevention
• Next: Security Headers in Spring Boot