More file upload issues
Earlier in the chapter, we had a look at how file upload can help us to compromise an application and the server it sits on. We were able to upload a valid PNG file containing an embedded PHP shell. The LFI vulnerability allowed us to execute that code.
There are other problems with allowing users to upload arbitrary files to the application. You could very well prevent users from uploading PHP, JSP, or ASP shells by simply blacklisting the extension. PHP only executes code in files with a particular extension (or two) if they are called directly. Barring any LFI vulnerability somewhere else in the application, the file upload feature should be fairly safe from a code execution perspective.
If one of the application features is to allow file storage for users, whitelisting may be difficult and cumbersome to implement. In this scenario, blacklisting extensions may be the most cost-effective solution. When we can't upload a shell or execute server...
