File Upload Security Testing: Preventing Remote Code Execution and Data Exfiltration — 日本企業向けガイド

File upload functionality — allowing users to upload images, documents, or other files — is one of the most common sources of critical vulnerabilities. If not properly validated, an attacker may upload executable files (web shells, server-side scripts) that grant remote code execution on the server. Testing covers: file type validation (can the application be tricked into accepting executable files disguised as images?), file content validation (does the server inspect file contents or only the extension?), storage location (are uploaded files stored in web-accessible directories where they can be directly executed?), filename handling (can path traversal in filenames overwrite critical files?), and file size limits (can oversized uploads cause denial of service?). The most dangerous uploads are those that bypass extension and MIME-type checks through double extensions, null bytes, or content-type manipulation.