Path Traversal Exploits Built-In Functions: Why Next.js File APIs Fall to Directory Attacks

The article reveals a dangerous misconception: developers often mistake general-purpose utility functions like path.join() for security boundaries, when they're merely string helpers. This vulnerability in Next.js file-serving endpoints demonstrates how unsanitized user input combined with false assumptions about built-in functions enables unauthorized file access, underscoring the critical need for explicit path validation in API design.