Secure file upload in PHP web applications

Various web applications allow users to upload files. Web forums let users upload avatars. Photo galleries let users upload pictures. Social networking web sites may allow uploading pictures, videos, etc. Blog sites allow uploading avatars and/or pictures.

Providing file upload function without opening security holes proved to be quite a challenge in PHP web applications. The applications we have tested suffered from a variety of security problems, ranging from arbitrary file disclosure to remote arbitrary code execution. In this article I am going to point out various security holes occurring in file upload implementations and suggest a way to implement a secure file upload.
The examples shown in this article can be downloaded from
http://www.scanit.be/uploads/php-file-upload-examples.zip. If you want to test the examples please make sure that the server you are using is not accessible from the Internet or any other untrusted networks. The examples are provided to demonstrate various security holes. Installing them on a server where those holes can be exploited is not a good idea.
Download Secure file upload in PHP web applications (pdf)