A file upload vulnerability is a security flaw that allows attackers to upload files to a server without proper authorization or validation. This vulnerability can be exploited to upload malicious files, such as web shells, scripts, or executables, which can then be used to compromise the server or execute unauthorized commands. Common causes include insufficient validation of file types, lack of authentication, and improper handling of file paths. To mitigate this risk, developers should implement strict file validation, limit allowed file types, and use secure storage locations. Regular security testing and code reviews are also essential to detect and fix such vulnerabilities.