A file upload vulnerability is a weakness in how a service accepts, checks, stores, or processes uploaded files. The danger is not just that a user can place a file on the server, but that the file may be handled as trusted input. If validation is weak, an attacker may upload unexpected file types, embed malicious content, or trigger server-side parsing bugs.
In cyber security, this matters because upload features often sit near sensitive systems such as web apps, storage, metadata services, or internal processing jobs. A flawed upload path can lead to code execution, data exposure, malware hosting, or unauthorized access to records. Defenses include strict allowlists, content-type and file-signature validation, file size limits, renaming files, storing them outside the web root, sandboxing processing, and using access controls and logging. Treat every upload as hostile until it is safely verified.